Wednesday, February 27, 2013

The Yolk’s On You

Biology concepts – parthenogenesis, avian reproductive system

Some people practice a form of vegetariansim called veganism. The definition of vegan can be different from person to person, but generally it means that one does not consume or use animal products in which an animal was harmed to obtain them.

Vegans range from those who won’t eat animals or
products that require animals to be harmed, to those
that will not use any animal product whatsoever.
Plants make up their entire diet. How shocked will
they be to learn that research has uncovered that
plants might well have feelings and sensations.
But there are gray areas. Take chicken eggs for instance. Most chicken eggs laid by working hens all over the world are not fertilized. We will talk much more about this in a couple of paragraphs.

Unfertilized eggs can’t develop into a chick. Because of this, some vegans will eat chicken and quail eggs that are certified unfertilized. Now for a potential problem – chickens, quails, turkeys, and other birds are known to undergo parthenogenesis! Unfertilized eggs might have a partially developed embryo inside them. What is a vegan to do?!

The thought that birds might be able to undergo parthenogenesis is not that strange. Reptiles are the ancestors of bird species, and reptiles are famous for their number of parthenogenic species, both obligate and facultative.

The difference between parthenogenesis in birds and in reptiles is that the bird form rarely, very rarely, ends with a chick hatching from the egg. Most die at some point in development - usually early.

The first species in which bird parthenogenesis was studied was the Beltsville Small White (BSW) Turkey. It was recognized in the 1950’s that this breed had some eggs that had started to develop even though they had not been mated to a male.

The researchers then embarked on a long breeding project in which they increased the number of parthenogenic embryos. By breeding females that had a higher tendency to lay parthenogenic eggs  to males from mothers who were more likely to develop parthenogenic eggs, the scientists developed the breed for parthenogens that developed longer and longer.

In some cases (still less than 1%) the parthenogenic eggs would hatch. And some of those turkeys matured fully and lived to a ripe old age! The breed still exists and is still studied, so parthenogenic tom turkeys are born to these females every once in a while.

Zebra finches are native to central Australia, but have
been introduced into North America, Brazil, and
Portugal. They are good for research because they can
breed all year round, usually after strong rains, they
have clutches of five-seven eggs, and they are fast maturing.
Remember that they must be toms because of the sex determination system of birds. Males are ZZ, while their mothers have ZW sex chromosomes. By producing a diploid ovum (egg cell), the moms would double their own DNA, giving either a ZZ or a WW, but WW’s are nonviable. Therefore, all parthenogens would have to be male.

After studies in turkeys were publicized, it was recognized that parthenogenic development was also occurring in chickens. There was a single report of parthenogenesis in a pigeon. Then in the late 2000’s, parthenogenesis in both Chinese Painted Quails and Zebra Finches was recognized. It is important to note that in these later named species, only one parthenogen was noted to survive; that being a chicken in the 1970’s. At that time, molecular methods of genome identification were not available, so we are not sure if this was a true parthenogen.

The other point to note is that these are all domesticated, captive birds. We don’t know if parthenogenesis takes place in birds in the wild. Similar to the cases in other animals, we first recognized parthenogenesis in sharks and komodo dragons in zoos, because that is where people could control if females were exposed to males. Many assumed that parthenogenesis was caused by a lack of males and that they would not give birth from unfertilized eggs in the wild. We now know that isn’t true for komodos, and we have the report showing that pit vipers will undergo parthenogenesis in the wild, even if males are present. Who knows if this is the case for birds.

To understand parthenogenesis in birds, it would help to look at how eggs are produced; we’ll use the chicken as a model. Some weird things can happen with chicken eggs and their process of production is responsible for most of these oddities.

This isn’t the most pleasant of pictures, but it shows the
reproductive tract well. The yellow orbs on the left are the
ova with developing yolk in the ovary. The bigger ones will
be released first. The magnum is larger and adds albumen;
the isthmus is narrower and adds the membranes. Guess
what the shell gland does. The finished eggs exit via the vent.
First off, think of the laying of an egg as the equivalent of a human female’s menstrual cycle. Each month, a woman of child-bearing years will release a mature egg from her ovary (maybe two). In humans, the ovaries trade off each month, but in chickens, only the left ovary and oviduct are functional; the right is there when born, but degenerates over time.

If the human egg is fertilized, the embryo will implant into the wall of the uterus and the placenta will develop. If not, the uterine environment will flush itself out each month and the cycle will begin again. This is different in chickens. Whether the egg is fertilized or not, the ovum (and attached yolk) will be sent on along the oviduct and an egg will be formed.

Chicks are born with 13,000-14,000 ova and they produce no more. Not all will be laid as eggs, but every 26 hours or so, a new ovum with developed yolk (fatty nutrients for the developing chick) will be released from the ovary. The timing of the release is actually controlled by the laying of the egg. When an egg is laid, a new ovum will be released about 30-60 minutes later.

Ovulation is also controlled by the amount of sunlight in the day. Summer day lengths stimulate ovulation, so egg producers manipulate the lights so the hens always think it is summer.

