With or Without W? Molecular and Cytogenetic Markers are Not Sufficient for Identification of Environmentally-Induced Sex Reversal in the Bearded Dragon

Transitions from environmental sex determination (ESD) to genotypic sex determination (GSD) require an intermediate step of sex reversal, i.e., the production of individuals with a mismatch between the ancestral genotypic and the phenotypic sex. Among amniotes, the sole well-documented transition in this direction was shown in the laboratory in the central bearded dragon, Pogona vitticeps, where very high incubation temperatures led to the production of females with the male-typical (ZZ) genotype. These sex-reversed females then produced offspring whose sex depended on the incubation temperature. Sex-reversed animals identified by molecular and cytogenetic markers were also reported in the field, and their increasing incidence was speculated as a climate warming-driven transition in sex determination. We show that the molecular and cytogenetic markers normally sex-linked in P. vitticeps are also sex-linked in P. henrylawsoni and P. minor, which points to quite ancient sex chromosomes in this lineage. Nevertheless, we demonstrate, based on a crossing experiment with a male bearded dragon who possesses a mismatch between phenotypic sex and genotype, that the used cytogenetic and molecular markers might not be reliable for the identification of sex reversal. Sex reversal should not be considered as the only mechanism causing a mismatch between genetic sex-linked markers and phenotypic sex, which can emerge also by other processes, here most likely by a rare recombination between regions of sex chromosomes which are normally sex-linked. We warn that sex-linked, even apparently for a long evolutionary time, and sex-specific molecular and cytogenetic markers are not a reliable tool for the identification of sex-reversed individuals in a population and that sex reversal has to be verified by other approaches, particularly by observation of the sex ratio of the progeny.

First Description of the Karyotype and Sex Chromosomes in the Komodo Dragon (Varanus komodoensis)

The Komodo dragon (Varanus komodoensis) is the largest lizard in the world. Surprisingly, it has not yet been cytogenetically examined. Here, we present the very first description of its karyotype and sex chromosomes. The karyotype consists of 2n = 40 chromosomes, 16 macrochromosomes and 24 microchromosomes. Although the chromosome number is constant for all species of monitor lizards (family Varanidae) with the currently reported karyotype, variability in the morphology of the macrochromosomes has been previously documented within the group. We uncovered highly differentiated ZZ/ZW sex microchromosomes with a heterochromatic W chromosome in the Komodo dragon. Sex chromosomes have so far only been described in a few species of varanids including V. varius, the sister species to Komodo dragon, whose W chromosome is notably larger than that of the Komodo dragon. Accumulations of several microsatellite sequences in the W chromosome have recently been detected in 3 species of monitor lizards; however, these accumulations are absent from the W chromosome of the Komodo dragon. In conclusion, although varanids are rather conservative in karyotypes, their W chromosomes exhibit substantial variability at the sequence level, adding further evidence that degenerated sex chromosomes may represent the most dynamic genome part.

Sex determination, longevity, and the birth and death of reptilian species

Vertebrate sex-determining mechanisms (SDMs) are triggered by the genotype (GSD), by temperature (TSD), or occasionally, by both. The causes and consequences of SDM diversity remain enigmatic. Theory predicts SDM effects on species diversification, and life-span effects on SDM evolutionary turnover. Yet, evidence is conflicting in clades with labile SDMs, such as reptiles. Here, we investigate whether SDM is associated with diversification in turtles and lizards, and whether alterative factors, such as lifespan's effect on transition rates, could explain the relative prevalence of SDMs in turtles and lizards (including and excluding snakes). We assembled a comprehensive dataset of SDM states for squamates and turtles and leveraged large phylogenies for these two groups. We found no evidence that SDMs affect turtle, squamate, or lizard diversification. However, SDM transition rates differ between groups. In lizards TSD-to-GSD surpass GSD-to-TSD transitions, explaining the predominance of GSD lizards in nature. SDM transitions are fewer in turtles and the rates are similar to each other (TSD-to-GSD equals GSD-to-TSD), which, coupled with TSD ancestry, could explain TSD's predominance in turtles. These contrasting patterns can be explained by differences in life history. Namely, our data support the notion that in general, shorter lizard lifespan renders TSD detrimental favoring GSD evolution in squamates, whereas turtle longevity permits TSD retention. Thus, based on the macro-evolutionary evidence we uncovered, we hypothesize that turtles and lizards followed different evolutionary trajectories with respect to SDM, likely mediated by differences in lifespan. Combined, our findings revealed a complex evolutionary interplay between SDMs and life histories that warrants further research that should make use of expanded datasets on unexamined taxa to enable more conclusive analyses.