Year: 2013 Page 34 of 37

Uta stansburiana mating. Image from http://cabezaprieta.org/

The side-blotched lizard, Uta stansburiana, is one of the most widely-studied lizard species, thanks largely to work by Barry Sinervo and colleagues on the evolution of  alternative mating strategies (a.k.a. the rock-paper-scissors game in lizards).  The most recent report on the evolution of this interesting species investigates reproductive isolation between two populations of Uta that diverged within the last 22,500 years.  One of these populations is found on lava flows and the others if found off the lava flows.  This report by Corl et al. (2012) is noteworthy because recent work on a range of other organisms suggests that some “rules” for the evolution of reproductive isolation are shared across the tree of life.  Do these rules also apply to lizards?

To my knowledge, patterns of reproductive isolation have only been investigated experimentally in one other genus of lizards: Lacerta (Rykena 1991, 1996; Olsson et al.  2004). This work with Lacerta suggest substantial intrinsic reproductive isolation between species resulting from low fertility and high rates of developmental defects in hybrid crosses. Studies of Lacerta also support Haldane’s Rule because females hybrids (ZW) suffer more fitness consequences than male hybrids (ZZ).

By conducting experimental hybridization studies between these two populations of Uta, Corl et al. (2012) were able to show that significant reproductive isolation has evolved between populations, largely in the form of pre-zygotic post-mating isolation; inter-population crosses produce significantly more unfertilized than fertilized eggs relative to intra-population crosses.  Corl et al.’s results are also consistent with at least one general rule for the evolution of reproductive isolation that has been reported in other organisms; asymmetric reproductive of isolation between the two Uta populations is consistent with Darwin’s Corollary to Haldane’s Rule.

How does all this relate to anoles?  My lab is interested in this work because we’re in the midst of a major project designed to answer questions about intrinsic reproductive isolation in Anolis.  Anthony Geneva reported on some preliminary results of this work earlier this year and we hope to have more to report sometime in the near future.

Rykena, S. 1991. Hybridization experiments as tests for species boundaries in the genus Lacerta sensu stricto. Mitteilungen aus dem Zoologischen Museum Berlin 67:55–68.

Rykena, S. 1996. Experimental interspecific hybridization in the genus Lacerta. Israel Journal of Zoology 42:171–184.

Get all the details in the newly posted paper by Eckalbar et al. in BMC Genomics “Genome reannotation of the lizard Anolis carolinensis based on 14 adult and embryonic deep transcriptions,” just posted on BMC Genomics. Here’s the low-down: “The green anole lizard, Anolis carolinensis, is a key species for both laboratory and field-based studies of evolutionary genetics, development, neurobiology, physiology, behavior, and ecology. As the first non-avian reptilian genome sequenced, A. carolinensis is also a prime reptilian model for comparison with other vertebrate genomes. The public databases of Ensembl and NCBI have provided a first generation gene annotation of the anole genome that relies primarily on sequence conservation with related species. A second generation annotation based on tissue-specific transcriptomes would provide a valuable resource for molecular studies. Here we provide an annotation of the A. carolinensis genome based on de novo assembly of deep transcriptomes of 14 adult and embryonic tissues. This revised annotation describes 59,373 transcripts, compared to 16,533 and 18,939 currently for Ensembl and NCBI, and 22,962 predicted protein-coding genes. A key improvement in this revised annotation is coverage of untranslated region (UTR) sequences, with 79% and 59% of transcripts containing 5′ and 3′ UTRs, respectively. Gaps in genome sequence from the current A. carolinensis build (Anocar2.0) are highlighted by our identification of 16,542 unmapped transcripts, representing 6,695 orthologues, with less than 70% genomic coverage. Incorporation of tissue-specific transcriptome sequence into the A. carolinensis genome annotation has markedly improved its utility for comparative and functional studies. Increased UTR coverage allows for more accurate predicted protein sequence and regulatory analysis. This revised annotation also provides an atlas of gene expression specific to adult and embryonic tissues.”

In recent years, a quirky area of research has developed in which researchers measure the length of the second and fourth digits on the hand and foot, calculate the ratio (2d:4d) and then compare this ratio between the sexes. Surprisingly, in many species there are consistent differences between males and females. In mammals, that ratio is smaller for males, whereas in birds, the opposite occurs. But few studies have looked at the other vertebrate classes.

With this in mind, Direnzo and Stynoski recently calculated digit ratios for several common Costa Rica anoles and frogs. The abstract of their paper, published in Anatomical Record last year, tells the story:

“It is now well documented that androgen and estrogen signaling during early development cause a sexual dimorphism in second-to-fourth digit length ratio (2D:4D). It is also well documented that males of mammalian species have a smaller 2D:4D than females. Although there are discrepancies among 2D:4D studies in birds, the consensus is that birds exhibit the opposite pattern with males having a larger 2D:4D than females. The literature currently lacks substantial information regarding the phylogenetic pattern of this trait in amphibians and reptiles. In this study, we examined 2D:4D in two species of frogs (Oophaga pumilio and Craugastor bransfordii) and two species of lizards (Anolis humilis and Anolis limifrons) to determine the existence and the pattern of the sexual dimorphism. Male O. pumilio and C. bransfordii displayed larger 2D:4D than females in at least one of their two forelimbs. Male A. humilis had larger 2D:4D than females in both hindlimbs, but smaller 2D:4D than females in both forelimbs. Male A. limifrons may also have smaller 2D:4D than females in the right forelimb. Finally, digit ratios were sometimes positively related to body length, suggesting allometric growth. Overall, our results support the existence of the 2D:4D sexual dimorphism in amphibians and lizards and add to the knowledge of 2D:4D trait patterning among tetrapods.”