Of course, there is an exception to this. Chickens won’t ovulate after about 3:00 pm! They must where watches. And the entire process for laying an egg takes about 26 hours. This is longer than a day – duh - so each day the chicken will ovulate about 2 hours later. This keeps up until she would be due to ovulate after 3:00. In this case, she just won’t do it, and will wait until the next morning to ovulate. As a result, a chicken will not lay an egg once every six days or so.

Double yolks aren’t really that uncommon, occurring
in about 1in 1000 eggs. However, they are usually
caught in the production process and used for other
egg products instead of putting them in your Styrofoam
box. Quadruple yolks are much less common, but they
do occur, and some breeds are more likely to give
them than others.
When ovum and mature yolk exit the ovary, they enter the oviduct. The first portion is called the infundibulum, and is where the ovum would be fertilized, if a rooster has been in the hen house recently. Whether or not it has been fertilized, the egg then passes into the magnum, which is about 4 inches or so long. In the magnum, the albumen is added around the yolk and ovum. The albumen is clear and provides protection and nutrients to the embryo. It is 90% water and about 10% protein.

After the magnum is the isthmus. This is where the egg is surrounded by the inner and outer membranes. The next stop is the shell gland, and you can guess what is added here. The calcium carbonate shell takes about 20 hours to form around the egg, so this is where the egg spends the majority of its time. Then it is laid by being squeezed out with muscle movement.

Like I mentioned above, weird things can happen during this 25-27 inch trek through the chicken. Sometimes two ova may be released at once. These can both be surrounded by a single albumen and shell and come out as a double yolk egg. There are instances where one or both yolks may be fertilized, but the lack of space and nutrients usually leads to at least one of the chicks dying in the shell, and usually both. The record is nine yolks in a single egg!

On a different note, when the hen is young no ovum may be released, and a small piece of loose tissue could be mistaken for an ovum. In this case, it will be wrapped in albumen, membranes and shell, and a yolkless egg will be produced. I have a student who is seriously considering investigating a way to manipulate chickens to give yolkless eggs all the time – could be a million dollar idea.

There was a recent story that illustrated one more weird possibility. If the muscular movement shoots the egg backward instead of out, it can happen that the developed egg will go through another round of the process. It can also meet up with the next developing ovum. In this case, the developed egg could be surrounded with more albumen, membrane, and then be wrapped in another shell - an egg within an egg! Don’t believe me? Watch the video.

In parthenogenesis, the ovum + yolk will be diploid, the result of endomitosis or fusion of two ova. They will be sent along the path of egg production, and once laid, they look like regular eggs. The embryo will not develop beyond three days or so, so they are hard to tell from unfertilized eggs or those eggs that are fertilized and undergo spontaneous early embryonic death. You probably wouldn’t know if you were eating one.

The Beltsville Small White turkey was developed in the
1930’s and quickly became the most popular turkey
on American plates. The name comes from the USDA
research farm n Beltsville Maryland, where they were
developed. However, in the 1940’s the broad breast
turkey was bred and the BSW quickly faded, except for
in research – they have high rates of parthenogenesis!
That should keep them popular.
The stimulus for diploid egg production is not known; however, the increase in parthenogenesis in the BSW turkeys after breeding them indicates that there is a genetic component to the development of unfertilized eggs. What that component might be is up for grabs. Maybe the breeding selected for females that have an odd hormone profile, or are more apt to undergo endomitosis in their gametes, or ….. you find out and get rich.

In the BSW turkeys, breeding led to later development and finally some live hatchings. This is now being tried in quails as well. Dr. C.D. McDaniel at Mississippi State University is investigating the idea that parthenogenic development actually reduces the hen’s ability to hatch fertilized eggs.

After nine generations of cross breeding females and males to increase parthenogenic development, McDaniel reported in late 2012 that quail that have more parthenogenic events do indeed have fewer fertilized eggs that hatch and develop to mature quails. Late embryonic death decreased, but early death increased dramatically. This is a significant economic question, as it would seem that lower rates of parthenogenesis will lead to greater production of quails.

Next week, we will see that parthenogenesis is not always the “choice” of the female. Sometimes, parthenogenesis can be forced on an animal.

Parker, H., Kiess, A., Robertson, M., Wells, J., & McDaniel, C. (2012). The relationship of parthenogenesis in virgin Chinese Painted quail (Coturnix chinensis) hens with embryonic mortality and hatchability following mating1 Poultry Science, 91 (6), 1425-1531 DOI: 10.3382/ps.2011-01692

Wednesday, February 20, 2013

Males – Can’t Live Without Them?

Biology concepts – parthenogenesis, gynogenesis, kleptogenesis, sperm-dependent parthenogenesis, pseudogamy, Muller’s ratchet

Last week we introduced the idea that species can be (facultative) or must be (obligate) parthenogenic. Both facultative and obligate species are diverse, interesting, and full of exceptions – what a surprise.

The pea aphid is a wonder of biology. Here, you see a winged male 
with offspring nearby. It is hard to tell if these are clonal 
offspring, but they are likely to be found in the Fall, 
as winged males are produced from late summer eggs. 
This is so they can fly to new food if necessary, before 
mating and the females laying eggs that will overwinter.
Pea aphids are a wonderful example of facultative parthenogenesis. There are several different cues that trigger parthenogenesis in animals that can produce both sexually and asexually, including temperature, behavior and a lack of males. In the case of aphids, they are only sexually in the summer. The rest of the year they reproduce by parthenogenesis.

