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Parthenogenesis is a mode of asexual reproduction in which offspring are produced by females without the genetic contribution of a male. Among all the sexual vertebrates, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate reptiles (snakes and lizards). [1]
The hemipenis is the intromittent organ of Squamata, [4] which is the second largest order of vertebrates with over 9,000 species distributed around the world. They differ from the intromittent organs of most other amniotes such as mammals, archosaurs and turtles that have a single genital tubercle, as squamates have the paired genitalia remaining separate. [5]
Some species reproduce exclusively by parthenogenesis (such as the bdelloid rotifers), while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis (other terms are cyclical parthenogenesis, heterogamy [ 14 ] [ 15 ] or heterogony [ 16 ] [ 17 ] ).
The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including Komodo dragons.
The male turtle headed sea snake, Emydocephalus annulatus, provides an interesting case study because of their unique scales. [35] Females of this species display smooth scales year round. Males, on the other hand, show a shift from smooth scales to ridged scales upon the arrival of the winter breeding season.
Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available. [78]
Yet, the genetic sexual determination pathway in TSD turtles is poorly understood and the controlling mechanism for male or female commitment has not been identified. [ 28 ] While sex hormones have been observed to be influenced by temperature, thus potentially altering sexual phenotypes, specific genes in the gonadal differentiation pathway ...
Growth continues until the turtle reaches sexual maturity, which occurs after approximately 2–3 years (or at a plastron length of 75–80 mm (2.95–3.15 in)) for males, and around 6–8 years (plastron length 141–155 mm (5.55–6.10 in)) for females. [2] The turtle continues to grow after reaching maturity, although considerably more slowly.