Sex determination in animals is amazingly plastic. Vertebrates display contrasting strategies ranging from complete genetic control of sex (genotypic sex determination) to environmentally determined sex (for example, temperature-dependent sex determination)1. Phylogenetic analyses suggest frequent evolutionary transitions between genotypic and temperature-dependent sex determination in environmentally sensitive lineages, including reptiles2. These transitions are thought to involve a genotypic system becoming sensitive to temperature, with sex determined by gene–environment interactions3. Most mechanistic models of transitions invoke a role for sex reversal3, 4, 5. Sex reversal has not yet been demonstrated in nature for any amniote, although it occurs in fish6 and rarely in amphibians7, 8. Here we make the first report of reptile sex reversal in the wild, in the Australian bearded dragon (Pogona vitticeps), and use sex-reversed animals to experimentally induce a rapid transition from genotypic to temperature-dependent sex determination. Controlled mating of normal males to sex-reversed females produces viable and fertile offspring whose phenotypic sex is determined solely by temperature (temperature-dependent sex determination). The W sex chromosome is eliminated from this lineage in the first generation. The instantaneous creation of a lineage of ZZ temperature-sensitive animals reveals a novel, climate-induced pathway for the rapid transition between genetic and temperature-dependent sex determination, and adds to concern about adaptation to rapid global climate change.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: C-banded P. vitticeps chromosomes. (163 KB)
a, Mitotic metaphase chromosomes of a ZW control female individual. Arrowhead indicates the presence of a W chromosome identified by dense black staining of a single microchromosome. b, Mitotic metaphase chromosomes of a female putative ZZ sex-reversed individual. No evidence of a W chromosome was detected. c, Mitotic metaphase chromosomes of a control ZZ male individual. No evidence of a W chromosome was detected. Scale bar, 10 μm.
- Extended Data Figure 2: Comparative genomic hybridization in P. vitticeps. (193 KB)
Genomic DNA was labelled by nick translation incorporating SpectrumGreen-dUTP for males and SpectrumOrange-dUTP for females. a, Mitotic metaphase chromosomes of a ZW control female individual. Arrowhead indicates the presence of a single W microchromosome identified by the enriched orange fluorescence of female specific genomic DNA labelled with SpectrumOrange-dUTP. b, Mitotic metaphase chromosomes of a female putative ZZ sex-reversed individual. No evidence of a W chromosome was detected. c, Mitotic metaphase chromosomes of a control ZZ male individual. No evidence of a W chromosome was detected. d–f, DAPI staining of the same metaphases, control ZW female, sex-reversed ZZ female and control ZZ male, respectively. Scale bar, 10 μm.
- Extended Data Figure 3: Physical mapping of a W-chromosome-linked microsatellite motif in P. vitticeps. (196 KB)
a, Mitotic metaphase chromosomes of a ZW control female individual. Arrowhead indicates the presence of a W chromosome identified by a strong hybridization of (AAGG)8-Cy3 florescence (orange) on a single microchromosome. b, Mitotic metaphase chromosomes of a female putative ZZ sex-reversed individual. No evidence of a W chromosome was detected. c, Mitotic metaphase chromosomes of a control ZZ male individual. No evidence of a W chromosome was detected. d–f, DAPI staining of the same metaphases, control ZW female, sex-reversed ZZ female and control ZZ male, respectively. Scale bar, 10 μm.
- Extended Data Figure 4: Modelling the decline of the ZW genotype resulting from frequency-dependent selection. (74 KB)
Frequency of the ZW genotype declines precipitously with increasing incubation temperature. Our wild population (shown in red, 14.3% sex reversal) resides on the precipice between GSD and TSD and requires only a small change in environmental temperature to precipitate loss of the W chromosome.