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. |
Parthenogenesisis 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. |
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. |
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. |
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. |
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. |
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 –