Gynogenesis: Attack of the clones

Now for part two of our exploration into the fascinating methods of parthenogenesis found in poecilid fish. In case you missed it, I covered the process of hybridogenesis a few days ago. That method produces hemiclones, but is there any way for a hybridization event to yield a population of truly clonal hybrids?

The answer, as you might have guessed considering I have devoted a post to the topic, is yes. An example of this reproductive strategy can be found within Poecilia, the same genus as the fish using hybridogenesis. This time the key players are P. latipinna (the sailfin molly) and P. mexicana.

It appears that at some point in the past a P. mexicana individual mated with a P. latipinna, and the offspring they produced are considered a separate species with a new scientific name, P. formosa. As in the crosses between P. monacha and P. lucida that we examined before, all of the hybrid offspring are female. In this case, however, all of the females are clones, not hemiclones as in P. monacha x lucida.

So how do they achieve this? This is where things get interesting. Apparently an ancestral P. formosa underwent a chromosome doubling event during meiosis, which results in the production of diploid eggs....the perfect recipe for parthenogenesis. You've got eggs with a full diploid set of genes, no need for fertilization to produce viable offspring, right?

Well, it's not quite that easy. It appears that although the eggs of P. formosa contain a full complement of DNA, they lack centrioles. These structures are crucial in organizing the spindle fibers that direct chromosomal movements during mitosis and meiosis. So what is a centriole-less egg to do?

The solution is for the female P. formosa to mate with a male. Obviously since all members of this species are female, they cannot mate with each other and must backcross to a member of the parent species (apparently either P. mexicana or P. mexipinna will work). The sperm penetrates the egg and is then dissolved, but the centriole remains. Now that the egg has that obtained that missing key to successful replication, it is able to hatch and develop into an adult fish. This method produces a population of clones, with each female producing eggs that are exact matches to themselves, and their mother before them, and her mother, etc etc all the way back to that first bastard child that got lucky with a rare chromosomal dysfunction leading to the production of diploid eggs.

So, there you have it, clonal reproduction in which a single individual is the parent of all the offspring, in spite of the need for a fertilization event. Both this story and the example of hybridogenesis seem to involve pretty dupey males. Why mate with an individual of a different species, if your genome is just going to be discarded? What is the benefit to the males?

There have actually been studies on this, and it appears that the males do actually gain something from the process also. Researcher Ingo Schlupp from the University of Hamburg showed that P. latipinna females are much more interested in mating with P. latipinna males that they have seen score successful copulations with P. formosa females. In a nutshell, it seems that it is worth a male's time to sacrifice some sperm to P. formosa, because it leads to higher success in intraspecific copulations afterwards.

There is a huge diversity of reproductive modes within this family, it's truly amazing!