"Sex’s history, its forms, and its uses all turn out to be far stranger and more various than biologists had imagined...."

"Lixing Sun... a biologist at Central Washington University, begins his story with a protozoan called Tetrahymena thermophila. T. thermophila are tiny—just a twentieth of a millimetre long—yet can reproduce two different ways: by splitting themselves in half or, when they’re starving, by engaging in a kind of proto-sex called conjugation, in which two cells briefly fuse and swap genetic material. The protozoa come in seven distinct 'mating types,' which means that there are twenty-one possible pairings... The common white-button mushroom... comes in eighteen mating types, the fairy inkcap mushroom a hundred and forty-three, and the split-gill mushroom an astonishing twenty-three thousand three hundred and twenty-eight. In organisms that reproduce via the union of types—this group also includes yeasts and slime molds—partners are functionally equivalent and the exchange of genetic material is symmetrical, an arrangement called isogamy. The way Sun tells it, the shift from isogamy to sex as we know it began with a cheat. Some 'crafty' creature figured out a way to game the system by skimping on its reproductive contribution. A two-sided scramble ensued. On the one hand, an edge could be gained by pumping out ever smaller, nimbler gametes; on the other, there was an advantage to be had in manufacturing fewer, larger gametes for the small fry to vie for. Eventually, Sun writes, 'a minor size gap' turned into an 'uncrossable divide.' The 'go-smallers' evolved into sperm-makers; the 'go-largers' into egg-bearers...."

From "What’s the Point of Sex, Anyway? The world’s life-forms reproduce sexually in a bewildering variety of ways, even though scientists still aren’t sure why they bother" (The New Yorker).