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ROLE OF GENES. “With genotypic sex determination (GSD), which occurs in the majority of species with known sex-determining mechanisms, genetic elements specify whether individuals are female or male. In many animals and some plants, however, the switch to develop into a female or male does not lie in the genes.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

RAPID EVOLUTION. “Sex determination is a rapidly evolving trait in many lineages, and sometimes closely related species, or populations of the same species, have different modes of sex determination. Houseflies, for example, normally have XY sex chromosomes, but dominant masculinizing and feminizing alleles on other chromosomes exist in some populations that override sex determination by the XY chromosomes.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

METHODS TO CHANGE SEX. “Diverse mechanisms are used to determine sex. All crocodiles, most turtles, and some fish exhibit temperature-dependent sex determination; Wolbachia infections override existing sex determination systems in many arthropod species and either kill/sterilize males or transform them into phenotypic females; male scale insects eliminate their father's genome after fertilization; marine worm Bonellidae larvae develop as males only if they encounter a female; and many plants and animals—including some snails and fish—change sex during their lifetime in response to environmental or social cues.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

BACKUP IN SESSILES. “This advantage of reproductive assurance is particularly pronounced in sessile animals—like corals—and plants, which cannot move to find a mate. Thus there is a clear advantage to combining both male and female functions within an individual, especially in taxa with low mobility.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

BACKUP SYSTEM. “Hermaphrodites can mate with each other and benefit from the advantages of sex by mixing their genomes, but when mates are difficult to find, hermaphrodites can also escape the need for a reproductive partner by self-fertilization (which, however, may produce low-fitness offspring due to “inbreeding depression.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

MODIFYING GENES. “In species with genotypic sex determination, the chromosome pair that determines sex can change rapidly over time. Transitions are particularly likely when the ancestral sex chromosome exhibits little genetic differentiation, since WW or YY combinations are then less likely to be lethal. New sex-determining genes (or copies of the original gene in a new location) can lead to transitions within and between different XY and ZW system.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn