Biology concepts – apomixis, automixis, genomic imprinting,
haplodiploid, facultative and obligate parthenogenesis
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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!
For
more information or classroom activities, see:
Genomic
imprinting –