Overwintered eggs hatch in the spring and become wingless females. These individuals immediately begin to give birth to clones of themselves, apomictic, thelytokic, parthenogens. These are all females, due to the sex determination system that aphids use, the XX/XO system. When diploid develop, they double their haploid chromosomes, so all are XX females.

The parthenogenic females reproduce quickly, giving birth to dozens of females over a period of just days. These females immediately begin to give birth to more clonal females. The reason it can be so fast is that the females are born pregnant! The process is called telescoping generations, because there is less and less time between birth and birth. This is one form of paedogenesis (paedo = child), reproduction by sexually immature forms.

The life cycle of the pea aphid is complicated, having
both sexual and asexual components. In the spring to
summer, females will produce off spring by
parthenogenesis. In the late summer and Fall, the
parthenogenic females will mate with males and lay
eggs that will hatch in Fall and later eggs that will
hatch the next spring.
In the heat of the summer, the aphid females will undergo a change of their egg production. Adding an extra step to their meiosis reduces their XX to an XO and produce males. These males then mate with the females and they lay eggs that will overwinter to produce next year’s females. Many generations of parthenogenic offspring are interrupted by one generation of sexually produced offspring.

The result is that millions of offspring can be produced from a single female in the spring (although they live only about 10-40 days). A comparison is warranted. If all the offspring from a female lasted an entire summer and they were lined up in a single line, they could circle the Earth more than four times! Maybe there is something to this parthenogenesis.

Bees are also facultative parthenogens, but with a different twist. Bees are haplodiploid, meaning that all the males develop from unfertilized haploid eggs, while the females come from fertilized eggs. Even the sterile female workers are the result of fertilization. The twist comes when in some species, the queen dies without an heir. In this case, some of the sterile worker bees can start to lay eggs. It is a futile effort though, they produce only males because they are sterile and have not mated. The hive dies out anyway.

The exception to this unfortunate affair is one species of South African bee, Apis melifera capensis, who can repopulate by hiring a new queen. The female workers of this species will fight it out when a queen dies, and some will start to produce diploid eggs to produce a new queen by parthenogenesis. She will be a clone of a worker, but she will mate with a male and introduce more genetic diversity into the hive.

Some species of whiptail lizards are females only –
no males at all. But they need the stimulation of
feigned mating to start development of the
unfertilized eggs. So females who have just laid
eggs act as males and perform male behaviors.
Females that are acted on by these “male fakers” are
more likely to lay eggs and have the young survive.
In some cases, females need some help to stimulate egg development for parthenogenesis. In a few instances, this help insures that maximal reproductive success is met. In the whiptail lizard, this takes the form of feigned mating. “But wait,” you say, whiptails are obligate parthenogens – they’re all female! Yep, but after they give birth they have a short burst of male hormones, and start to mimic male behaviors, including mating. The funny thing is, females who are not “mated” by these other females do not produce as many offspring. Something in the behavior helps stimulate more egg development.

Other parthenogenic species need more help to jump-start the egg development. Many species require sperm in order to stimulate egg development. The sperm does not contribute any DNA to the embryo, but it contains a chemical, hormonal, or physical property that makes the egg develop into a whole animal.

If many obligate parthenogens are strictly female, where does the sperm come from? A male of a closely related species usually does the honor, but it doesn’t really matter, since the DNA is not incorporated into the egg. This process has many names, and they all mean pretty much the same thing - sperm-dependent parthenogenesis, kleptogenesis, pseudogamy, gynogenesis – more names than those two fellas on “Psych” (when are they going to bring that show back?).

The triploid Amazon molly fish (Poecilia formosa) is a good example of a gynogenetic species. It is the result of a hybridization of the Mexican and Atlantic molly species, and now lives in harmony with those species in an overlapping habitat It is good for P. formosa that they all get along so well, since they would die off with out the males of the other species. It is the mating process with those males that stimulates the amazon molly eggs to develop and hatch.


The amazon molly doesn’t live in the Amazon River.
It was named for the Amazon warriors of Greek
mythology, an all female warrior society. The amazon
molly is an all female species that reproduces by
gynogenesis. They mate with a closely related male,
but do not incorporate his DNA into the developing
embryo. The sperm is needed to stimulate egg development.
A 2011 study showed that male mollies of a close relative species fertilized P. formosa eggs about 50% as often as the eggs of females of its own species. The authors suggested that male-male competition for females was responsible for fertilization of the P. formosa eggs. These were the losers of the contest for females of their own species, but it really doesn’t matter, since the losers are not contributing DNA to the amazon molly offspring. Therefore, they are not weakening the species. Apparently this arrangement is enough to make P. formosa reproductively successful.

Many times, parthenogenesis is an animal’s only choice, but there are definite advantages to this mode of asexual reproduction. One, the offspring are clones, produced under a certain set of environmental conditions. Since the conditions were good enough to let the mother survive and reproduce. That means that offspring exactly like her should thrive in those conditions too. Little effort – maximum effect.

