Reproduction, TSD & Parthenogenesis

All crocodilians, most turtles, Sphenodon, and some lizards determine sex by incubation temperature during a sensitive window in mid-gestation rather than by sex chromosomes (Charnier 1966, Bull 1980). The three TSD patterns are:

• Pattern Ia (low → male, high → female): most turtles, Chelonia mydas.
• Pattern Ib (low → female, high → male): a few turtles, tuatara.
• Pattern II (both extremes → female, intermediate → male): crocodilians, some turtles.

The molecular mediator is the CIRBP cold-inducible RNA-binding protein and its downstream activation of SOX9, DMRT1, and the aromatase CYP19A1 that converts testosterone to estradiol (Schroeder 2016, Ge 2018). The pivot temperature T_p is the incubation temperature that yields 50% of each sex.

\[ P(\text{female}\mid T) \;=\; \frac{1}{1+e^{-k(T-T_p)}} \]

A TSD species whose pivot temperature sits just below the mean nest temperature is at acute climate risk: a few tenths of a degree of warming tip the hatchling ratio toward near-100% female. Jensen 2018 documented the Great Barrier Reef northern Chelonia mydas rookery: >99% of juvenile hatchlings have been female for over two decades, implying effective reproductive dead-end absent dispersal from cooler southern rookeries.

Janzen 1994 (PNAS) first formalised the mismatch model and argued for rapid selective pressure on pivot temperature or nest-site choice. Some species appear to adapt by shifting nesting to cooler microhabitats (shade, higher elevation, cooler sand); others have lost TSD and re-evolved genetic sex-determination (Sarre 2004 GSD-to-TSD transitions).

Obligate parthenogenesis (all- female, egg without meiosis) has evolved independently in at least 50 squamate species across geckos, lacertids, xantusiids, and notably the whiptail lizards (Aspidoscelis). Whiptail parthenogens are allopolyploid hybrids whose hybrid origin generates heterozygosity that purely asexual clones usually lack, offsetting Muller’s ratchet (Moritz 1993, Dawley & Bogart 1989).

Facultative parthenogenesis has been documented in captive pythons, boas, and even a Komodo dragon (Watts 2006, Nature); allele homozygosity in the offspring confirmed terminal-fusion automixis. Parthenogenesis in sharks and some birds appears to use the same mechanism.

Viviparity has evolved >100 times in squamates (Blackburn 2015), most often as an adaptation to cold climates where embryonic thermoregulation inside a basking mother is safer than in a buried nest. The scale ranges from ovoviviparity (retained yolk-provisioned eggs) to true matrotrophy: Pseudemoia entrecasteauxiiand other skinks have a complex placenta with nutrient transport comparable to eutherian mammals (Thompson 2009). True viviparous snakes include garter snakes (Thamnophis) and European adder (Vipera berus).