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Chromosoma. 2017 Feb;126(1):105-113. doi: 10.1007/s00412-016-0576-7. Epub 2016 Feb 3.

Discovery of the youngest sex chromosomes reveals first case of convergent co-option of ancestral autosomes in turtles.

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Department of Ecology, Evolution and Organismal Biology, Iowa State University, Ames, IA, 50011, USA.
Department of Biology, University of Northern Iowa, Cedar Falls, IA, USA.
Department of Biology, Hofstra University, Hempstead, NY, USA.
Department of Ecology, Evolution and Organismal Biology, Iowa State University, Ames, IA, 50011, USA.


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.


Comparative genome and BAC in situ hybridization molecular cytogenetics; Convergent ancestral reconstruction; Evolution of genome and sex chromosome organization; Genotypic and temperature-dependent sex determination; Turtle reptile vertebrates; Wt1 chromosomal rearrangement

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