Two, we talked last week how rapid reproduction by parthenogenesis can help komodos colonize new territory quickly, much faster than they could by sexual reproduction alone. And three, parthenogenesis doesn’t waste community resources and energy on animals that don’t give birth – males. I don’t think I like this advantage.
But there are also definite disadvantages to parthenogenesis. One disadvantage is that the very clonality that helps them in steady state conditions is a hindrance if the environment changes. Genetic diversity is important for adaptation, but parthenogenesis offers no chance for genetic diversity.

Another potential disadvantage to parthenogenesis is the loss of traits that are needed for sex, like mating behaviors, mating calls, etc. An example is a facultatively parthenogenic fruit fly. In 1961 they were separated from males and raised separately. Ten years later they were reintroduced to males. Only some mated, but they still had the genes that controlled mating behaviors. I 1981 they were reintroduced again, and none of the females participated in the mating behaviors; they had been lost completely.

Muller’s ratchet has more to say than just that unused
genes will drift. In terms of becoming parthenogenic, it
does surmise that genes that have to do with sexual
reproduction will mutate at a higher rate. However, it
also states that there will be deleterious mutations in
asexual organisms, resulting in a drop off in births. As
such, the ratchet is a commonly held argument for
why sexual reproduction is so evolutionarily important.
This is evidence for something called Muller’s ratchet. Muller states that if positive evolutionary pressure is not kept on a trait, mutations will build in that trait until it is lost or non-functional. This seems to be what happened in the fruit flies.

One last disadvantage - parthenogenic species seem to last only about 100,000 years on average, probably due to the lack of genetic diversity. However, some salamanders have been gynogenic for 1 million years, suggests that they have had a few indiscriminate fertilizations along the way that have introduced new DNA, about 1 in a million births. Some orbatid mites (1 mm soil mites that help recycle dead material) have been parthenogenic for 100 million years!

Even though species have been parthenogenic for millions of years, it is only in the last few decades that we have really learned anything about these behaviors. Now that we have some knowledge, it seems time to put it to use.

For instance, human eggs can now be induced to develop in the absence of sperm. Before release, pre-eggs are frozen in time in metaphase II stage of meiosis. This means that they are still diploid, it isn’t until anaphase and telophase that the chromatids are pulled apart and the eggs become haploid.

In this stage, if you prick the eggs with a needle on their membrane, or treat them with some chemicals, or apply a mild electric shock, it seems to bring the same response that penetration of a sperm head does. This triggers the initial stages of development in the egg (blastocyst), regardless of the fact that it doesn’t have dad’s DNA.

Under these conditions in the lab, the eggs will develop to the 500-1000 cell stage, and then they will die out. Remember that they do not have the paternally imprinted genes available to them, so they can never become a full-fledged embryo.

Human stem cells are produced by teasing out the cells of a
blastocyst and growing them separately. Then you can treat
them with different growth hormones and make them into
different types of cells. One way to get the blastocyst cells is
from fertilized eggs. But to avoid those ethical headaches,
now scientists often stimulate the egg to develop
parthogenetically, and then harvest the stems cells.
But, they can be teased apart and used as stem cells. Using these human parthenogenic embryonic stem cells (hpESC’s) avoids the ethical issues of creating stem cells from fertilized eggs. In the past five years or so, many efforts have been made to get these pluripotent (can become any time of cell) stem cells to mature into different kinds of cell types so that they can be used for research and as medical treatments.

For instance, one 2012 study showed that hpESC’s could be used to generate mesenchymal stem cells, that had the ability to differentiate into several different type of cells, include bone making cells and fat making cells. They compared the hpESC’s to stem cells generated from embryos and found they expressed very similar marker proteins. Because they are homozygous for immune markers, it is hoped that hpESC’s will be good replacement cells in tissue therapies.

Next week – birds can undergo parthenogenesis, but it is usually not a happy ending, unless you like omelets.

Chen, Y., Ai, A., Tang, Z., Zhou, G., Liu, W., Cao, Y., & Zhang, W. (2012). Mesenchymal-Like Stem Cells Derived from Human Parthenogenetic Embryonic Stem Cells Stem Cells and Development, 21 (1), 143-151 DOI: 10.1089/scd.2010.0585

Alberici da Barbiano, L., Aspbury, A., Nice, C., & Gabor, C. (2011). The impact of social context on male mate preference in a unisexual-bisexual mating complex Journal of Fish Biology, 79 (1), 194-204 DOI: 10.1111/j.1095-8649.2011.03009.x

For more information or classroom activities, see:

Sperm-dependent parthenogenesis –

Mueller’s ratchet –

Human parthenogenic embryonic stem cells –

Wednesday, February 13, 2013

Just Leave The “Father” Line Blank

Biology concepts – apomixis, automixis, genomic imprinting, haplodiploid, facultative and obligate parthenogenesis

Kids questions can be exasperating, exhilarating,
and problematic –all at the same time. Questions
about biology are especially difficult because you
never know how much information to give at what
age. My advice – give the simplest answer that will
stimulate additional questions. Too much detail can
be a turn off to a kid and can lead you into subjects
that you aren’t ready to tackle with them. If you don’t
know the answer – find it out. Your kids should see
you demonstrate looking for an answer and learning.
“Mommy, why is the sky blue? Daddy, if atoms are mostly empty space, then why are objects solid?” These are questions with which every parent must deal. Unless you are familiar with Raleigh scattering or the quantum structure of the atom, you’re going to have to make something up. Use big words – it will confuse them into moving on to something else, and you get to look like you know something.

