Gamble and Zarkower (2012) Current Biology
Tony Gamble, a postdoctoral researcher working with Dave Zarkower at the University of Minnesota, presented his work on uncovering sex-specific markers in geckos and anoles. Recent years have seen a large impetus to understand how sex chromosomes evolve. Sex chromosomes can be involved in sex-specific adaptation, genetic conflict, and other important modes of evolution. This line of research is particularly imperative in reptiles because not only do we have comparatively little information about sex chromosomes in this group, but different types of sex determining mechanisms have evolved multiple times and so there are likely multiple sex-specific mechanisms and multiple evolutionary transitions are at play (see Figure above).
Traditionally sex chromosomes were discovered by karyotyping, which is a method of separating and identifying the chromosomes. This is problematic in reptiles because the sex chromosomes of many species are homomorphic, meaning they are similarly shaped and, oftentimes, quite small. Gamble and Zarkower tried a different approach – RADseq – for identifying sex chromosomes. RADseq uses restriction enzymes to identify sex-specific markers. Their reasoning is that in XY systems (i.e., males are the heterogametic sex), you would expect males and females to exhibit X-specific markers and males to exhibit sex-specific markers unique to the Y (i.e., the non-recombining region). In ZW systems (i.e., females are the heterogametic sex), you would expect the opposite. In theory, this could prove a cheap and fast way to determine the sex chromosomes of different species and develop sex-specific markers.
The challenge for this study was to determine the sex chromosomes for the crested gecko and for the anole. Unlike the crested gecko, Anolis is genome-enabled and we have evidence that they are an XY system, and so they used anoles to pilot their method and confirm that it works before trying it on the crested gecko. However, anoles are not without their challenges. The sex chromosomes are not only homomorphic, but they are also micromorphic, meaning they are quite small. Furthermore, the Anolis genome was built using a female anole, making finding sex-specific markers on the non-recombining region (i.e., the Y chromosome) that much more challenging. Their RADseq approach worked quite well, however, as they were able to recover a male-specific marker in A. carolinensis, which they were able to confirm with PCR amplification. They repeated their results using more A. carolinensis (from a different clade), A. sagrei, and A. lineatopus, and were able to recover the same locus. When they conducted this method in the crested gecko, they found evidence for a ZW system and, correspondingly, recoverd two female-specific markers. Thus, they found that RADseq will work in a variety of taxa, even if they are not genome-enabled, and can successfully be used to uncover sex-specific markers. A neat application of this method is that, using their sex-specific primers, you can sequence an embryo to determine its sex, something that was not previously possible.
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bolko
Congratulations for that excellent blog. I am checking on it regularly and everytime I find surprising and fascinating information. Lizards are a very successful group of reptiles, of wich the success and variability is masked by the lumped taxonomy used. If squamates were mammals, I am sure that they would be a superorder or an even higher taxonomic entity all their own, with the major subgroups like iguanians, gekkotans, etc being separate orders.
The only problem with the blog is the bad attitude towards the little geckos. Day geckos aren’t anole wanabes, anoles are day gecko wanabes, at least that is what a gecko lover believes. Or probably they are insectified/small-mammalified iguanians (small size, short lifespan, high reproductive output).
I am interested in the study about the sex chromosomes in crested geckos. Is there conclusive evidence that this species has genetic sex determination? I ask because anecdotally it is said that in captivity females are produced in lower temperatures and males at higher, with no scientifically rigorous study having being undertaken however. I would like to have the reference for the study. Thank you very much.
Kind Regards,
Stefanos from Greece
ps. A think that perplexes me so much is the nearly complete absence of the crested gecko in scientific studies, even if it is bred in captivity from 1994. As a crested gecko owner, I am finding myself extremely frustrated when I must search about Rhacodactylus leachianus or more distantly related diplodactylids to answer some questions about that species’ biology, habitat, diet etc. Why hasn’t the crested gecko become a model reptile?