Temperature-sex determination in Podocnemis expansa (Testudines, Podocnemididae)

Abstract

This study has been carried out at the central region of the Araguaia river on the border between the states of Goiás and Mato Grosso in the Brazilian Amazon Basin from September to December 2000. We recorded temperature fluctuation, clutch-size, incubation period and hatching success rate and hatchlings' sex ratio of five nests of Podocnemis expansa (Schweigger, 1812). Despite the relatively small sample size we infer that: a) nests of P. expansa in the central Araguaia river have a lower incubation temperature than nests located further south; however, incubation period is shorter, hatching success rate is lower and clutch-size is larger; b) Podocnemis expansa may present a female-male-female (FMF) pattern of temperature sex-determination (TSD); c) thermosensitive period of sex determination apparently occur at the last third of the incubation period; and, d) future studies should prioritize the relationship between temperature variation (i.e., range and cycle) and embryos development, survivorship and sex determination.

Podocnemis expansa Turtles Hint to a Unifying Explanation for the Evolution of Temperature-Dependent Sex Determination in Long-Lived and Short-Lived Vertebrates

“…One hypothesis is that TSD might be retained in P. expansa via phylogenetic inertia due to lack of genetic variation that precludes this lineage from evolving out of TSD. However, this explanation can be ruled out because variation exists among populations of P. expansa in their TSD pattern, as observed also within and among congeneric species (P. unifilis, P. erythrocephala, and P. leywana) [Lance et al, 1992;de Souza and Vogt, 1994;Valenzuela et al, 1997;Valenzuela, 2001a, c;Lubiana and Ferreira, 2009;Paez et al, 2009;Bonach et al, 2011;Gallego-Garcia and Paez, 2016;Gómez-Saldarriaga et al, 2016], as well as in their incubation time [Hildebrand et al, 1997;Valenzuela, 2001c;Lubiana and Ferreira, 2009], clutch size, and other life history traits [Vanzolini, 2003]. Furthermore, variation in the thermal response of the gene network regulating sexual development in turtles and the general lability of sex determination in this clade (TSDIa, TSDII, XX/XY, and ZZ/ZW systems have evolved in turtles) also argue against this hypothesis as an overall explanation for chelonians [Bull, 1980;Ewert et al, 2004;Valenzuela, 2004;Grossen et al, 2010;Uller and Helantera, 2011;Valenzuela et al, 2013;Holleley et al, 2015].…”

“…P. expansa's sex ratios at birth vary among years and populations. For instance, reports range from a highly feminized sex ratio (11:1 female:male) detected in a longer-term study of a population in the Orinoco river [Mogollones et al, 2010] to slighter female-bias or malebias recorded with lesser sampling in single years in oth-er populations [Valenzuela, 2001a;Bonach et al, 2011]. Further, no systematic female-biased sex ratios at a population-wide scale have been reported for P. expansa that are tied to contemporary global warming.…”

Ecotoxicological Perspectives of Sex Determination

“…Gonads may still be sex-reversed after the thermosensitive period when exposed to exogenous hormones or inhibitors that affect the maintenance of gonadal differentiation [Dorizzi et al, 1996;Belaid et al, 2001]. The duration of the thermosensitive period may vary by species, by temperature, and even by population [Yntema, 1979;Pieau and Dorizzi, 1981;Wibbels et al, 1991;Valenzuela, 2001;Hewavisenthi and Parmenter, 2002;Bonach et al, 2011;Rhen et al, 2015]. Under fluctuating temperature regimes, as occur in natural nests, sex is determined by the cumulative effect of the temperatures above the pivotal temperature (or above the value that causes development to halt) experienced during the thermosensitive period Neuwald and Valenzuela, 2011].…”

Morphology and immunolocalization of intertubular steroidogenic cell in mesonephros of Podocnemis expansa during gonadal differentiation

“…The nesting of this species is influenced by the water levels of the river, in which spawning, and hatching are carried out in the dry season and the hatching of eggs coincides with the beginning of the rainy season and the rise of rivers (Vanzolini, 2003;Piña et al, 2006). This species lays an average of 92 eggs (63-134) and the hatching period is between 55 and 70 days (Bonach et al, 2011). It presents sexual determination influenced by incubation temperature (Temperature-dependent sex determination -TSD), in which high temperatures promote birth more females, while low temperatures increase male births (Valenzuela, 2001;Bonach et al, 2011).…”

“…This species lays an average of 92 eggs (63-134) and the hatching period is between 55 and 70 days (Bonach et al, 2011). It presents sexual determination influenced by incubation temperature (Temperature-dependent sex determination -TSD), in which high temperatures promote birth more females, while low temperatures increase male births (Valenzuela, 2001;Bonach et al, 2011). There is usually a time window known as thermosensitive period (TSP), during the second third of incubation period, in which physical temperature stimulus is transformed into biological stimulus acting on gonadal tissues and triggering sexual determination (Radhakrishnan et al, 2018).…”