Parthenogenesis is a subject that baffles a lot of people for a lot of reasons, mostly because we know little about it yet. What has science’s response been to this lack of knowledge – give everything a new name and bury people in mountains of terminology. Jargon is job security after all.

Last week we saw that parthenogenesis, while an exception, is not as rare as we once thought. Now let’s take some time to get down and dirty and look at the in and outs of abandoning sex. We’ll use examples to keep the vocabulary monster at bay.

The whole point of parthenogenesis is to make an unfertilized egg develop into a whole organism. How can an egg develop on it own? In general, haploid eggs are useless (exception alert!) so moms need to construct a diploid or polyploid egg in order for parthenogenesis to have a chance.

One way is for mom to forego meiosis and produce diploid eggs. The term for this is apomixis (apo = free from, and mixis = mixing). It basically means "with no mixing of chromosomes;" neither by homologous recombination nor by random assortment in meiosis. Therefore, offspring produced by apomictic parthenogenesis are clones of their mothers.

Automixis results in half clones. The mother’s eggs
go through meiosis, so the joining of different
products to regain diploidy will necessarily join
unlike chromosomes. Therefore, the offspring can’t
be a full clone of the mother. The mixing can come
from joining two eggs that went through different
random assortment stages, or through
recombination that mixes different parts of
homologous chromosomes in meiosis.
On the other hand, if the mom’s gametes do go through meiosis, then the haploid egg has to be manipulated so that it is once again diploid. This is called automixis (auto = self) and comes in a couple of flavors. Two eggs can fuse, each being haploid, to produce a diploid super egg. Another way is for the egg to start to develop, go through a few divisions to form what is called a blastomere, and then two blastomere cells will fuse. Finally, a haploid egg or blastomere cell can fuse with a polar body, one of the meiotic products that was cheated of some cytoplasmic factors and did not become a full fledged egg.

The result of any of these fusions is the same, a diploid egg that can develop into a whole organism without fertilization. BUT---- they are not equal to the apomictic egg described above. In meiosis, there is a division of chromosomes and possible recombination to form new sequences. Therefore, no two eggs will have exactly the same DNA, even if produced at the same time by the same mom.

If you fuse these two different eggs (or blastomeres), the sets of chromosomes ARE NOT the same, so even though the offspring will have only maternal DNA, they will not be exact clone of the mom. Automictic parthenogens are therefore called half clones; apomictic parthenogens are full clones.

The fly in the ointment here is sex determination. It is possible for a clonal offspring of a parthenogenic mom to be of the opposite sex – weird enough for you? It all depends on the system that the particular group of animals uses to determine sex.

In mammals, the sex determination system is XX/XY. Females don’t have a Y, so even if by some miracle a mammal could give birth parthenogenically (it doesn’t happen, see below for why), the offspring could be only female. In other animals, this is not so.

There are different sex determination systems in different
groups of animals. The difference between human and
insects is that in humans males have two different sex
chromosomes, while in insects, the male just gets one copy
of the only type of sex chromosome. In the komodo dragon,
the sex determination system is the same as in birds – that
makes sense, birds and reptiles have a common ancestry.
Going back to last week’s example of the Komodo Dragon, their sex determination system is ZZ/WZ, with ZZ = male. Therefore, the automictic fusion of a Z egg with a W egg could produce a female, while two Z eggs fusing would produce a male. On the other hand, apomictic parthenogenesis could produce only males, a Z egg doubles to become a ZZ egg, but a WW egg is not viable.

So komodos would produce more males than females, and their wild populations bear this out. Communities of Komodos can be up to 75% male. It would seem that this is an evolutionary strategy to help the Komodos colonize new islands. Say a female carjacks a log and lands on a new island. She undergoes parthenogenesis because no males are around, and produces males and a few females. Since parthenogenesis is quick (no time wasted on mating and seasonal fertile times) they can build a presence on the island quickly.

Then sexual reproduction can take over, increasing the genetic diversity of the species (because some drift and mutations will have taken place in the offspring). Now the Komodos might be more likely to survive an environmental change that would put on pressure for adaptation.

Another sex determination system is the XX/XO system of many insects. Pea aphids use this system, where XX = female, but those with only one X are male. Parthenogenesis in aphids can also produce only females. And wouldn’t you know it, there are terms for each. If only females are produced, it is called thelytoky; if only males, arrhenotoky, and if both can be produced, deuterotoky.

