A tropical oviparous lizard, Calotes versicolor, exhibiting a potentially novel FMFM pattern of temperature‐dependent sex determination

Abstract

Among squamate reptiles, lizards exhibit an impressive array of sex-determining modes viz. genotypic sex determination, temperature-dependent sex determination, co-occurrence of both these and those that reproduce parthenogenetically. The oviparous lizard, Calotes versicolor, lacks heteromorphic sex chromosomes and there are no reports on homomorphic chromosomes. Earlier studies on this species presented little evidence to the sex-determining mechanism. Here we provide evidences for the potential role played by incubation temperature that has a significant effect (P < 0.01) on gonadal sex and sex ratio. The eggs were incubated at 14 different incubation temperatures. Interestingly, 100% males were produced at low (25.5 ± 0.5 ° C) as well as high (34 ± 0.5 ° C) incubation temperatures and 100% females were produced at low (23.5 ± 0.5 ° C) and high (31.5 ± 0.5 ° C) temperatures, clearly indicating the occurrence of TSD in this species. Sex ratios of individual clutches did not vary at any of the critical male-producing or female-producing temperatures within as well as across the seasons. However, clutch sex ratios were female- or male-biased at intermediate temperatures. Thermosensitive period occurred during the embryonic stages 30-33. Three pivotal temperatures operate producing 1:1 sex ratio. Histology of gonad and accessory reproductive structures provide additional evidence for TSD. The sex-determining pattern, observed for the first time in this species, that neither compares to Pattern I [Ia (MF) and Ib (FM)] nor to Pattern II (FMF), is being referred to as FMFM pattern of TSD. This novel FMFM pattern of sex ratio exhibited by C. versicolor may have an adaptive significance in maintaining sex ratio.

Differential expression of estrogen receptor alpha in the embryonic adrenal–kidney–gonadal complex of the oviparous lizard, Calotes versicolor (Daud.)

“…Care was taken to ensure that the fluctuation in the incubation temperature did not exceed 0.5°C and a relative humidity of 62%. Other incubation methods have already been described elsewhere (Inamdar et al, 2012a). The eggs were incubated from oviposition (stage 27) to hatching (stage 42).…”

“…Gonadal sex differentiation occurs at stage 34 (Inamdar et al, 2012a). During this stage ERa expression was predominantly confined to cytoplasm of medullary region as well as to the oogonial cells of the cortex.…”

“…Since estrogen signalling is required for sexual differentiation, the expression of estrogen receptor (ERa) was studied in the embryonic adrenal-kidney-gonadal (AKG) complex of lizard, Calotes versicolor which exhibits a novel FMFM pattern of TSD i.e., the incubation of eggs at 23.5 ± 0.5 and 31.5 ± 0.5°C produces 100% female hatchlings and incubation of eggs at 25.5 ± 0.5 and 34 ± 0.5°C produces 100% male hatchlings (Inamdar et al, 2012a). To elucidate the estrogen action during sex differentiation and to establish the onset of gonadal estrogen activity, the ontogenic expression, abundance and localization of ERa was examined in the embryonic AKG complex through immunolocalization analysis in C. versicolor at FPT of 31.5 ± 0.5°C.…”

Molecular Cytogenetic Search for Cryptic Sex Chromosomes in Painted Turtles <b><i>Chrysemys picta</i></b>

“…Data on sex determination was collected from the literature [Olmo, 2005;Olmo and Signorino, 2005;Vogt, 2008;Leaché and Sites, 2009;Paez et al, 2009;Pokorná and Kratochvíl, 2009;Shibaike et al, 2009;Bernhard, 2010;Okada et al, 2010;Zhang et al, 2010;Hare et al, 2011;Valenzuela and Adams, 2011;Inamdar et al, 2012;Rojo et al, 2012;Tang et al, 2012;Valenzuela et al, 2013]. Lizards are paraphyletic with snakes nested within this clade.…”

Revisiting the first report of temperature‐dependent sex determination in a vertebrate, the African redhead agama

“…a few turtles, crocodilians, some lizards; Harlow & Taylor, ; El Mouden, Znari & Pieau, ; Kraak & Pen, ). Importantly, the level of sex ratio bias at different temperatures varies considerably across species, among populations, and even among family groups within populations (Rhen & Lang, ; Warner, ; Inamdar Doddamani, Vani & Seshagiri, ; Bull, ). Thus, the TSD patterns described above are generalizations of sex‐determining reaction norms based on constant temperature incubation in the laboratory, but there is considerable variation at different levels.…”