Molecular evolution of Dmrt1 accompanies change of sex-determining mechanisms in reptilia.

In reptiles, sex-determining mechanisms have evolved repeatedly and reversibly between genotypic and temperature-dependent sex determination. The gene Dmrt1 directs male determination in chicken (and presumably other birds), and regulates sex differentiation in animals as distantly related as fruit flies, nematodes and humans. Here, we show a consistent molecular difference in Dmrt1 between reptiles with genotypic and temperature-dependent sex determination. Among 34 non-avian reptiles, a convergently evolved pair of amino acids encoded by sequence within exon 2 near the DM-binding domain of Dmrt1 distinguishes species with either type of sex determination. We suggest that this amino acid shift accompanied the evolution of genotypic sex determination from an ancestral condition of temperature-dependent sex determination at least three times among reptiles, as evident in turtles, birds and squamates. This novel hypothesis describes the evolution of sex-determining mechanisms as turnover events accompanied by one or two small mutations.

Genome evolution in Reptilia, the sister group of mammals.

The genomes of birds and nonavian reptiles (Reptilia) are critical for understanding genome evolution in mammals and amniotes generally. Despite decades of study at the chromosomal and single-gene levels, and the evidence for great diversity in genome size, karyotype, and sex chromosome diversity, reptile genomes are virtually unknown in the comparative genomics era. The recent sequencing of the chicken and zebra finch genomes, in conjunction with genome scans and the online publication of the Anolis lizard genome, has begun to clarify the events leading from an ancestral amniote genome--predicted to be large and to possess a diverse repeat landscape on par with mammals and a birdlike sex chromosome system--to the small and highly streamlined genomes of birds. Reptilia exhibit a wide range of evolutionary rates of different subgenomes and, from isochores to mitochondrial DNA, provide a critical contrast to the genomic paradigms established in mammals.

Thoracic epaxial muscles in living archosaurs and ornithopod dinosaurs.

Crocodylians possess the same thoracic epaxial muscles as most other saurians, but M. transversospinalis is modified by overlying osteoderms. Compared with crocodylians, the thoracic epaxial muscles of birds are reduced in size, disrupted by the synsacrum, and often modified by intratendinous ossification and the notarium. A phylogenetic perspective is used to determine muscle homologies in living archosaurs (birds and crocodylians), evaluate how the apparent disparity evolved, and reconstruct the thoracic epaxial muscles in ornithopod dinosaurs. The avian modifications of the epaxial musculoskeletal system appear to have coevolved with the synsacrum and notarium. The lattice of ossified tendons in iguanodontoidean dinosaurs (Hadrosauridae and Iguanodontidae) is homologized to M. transversospinalis in crocodylians and M. longus colli dorsalis, pars thoracica in birds. Birds have an arrangement of tendons within M. longus colli dorsalis, pars thoracica identical to that observed in the epaxial ossified tendons of iguanodontoid dinosaurs. Moreover, many birds (such as grebes and turkeys) ossify these tendons, resulting in a two- or three-layered lattice of ossified tendons, a morphology also seen in iguanodontoid dinosaurs. Although the structure of M. transversospinalis appears indistinguishable between birds and iguanodontoid dinosaurs, intratendinous ossification within this epaxial muscle evolved convergently.

New resources inform study of genome size, content, and organization in nonavian reptiles.

Genomic resources for studies of nonavian reptiles have recently improved and will reach a new level of access once the genomes of the painted turtle (Chrysemys picta) and the green anole (Anolis carolinensis) have been published. Eleven speakers gathered for a symposium on reptilian genomics and evolutionary genetics at the 2008 meeting of the Society for Integrative and Comparative Biology in San Antonio, Texas. Presentations described results of reptilian genetic studies concerning molecular evolution, chromosomal evolution, genomic architecture, population dynamics, endocrinology and endocrine disruption, and the evolution of developmental mechanisms. The presented studies took advantage of the recent generation of genetic and genomic tools and resources. Novel findings demonstrated the positive impact made by the improved availability of resources like genome annotations and bacterial artificial chromosomes (BACs). The symposium was timely and important because it provided a vehicle for the dissemination of novel findings that advance the field. Moreover, this meeting fostered the synergistic interaction of the participants as a group, which is anticipated to encourage the funding and creation of further resources such as additional BAC libraries and genomic projects. Novel data have already been collected and studies like those presented in this symposium promise to shape and improve our understanding of overall amniote evolution. Additional reptilian taxa such as the American alligator (Alligator mississippiensis), tuatara (Sphenodon punctatus), and garter snake (Thamnophis sirtalis) should be the foci of future genomic projects. We hope that the following articles in this volume will help promote these efforts by describing the conclusions and the potential that the improvement of genomic resources for nonavian reptiles can continue having in this important area of integrative and comparative biology.

Evolution of sex chromosomes in Sauropsida.

Reptiles (sauropsids) represent the sister group to mammals, and the basal members of Reptilia may provide a good model for the condition of the common ancestor of both groups. Sex-determining mechanisms (SDM) and organizations of sex chromosomes among genotypically sex-determining (GSD) species vary widely across reptiles. Birds and snakes, for example, are entirely GSD whereas other reptiles, like all crocodilians, exhibit temperature-dependent sex determination (TSD). Here we explore the evolution of sex chromosomes and SDM within reptiles, using family-level analyses of character evolution and applying parsimony, likelihood, Bayesian, and stochastic methods. We find support for the common ancestor of amphisbaenians and whiptail lizards (Laterata) possessing the XY (male heterogametic) GSD mechanism, while the ancestors of Testudines and Crocodylia, as well as the larger group Archosauromorpha (here containing turtles) are inferred to have exhibited TSD. We also find evidence consistent with the hypothesis that the XY system is more labile and evolves faster than does the ZW (female heterogametic) system. Phylogenetic-based speciation tests do not support an association between GSD and speciation, and reject the hypothesis that the presence of the XY system is associated with speciation in reptiles.