Most hymenopterans (bees, wasps) are haplodiploid.
This means that the two sexes have different number of
chromosomes. All the males are the product of unfertilized
haploid egg development, but despite this they are sexually
mature. The females are the result of sexual mating and
fertilization of haploid eggs to make them diploid. Even so,
only the rare diploid egg gets the right environment to
become a new queen.
I said above that there is an exception to haploid eggs being worthless, and here it is. Bees are haplodiploid in their sex determination. Males develop from haploid, unfertilized eggs, while females develop from fertilized, diploid eggs. The queen will mate with one or more males to produce new eggs that will be female, while she will lay unfertilized eggs to produce males, In some cases, the female workers will produce unfertilized eggs to become males as well. This is an example of arrhenotoky.

O.K., we’re almost through the terminology, one more set still to get through. Some animals, like the komodos and the pit vipers we have talked about, reproduce through sexual means, but can also reproduce by parthenogenesis under special circumstances. This is called facultative parthenogenesis. On the other hand, some species have abandoned sex all together and ONLY reproduce by parthenogenesis. This is called obligate parthenogenesis and their populations consist of only females – can you imagine the amount of gossip that must go on.

The vast majority of species that have completely abandoned sex (obligate parthenogens) are polyploid. Whiptail lizards are a good example. Of the all the species of whiptails, parthenogenic and sexual, 15 species are obligate parthenogens. And of these, all are polyploid.

Polyploid whiptails have trouble segregating chromosomes because of the increased number of them, and their spindle apparatuses are usually screwed up. If meiosis is going to fail, why use it? And if you aren’t going to use meiosis, why mate with males to produce embryos, just do it yourself? In addition, these species do tend to be found in extreme climates, where males finding females would be more difficult. Parthenogenesis is a way to keep the species going.

In genomic imprinting, genes from mother and dad are
differently regulated. Only one will be active in the
embryo, so you need inputs from ma and pa. The
silencing of the genes in one sex often are the result of
adding methyl groups to the cytosine or adenine bases,
so that they cannot be transcribed into mRNA;
therefore no protein is made.
Facultative parthenogens only resort to asexual reproduction under certain circumstances, usually when males are in short supply, or when increasing numbers quickly is in the species best interest. I say this is usually true, but the paper we talked about last time concerning pit vipers showed that they use parthenogenesis even when males are present. May be they are just fed up with men.

What is common to facultative parthenogens is a lack of genomic imprinting, ie. there are not specific genes provided ONLY by the mom and other genes provided ONLY by the dad. If genes of the different parent must interact to work properly, this is one type genomic imprinting. If the genes exist in both sperm and egg, but one or the other is always silenced, this another type of imprinting.

If there is no genomic imprinting, an individual can survive with just the genes from one parent. However, imprinting is an important regulatory mechanism in all mammals, so we won’t be adopting parthenogenesis any time soon.

Mammals are the only group of animals in which we find genomic imprinting. Of course there is an exception- the monotremes, the platypuses and echidnas. But they’re known for being difficult to put into any one box. They’re mammals, but they lay eggs for gosh sakes!  In fact, it’s their egg laying that negates their necessity for imprinting.

A 2013 review paper looks into the evolution and mechanisms of genomic imprinting in mammals. The imprinted genes are largely involved in transfer of nutrition from the mother to the embryo, ie. the placenta. All mammals have a placenta of one type or another, but monotreme placentas are very short lived, just until the yolk sac forms.

The only surviving monotremes are the platypus and
four species of echidnas. They both look like science
projects gone horribly wrong. They both are mammals,
but they lay eggs. The platypus male has poison spikes
on its hind feet, but its bill is not like a bird bill. The
mouth is on the underside. The echidna has spines and
a long, narrow snout that house both nose and mouth.
Both monotremes have electrosensors in their
bills to find prey.
On the other hand, marsupial mammals (kangaroos, etc.) give birth to very immature young, which then grow bigger and stronger in their pouches. But while in utero, they are still tethered to mama by a placental connection. This makes sense, since monotremes diverged from placental mammals long before the marsupials did.

Since the placenta is so short-lived in monotremes, many of the reasons for imprinting of genes (placental nutrition) are not required. This would leave them free to pursue parthenogenesis as a reproductive strategy, but I am not aware of any documented instances of this.

Genomic imprinting is much more involved than we have described here, and it is involved in more processes than just placental function, including the size of offspring and the competition between males for female eggs. It’s the reason that ligers are so much bigger than tigons! I encourage you to read more about it.

Next week we can look at some very interesting examples of facultative and obligate parthenogenesis, and then some exceptions as to how parthenogenesis works. Exceptions to an exception!

Renfree, M., Suzuki, S., & Kaneko-Ishino, T. (2012). The origin and evolution of genomic imprinting and viviparity in mammals Philosophical Transactions of the Royal Society B: Biological Sciences, 368 (1609), 20120151-20120151 DOI: 10.1098/rstb.2012.0151

For more information or classroom activities, see:

Genomic imprinting –

Wednesday, February 6, 2013

Exceptions Give Birth To Exceptions

Biology concepts – parthenogenesis, polyploidy, geographic parthenogenesis

Komodo dragons are the largest lizards on Earth, reaching
more than 10 ft (3 m) in length and upwards of 300 lb.s
(136 kg). It was believed that they used the toxic bacteria in
their moths to infect the prey they bite, then wait for it to die.
But later research shows they have a toxin in their saliva as well.
In early 2006, a female Komodo dragon in the London Zoo laid a clutch of 22 eggs – no big deal right? Well, she hadn’t been housed with a male Komodo for more than 2.5 years! She had four offspring come to maturity from that clutch, all males.

