Environmental sex determination
Environmental sex determination is the establishment of sex by a non-genetic cue, such as nutrient availability, experienced within a discrete period after conception. This is in contrast to genotypic sex determination, which establishes sex at conception by genetic factors such as sex chromosomes. Environmental sex determination is different to some forms of sequential hermaphroditism in which the sex is determined flexibly after birth.
The amphipod crustacean Gammarus duebeni produces males early in the mating season, and females later, in response to the length of daylight, the photoperiod. Because male fitness improves more than female fitness with increased size, environmental sex determination is adaptive in this system by permitting males to experience a longer growing season than females.
The branchiopod crustacean Daphnia magna parthenogenetically produces male progeny in response to a combination of three environmental factors, namely a reduced photoperiod in autumn, shortage of food and raised population density.
The sex of most amniote vertebrates, such as mammals and birds, is determined genetically. However, some reptiles have temperature-dependent sex determination, where sex is permanently determined by thermal conditions experienced during the middle third of embryonic development. The sex of crocodilians and sphenodontians is exclusively determined by temperature. In contrast, squamates (lizards and snakes) and turtles exhibit both genotypic sex determination and temperature-dependent sex determination, although temperature dependence is much more common in turtles than in squamates.
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