Environmental sex determination

Environmental sex determination or ESD is any sex-determination system in which sex is established by a nongenetic cue (e.g. nutrient availability) experienced within a discrete period after conception. In contrast, genotypic sex determination (GSD) occurs when sex is established by genetic factors (e.g. sex chromosomes) at conception. Environmental sex determination should also not be confused with some forms of hermaphroditism in which the sex is determined flexibly after birth, such as dichogamy.

In its mating season in the amphipod Gammarus duebeni, photoperiod induces production of males earlier in the season, and females later. 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 adaptive significance of environmental sex determination is not so clear for amniotes, because most amniote vertebrates have genetic sex determination. In those animals that don't, environmental sex determination occurs as temperature-dependent sex determination (TSD), wherein sex of individuals is permanently determined by thermal conditions experienced approximately during the middle one-third of embryonic development. Mammals and birds have only genetic sex determination, whereas crocodilians and sphenodontians exclusively have temperature-dependent sex determination. In contrast, squamates (lizards and snakes) and turtles exhibit both genotypic sex determination and temperature-dependent sex determination, although TSD is much more common in turtles than in squamates.

The branchiopod crustacean Daphnia magna parthenogenetically produces male progeny in response to environmental conditions.

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