Later that same year, a Komodo Dragon in the Chester Zoo in England also laid a clutch of eggs, but she had never been house with a male! What gives? In both cases, DNA tests showed that the offspring had only their mother’s DNA – they were virgin births, technically called parthenogensis (parthenos = virgin, and genesis = birth).

The first incident had been attributed to storage of sperm from a past mating (many animals can do that), but the genetic tests proved that both mothers had resorted to asexual reproduction when faced with a lack of males.

A similar event occurred in 2008 in the Virginia Aquarium. A female black tip shark gave birth to several baby sharks, and they all had her DNA only. This made everyone go, “Hmmmm,” and then they started checking some other reports of shark births to females that hadn’t been housed with males. Like the Komodos, this had been reported, but they assumed they were cases of stored sperm. Low and behold, a 2001 family of bonnethead sharks from the Omaha Zoo showed that all the offspring had just their mother’s DNA as well.

Hammerhead shark come in different flavors. The bonnethead
has a curved front appendage (cephalofoil), while the
hammerheads have scalloped or straight edges. In the front
appendage houses their eyes, as well as an electrical sensor
and it also helps them to turn quickly.
By 2011 we knew of over 70 species of vertebrates could undergo asexual reproduction by some form of parthenogenesis (or related mechanism), including some captive birds, like turkeys and chickens. But there were still more surprises to be seen. Scientists surmised that this abandonment of sex was due to their environment, being held captive without males around – a last ditch effort to save the species as it were.

But a study published in December of 2012 showed that pit vipers, specifically cottonmouth and copperhead snakes, can revert to asexual reproduction and undergo parthenogenesis in the wild, even with males all around! There are many known one-sex species of fish, reptiles, and amphibians that only undergo parthenogenesis as a reproductive strategy; finding a sexual species that will randomly switch to asexual in the wild had not been seen before, especially not in a vertebrate. This was a daunting task, since following the snakes around and proving that they didn’t mate. And then proving that the offspring (if you can catch them) have the same DNA as the mom ain’t easy.

Pit vipers are a group of snakes that can sense prey and
predator by their heat signature. The pit organ is an
infrared heat sensor, controlled by a protein called
TRP1a, a protein that is usually a chemical sensor
in other animals.
So science is now becoming more aware that parthenogenesis is not a freak way of reproducing, it is more common and has more variants than we ever could have imagined. But how does this fit into our previous series on polyploid organisms? believe it or not, in many cases, the two exceptions are linked.

There are two links between polyploidy and parthenogenesis, and they themselves are linked together. First is the issue of meiosis. We have discussed before that polyploidy messes with meiosis. Homologous pairs of chromosomes are hard to align when they don’t come in pairs (odd ploidys) or when there are more than one pair of the same chromosome (tetraploidy and higher even ploidys). The pairing gets mixed up with some left out, or more than one segregating together in meiosis I. This doesn’t even take into account how high ploidys seem to alter the production of centrosomes (centrioles + spindles), the apparatus that pulls the chromosomes apart.

As a result, gametes are more likely to be defective, and dosage problems (how much protein is made due to increased copies of a gene) can render a polyploidy organism sexually immature. These difficulties make it less likely that the organism will successfully reproduce if it has to rely on sexual means of propagation.

Therefore, through genetic drift and natural selection of the sex genes that were being used less, parthenogenesis appeared. With this strategy, the problems of meiosis can be avoided by merely skipping that step and making diploid (or higher ploidy) eggs. Being diploid, the eggs don’t need the contribution of sperm DNA to be complete, they “just” need to be jump started to develop into an embryo. That’s a big “just”, and we will talk about it in a bit.

The second link between polyploidy and parthenogenesis has to do with geography, and is often called the “rule of geographical parthenogenesis.” As a model, let’s use Alaskan bachelors. Men that relocated to Alaska first find gold and later to find oil were moving to a harsh environment. They were spread out over large areas so that the population density was low. So what were the chances that they were going to meet a nice girl, settle down, and have a family out there in the wilds?

In this figure, the desert regions (yellow) and the ice
and snow regions (blue) correspond to where the most
polyploidy and parthenogenic animals are found. In
South America, the ice/snow region is located high in the
mountains – remember that elevation is similar to
movement to extreme latitudes.
It’s the same way with all other species. If they are located in cold or particular harsh climates, or if they find themselves in a geography that separates them from others of their species, then they will be less likely to find a mate. This means that to keep the species going, they have two choices (O.K., neither is really a choice, it's nature finding a way): parthenogenesis or hybridization by mating with a closely related species that they happen to come across.

Either way, parthenogenesis is going to become more common in these habitats. One - they switch to parthenogenesis because they can’t find a mate, or two - they switch to parthenogenesis because they have hybridized and are now quite likely to be polyploid. Our model fails here, at least I hope it does, because I don’t think the Alaskan bachelors did either; they didn’t have babies on their own and I really hope they didn’t hybridize with a local species!

