Most turtle species possess temperature-dependent sex determination (TSD), but genotypic sex determination (GSD) has evolved multiple times independently from the TSD ancestral condition. GSD in animals typically involves sex chromosomes, yet the sex chromosome system of only 9 out of 18 known GSD turtles has been characterized. Here, we combine comparative genome hybridization (CGH) and BAC clone fluorescent in situ hybridization (BAC FISH) to identify a macro-chromosome XX/XY system in the GSD wood turtle Glyptemys insculpta (GIN), the youngest known sex chromosomes in chelonians (8-20 My old). Comparative analyses show that GIN-X/Y is homologous to chromosome 4 of Chrysemys picta (CPI) painted turtles, chromosome 5 of Gallus gallus chicken, and thus to the X/Y sex chromosomes of Siebenrockiella crassicollis black marsh turtles. We tentatively assign the gene content of the mapped BACs from CPI chromosome 4 (CPI-4) to GIN-X/Y. Chromosomal rearrangements were detected in G. insculpta sex chromosome pair that co-localize with the male-specific region of GIN-Y and encompass a gene involved in sexual development (Wt1-a putative master gene in TSD turtles). Such inversions may have mediated the divergence of G. insculpta sex chromosome pair and facilitated GSD evolution in this turtle. Our results illuminate the structure, origin, and evolution of sex chromosomes in G. insculpta and reveal the first case of convergent co-option of an autosomal pair as sex chromosomes within chelonians.

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Sea stars of the Leptasterias species complex are common members of the rocky intertidal community of the temperate and subarctic North American Pacific coast. Variation in 53 morphological characters was studied in 645 Leptasterias spp. collected at 13 intertidal locations from the Aleutian Islands to Oregon. The sea stars were also examined for restriction-site polymorphisms in polymerase chain reaction amplified DNA representing the putative mitochondrial DNA (mtDNA) control region and flanking sequences; these polymorphisms have recently been shown to discriminate among Leptasterias species in the Puget Sound region and southern Alaska. Ten mtDNA haplotypes were identified (designated A-J)

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 temperaturedependent 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.

The North American wood turtle, Glyptemys insculpta, is a semi-aquatic species that is considered rare, threatened, or endangered over much of its range. In this study, a particularly vulnerable peripheral isolate population in Iowa has been monitored over a period of 7 years. Population census size, estimated from mark-recapture data, and age structure determined from morphology are compared with genetic variation assessed using microsatellites. For reference, the genetics and demographics of this peripheral isolate are compared to data from a more dense population nearer the core of the species range in West Virginia. Geneflow between the Iowa population and a nearby population in Minnesota also is assessed. Genetic data indicate that the Iowa population is isolated, unique, and diverse. Although the Iowa population has lower allelic richness, lower heterozygosity, and smaller genetic effective population size than does the West Virginia population, the difference is not dramatic despite its lower population size, position at the periphery of the species range, and biogeographic history. The Iowa population is not inbred, and there is no genetic signature of a recent population bottleneck. However, interpretations of recent population dynamics based on genetic data may be unduly encouraging in long-lived species such as G. insculpta. Field data suggest a nearly complete lack of recruitment in Iowa. A number of environmental and anthropogenic factors, including recent increases in summer flooding during egg incubation, may have a more negative impact on the Iowa population than on the West Virginia population.