Gynogenesis redux: Return of the Clones

A few months ago I blogged about the fascinating mating systems of poecilid fish, which can involve either of two mechanisms for parthenogenesis: hybridogenesis (which produces hemiclones or gynogenesis (which produces "true clones"). It appears that the topic of the day over at Scienceblogs is "virgin birth," aka parthenogenesis. I had actually been planning a post on parthenogenetic lizards for a while, so I figured that this might be an appropriate time to sit down and do it.

As it turns out, although I focused on poecilids earlier, fish are not the only vertebrates in which we find named species that are actually the results of hybridization and subsequent parthenogenesis. A classic example from the reptile world occurs within the genus Cnemidophorus. These are the whiptails and racerunners (there is some controversy over whether some actually belong in a separate genus, Apsidoscelis, but for now we'll just use the traditional classification).

Identitiy crises are sad things, but apparently if you're a lizard it is not necessarily catastrophic. Sometimes female Marbled whiptails (C. tigris) will hybridize with male Plateau Spotted whiptails (C. septemvittatus). The resulting offspring are all female, and are classified as a distinct species, C. tesselatus (pictured above)

How does this happen? The two parent species have different numbers of chromosomes, so normal development is impossible. There is a way around this, however. Chromosome doubling occurs, and this is followed by meiosis, with the end product being diploid eggs that can hatch and grow into female lizards without fertilization from a male.

This alone is fascinating: hybridization between two separate species producing viable offspring that is classified as a third species. The story doesn't end here, however. There is yet another species involved in this system: C. sexlineatus, the six-lined racerunner (a normal "bisexual" species). These guys (always males, because C. tesselatus is made up of only females) will occasionally breed with members of C. tesselatus (the hybrids). Because they have different numbers of chromosomes per gamete, as we saw before, chromosome doubling must occur , this time creating a 6N cell that undergoes meiosis to produce 3N offspring. These triploid individuals, confusingly, are also classified as C. tesselatus even though their genomes are very different from the offspring of C. tesselatus resulting from hybridization between C. tigris and C. septemvittatus.

Oh, the tangled webs we weave....but this case is not actually not exceptional, if you are a whiptail, at least. Cnemidophorus consists of about 50 species, and it turns out that about 1/3 of them are parthenogenetic, with entirely female populations.

Asexual reproduction is common among plants and invertebrates, but until recently it was thought to be very rare among vertebrates. Now, however, it seems like they are discovering new parthenogenetic species all the time. Just in the last year or two they've discovered that both komodo dragons and hammerhead sharks. It appears that parthenogenesis is a strategy that some species only resort to in extreme circumstances (such as being kept in captivity for years with no access to a mate), and the physiological response to these stresses results in a shift from sexual reproduction to asexual. Sex helps to maintain genetic diversity by combining alleles from two parents, but producing a few clones, even if they aren't quite as robust, is better than forgoing reproduction altogether.

There are many posts today in response to Seed's prompt for discussion of "virgin birth" over at Scienceblogs head over there to check them out!

(Image credit for C. tesselatus)