So geography is linked to polyploidy and it is linked to parthenogenesis. Here’s a simpler way of phrasing this geography idea - there is very little parthenogenesis and very few parthenogenic species in the tropics, but as you travel further north or south you gain more of both. In the polar and sub-polar regions, both polyploidy and parthenogenesis are much more popular.

Another factor is elevation. Most people, other than ecologists, don’t think much about it, but going up in elevation tends to mimic moving further north or south of the equator. In fact, every 300 feet of elevation equals one degree of latitude or 70 statute miles north. Elevation brings the same changes in climate and habitat as do changes in latitude. So as you go up a mountainside, you are likely to find more and more polyploid species and more parthenogenic species.

So there is little parthenogenesis where it is hot, and much less sex going on where and when it is cold. That is sort of the opposite of humans; you ever wonder why more babies in the US are born in July through September?

Platythyrea punctata is a ponerine ant of Central and
southern North America, as well as amny Caribbean
islands. It has a nasty sting; it belongs to the same
group as the very toxic bullet ant.
A recent study of a neotropical ant helps to illustrate the idea of geographical parthenogenesis. Platythyrea punctata is a stinging ant that lives in Florida, Texas and Central America, as well as on many Caribbean islands. The 2013 paper shows that those colonies found on islands are exclusively parthenogenic, producing only females from unfertilized eggs. However, the continentally located colonies reproduce almost exclusively by sexual means. Those on islands have lost the genes to have sex, and those on continents have never developed the genes to undergo parthenogenesis. O.K. not quite, some colonies in Central America can produce parthenogenic offspring; I wonder if they were transferred from an island back to the mainland and the traits just haven’t disappeared yet.

The point is that islands are geographically isolated, so finding mates that are genetically different will be difficult, and if an ant isn’t going to gain the advantage of genetic diversity by sex, why go to the cost and energy of having sex. Parthenogenesis allows them to populate much faster and easier.

In general, insects that are parthenogenic are almost exclusively polyploid. No study has been carried out to see if P. punctata on the islands is polyploid, but they do have an abnormally high number of chromosomes for ant (84). As with many species, polyploidy and parthenogenesis in insects seem to be linked; those in tough areas do both because they need to.  

Insect parthenogens and those in other taxa also tend to be less mobile. Parthenogenic insects, for instance, are often flightless. This makes moving around harder, and that means they are more likely to not find mates (one reason to be parthenogenic) or to hybridize with those they can find, and become polyploid (another reason for parthenogenesis).

The New Zealand mud snail, Potamopyrgus antipodarum,
is an invasive snail that can reach amazing densities in
temperate waters, even though each individual is very
small. It was first introduce to England in the 1850’s and
spread to North America and the rest of Europe from
there. Wasn’t identified in the USA until 1987, in Idaho. The
US dime is 18 mm across.
But that doesn’t mean that the relationship between ploidy, reproductive manner and geography is always that defined. Take Potamopyrgus antipodarum, a New Zealand freshwater snail. This single snail species exists in diploid, triploid, and higher ploidy cytotypes – and they all live in the same climate. The polyploid versions of the snail live next to the diploids, with some lakes being <10% male and others being nearly 50% male, so the rule of geographic isolation as a source of hybridization and polyploidy doesn’t seem to fit.

What is more, a 2010 paper that was a collaboration of Indiana University, University of Iowa, and the Swiss Federal Institute of Technology (studying a New Zealand snail!) showed that the P. antipodarum has sexual and asexual reproductive strategies. It is usually assumed that the diploids reproduce sexually and the triploids and higher reproduce by parthenogenesis, but that is not what the researchers found that many of the diploid, triploid and higher ploidy males are offspring of asexual females, while some higher ploidy individuals likely come from sexual reproduction. Leave it to nature to screw up a good pattern.

Now that we know that parthenogenesis is widespread occurs in many different kinds of animals (and plants), let’s dive in a bit deeper. Even though it is reproduction without sex, it is still a battle of the sexes.

Booth, W., Smith, C., Eskridge, P., Hoss, S., Mendelson, J., & Schuett, G. (2012). Facultative parthenogenesis discovered in wild vertebrates Biology Letters, 8 (6), 983-985 DOI: 10.1098/rsbl.2012.0666

Kellner, K., Seal, J., & Heinze, J. (2013). Sex at the margins: parthenogenesis vs. facultative and obligate sex in a Neotropical ant Journal of Evolutionary Biology, 26 (1), 108-117 DOI: 10.1111/jeb.12025

Neiman, M., Paczesniak, D., Soper, D., Baldwin, A., & Hehman, G. (2011). WIDE VARIATION IN PLOIDY LEVEL AND GENOME SIZE IN A NEW ZEALAND FRESHWATER SNAIL WITH COEXISTING SEXUAL AND ASEXUAL LINEAGES Evolution, 65 (11), 3202-3216 DOI: 10.1111/j.1558-5646.2011.01360.x

For more information or classroom activities, see:

Parthenogenesis –

Rule of geographical parthenogenesis –

Polyploidy and parthenogenesis -