Biology concepts – apomixis, automixis, genomic imprinting,
haplodiploid, facultative and obligate parthenogenesis
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.
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.
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.
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.
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.
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 –
Hi!
ReplyDeleteThere's something that's not entirely clear from this post (which I find quite interesting anyway, so thanks!), as you wrote:
"... 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..."
But if apomictic offsprings are FULL clones of their mommy (produced by mitosis) then how is this possible? i.e. all of them should be only females, shouldn't they?
RagTiger
It would seem that way, but we are biased by thinking in terms of XX/XY sex determination, like in humans. But in many species, it is the females that have different sex chromosomes, so when a Z gamete duplicates itself, it can only be ZZ (male), and when a W gamete duplicates, it is non viable, so only a male Komodo can be born parthenogenically by apomixis.
DeleteRemember that apomixis means no mixing of alleles by recombination or mixing of two gametes that have undergone different random assortments in meiosis. That is automixis, and could produce females, if one of the merged gametes was W and one was Z. But in that case, the mixing means that you lose the full clone - it is only a half clone - the sets of chromosomes in two gametes WILL NOT be the same.
Thanks for the answer, the automictic part was clear for me. I think this APOmixis confuses me a bit. I've thought that during APOmixis a diploid egg is produced by the female (ZW) comodo via mitosis - if that's it, there's no meiosis, no recombination and the egg will be the exact clone (also ZW) of its mother, thus being a female.
ReplyDeleteOr am I missing something...?
RagTiger