Sensory neurons in the spinal cord of nominal female embryos in the marine turtle Lepidochelys olivacea respond to shifts in incubation temperature:implications for temperature dependent sex determination

Gonadal determination in marine turtles depends on incubation temperature. The mechanisms that spark off this process remain unclear. Previously, we proposed that sensory nerves reaching the gonadal primordium in nominal female embryos of Lepidochelys (L) olivacea may sense and signal incubation temperature. These nerves could later trigger ovarian determination by releasing neurotransmitters in a code constructed based on the thermal information (Gutierrez-Ospina et al., Acetylcholinesterase-positive innervation is present at the undifferentiated stages of the sea turtle Lepidochelys olivacea embryo gonads: implications for temperature-dependent sex determination, J. Comp. Neurol. 410 (1999) 90-98). The hypothesis briefly described, however, has been recently refuted under weak theoretical grounds and experimental misinterpretations (see introduction). Here, we present preliminary results that show that nominal female embryos have sensory neurons located in the dorsal horn laminae I and II of the lumbar spinal cord that display increased c-Fos-like immuno-staining after being incubated either at 15°C or 50°C. Because these spinal neurons are the primary central target of dorsal root ganglion neurons that innervate the urogential crest, these observations keep open the possibility that gonadal sensory nerves indeed signal thermal information that could later be used to trigger or instruct ovarian specification in marine turtles.

Contributions of source population and incubation temperature to phenotypic variation of hatchling Chinese skinks

Phenotypic plasticity and local adaptation are viewed as the main factors that result in between-population variation in phenotypic traits,but contributions of these factors to phenotypic variation vary between traits and between species and have only been explored in a few species of reptiles.Here,we incubated eggs of the Chinese skink(Plestiodon chinensis)from 7 geographically separated populations in Southeast China at 3 constant temperatures(24,28,and 32℃)to evaluate the combined effects of dutch origin,source population,and incubation temperature on hatchling traits.The relative importance of these factors varied between traits.Nearly all examined hatchling traits,including body mass,snout-vent length(SVL),tail length,head size,limb length,tympanum diameter,and locomotor speed,varied among populations and were affected by incubation temperature.Measures for hatchling size(body mass and SVL)varied considerably among clutches.Source population explained much of the variation in hatchling body mass,whereas incubation temperature explained much of the variation in other examined traits.Our results indicate that between-population variation in hatchling traits of P.chinensis likely reflects the difference in natural incubation conditions and genetic divergence.

Endoscopic Imaging of Gonads,Sex Ratio and Temperature Dependent Sex Determination in Captive Bred Juvenile Burmese Star Tortoises Geochelone platynota

The possibility of temperature dependent sex determination makes it important to evaluate sex ratios in captive breeding programs of threatened tortoises. We assessed the sex ratio of juvenile Burmese Star Tortoises Geochelone platynota by direct observation of their gonads through an endoscope in the captive breeding program of the Behler Chelonian Center （BCC） in California. The gonads of small juvenile G. platynota are thin and elongate, and fixed to the dorsal part of body cavity, with ovaries appearing as transparent sheaths with some oocytes visible and testes appearing as small, transparent, thin, sausage-like structures with a net of fine blood vessels on the surface. With growth, ovaries expand and masses of pre-vitellogenic follicles appear on the surface. Testes are transparent in small juveniles and, with growth, turn pinkish-white and then yellowish, with tubuli structures visible through a thin, transparent theca containing a network of fine blood vessels. Egg incubation temperatures were not rigorously monitored, but a temperature of 28.9 ℃ produced a heavily male biased sex ratio whereas a temperature of 30 ℃ produced a balanced sex ratio. This suggests that G. platynota has temperature dependent sex determination.

Thermosensitive period for sex determination of the tropical freshwater turtle Malayemys macrocephala

Many egg-laying reptiles possess temperature-dependent sex determination(TSD)in which outcome of gonadogenesis is determined by incubation temperature during a temperature-sensitive period of development.Prior studies on Malayemys macrocephala showed that incubation temperatures influence gonadal development and suggested that M.macrocephala exhibits TSD.However,information on the temperature-sensitivity period in this species was unknown until the current study.Turtle eggs were collected from rice fields in central Thailand from December 2016 to February 2017.In the laboratory,eggs were incubated at male-biased temperature(26°C)and shifted to female-biased temperature(32°C),or vice versa.Single shift experiments were performed systematically during embryonic stages 13–21.After hatching,sex of individual turtles was determined by histological analysis.We found that the sex determination of M.macrocephala is affected by temperature up to stage 16 of embryonic development.