Saturday, November 13, 2010

Snake parthenogenesis II: Non-virgin virgin birth

So, in the last post, we went through some of the strange and interesting things associated with the Boa constrictor that gave parthenogenetic birth to 22 baby Boas. It turns out there's yet another crazy thing going on here. Etymologically speaking, parthenogenesis means "virgin birth." It is a combination of parthenos (παρθένος), meaning "virgin," like the parthenon, and genesis (γένεσις), meaning, well, genesis.

The thing is, though, while it seems clear that the baby Boas' genetic material comes entirely from the mother, she's likely not really a virgin. I don't mean that she's a born-again virgin who had some had some wild times back in snake college, repented, then ran for Senate. Instead, it appears that she only gave birth after being housed with a male snake. Of course, it's only two litters, so it could well be a coincidence. On the other hand, it could be that fertilization was required to initiate development of the diploid eggs produced by the female.

There is a somewhat related phenomenon of paternal genome loss that has been identified in several different species of creepy crawlies, including at least some species of Phytoseiid mites (click here for non-English text, but drawings of them preying on other mite species), scale insects, and sciarid flies. Typically, paternal genome exclusion is limited to males, which start of diploid, but then lose their paternally inherited genome at some point during development, often living much of their lives in a haploid state. These and related phenomena are nicely covered in chapter 10 of Genes in Conflict by Austin Burt and Robert Trivers. Of course, the difference here is that the snakes have two full maternal genomes. Also, we don't really know if they received, and then jettisoned paternal genes, or never got them in the first place.

It also bears some similarities to one of the mechanisms by which uniparental disomies arise in humans (among others). Normally, meiosis results in one copy of each chromosome going into each gamete. With some frequency, though, they don't sort out correctly, and two aneuploid gametes wind up being produced, one with an extra copy of one chromosome, and one that is missing that chromosome altogether. If one of these gametes winds up contributing to the offspring, that offspring may wind up missing one copy of a chromosome (e.g., the X chromosome in Turner's syndrome), or with an extra copy of a chromosome (e.g., the X chromosome in Kleinfelter's syndrome, or chromosome 21 in Down syndrome). Another possible outcome for the extra chromosome case is "trisomy rescue," where the zygote somehow recognizes the presence of the extra chromosome and kicks out one of the three copies.

There are a couple of different ways that this trisomy rescue can happen. Let's say the extra chromosome came in with the egg. If one of the two maternal copies is kicked out, you wind up back at the standard diploid genome. On the other hand, if the paternal copy gets kicked out, you have the standard number of chromosomes, but a uniparental disomy. If the chromosome contains one or more imprinted genes, this can have various developmental consequences.

So, one possibility is that this female snake, for whatever reason, produces diploid eggs. Fertilization triggers development, but then a triploid rescue mechanism kicks in. The key thing is that it would need to be kicking in before fusion of the maternal and paternal pronuclei, since it seems to be the paternal genome that goes missing in every case.

Or this could all be related to the fact that the males snakes housed with this female in 2009 and 2010 were all huge General Ripper fans.

Update: One more follow-up post here.

No comments:

Post a Comment