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OBLIGATE SEXUALITY AS A RACE. “One driving force may have been a need to outrun pathogens, either exogenous (bacteria, viruses, parasites, fungi, prions) or endogenous ones (transposons).” http://www.sciencedirect.com/science/article/pii/S1749461315000391 View in LinkedIn

OBLIGATE SEXUALITY. “At the root it suggests that recombinant progeny may be always better than the parental. If so, what selective force might have driven the transition from a facultatively sexual last common ancestor to a derived obligately sexual state?” http://www.sciencedirect.com/science/article/pii/S1749461315000391 View in LinkedIn

THE SECOND QUESTION. “Many animals are obligately sexual. Why? This is in marked and striking contrast to the vast majority of microbial eukaryotes which are facultatively sexual, and happily capable of vegetative mitotic clonal reproduction interspersed with bouts of sexual reproduction. Why this is not so for humans, mice, rabbits, is an interesting conundrum.” http://www.sciencedirect.com/science/article/pii/S1749461315000391 View in LinkedIn

THE CURVE. “These may seem exceptions, but considering what is shared and what is distinct between closely aligned species with two sexes vs. those with more, or those with only one (unisexual, to mean “uni” as in one sex, or “universal” as in pansexual and capable of mating with any other member of the population as well as itself), may prove enlightening.” http://www.sciencedirect.com/science/article/pii/S1749461315000391 View in LinkedIn

THOUSANDS OF SEXES. “There are examples in which there are more than two mating types or sexes. These include the basidiomycete fungi we have discussed that have literally thousands and thousands and thousands of different mating types (multisexual) including Coprinopsis cinerea, the slime mold Dictyostelium with its three mating types (trisexual), and the ciliate Tetrahymena with seven (septasexual).” http://www.sciencedirect.com/science/article/pii/S1749461315000391 View in LinkedIn

REVERSE TRANSITIONS. “While we have emphasized the evolutionary transition from hermaphroditism to separate sexes, several examples are known where the opposite transitions occur, indicating that the conditions favoring the separation of male and female function are not always present.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

TRANSITIONS. “When separate sexes are favored, the transition can occur via several evolutionary pathways. Separate sexes may evolve from hermaphrodites either by gradual increases in sex-specific investment or rapidly by the appearance of male- or female-sterility mutations. The latter occurs regularly in plants, often generating mixed sexual systems, such as gynodioecy (mixtures of females and hermaphrodites) and androdioecy (mixtures of males and hermaphrodites).” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

2ND MAIN CONCEPT. “The second major hypothesis is that separate sexes evolve as a means to avoid self-fertilization, which can produce low-fitness offspring because of the exposure of recessive deleterious alleles (“inbreeding depression”). Empirical evidence for inbreeding depression is widespread in animals and plants; for instance, in the Hawaiian endemic plant genus Scheidia, high inbreeding depression promotes the evolution of dioecy.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

EVIDENCE. “Indirect evidence of a trade-off comes from the fact that many asexual animals and plants that still have residual sperm/pollen production evolve reduced investment in male gametes over time, suggesting that doing so increases female function.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn

EVIDENCE. “Direct evidence for the trade-off hypothesis is sparse, and observations consistent with it come from hermaphroditic great pond snails, which reallocate resources to female function when sperm production is experimentally abolished, and from strawberries, in which increased pollen production comes at the cost of reduced seed set.” http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899 View in LinkedIn