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 -

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