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Snipsand snailsandpuppydogs’tails,orsugar and spice and all things nice? Fromour animal-centered point of view, thedistinction between males and female canseem rather obvious—many animals havemale individuals and female individualswhose differences are often pretty evidentat a glance. However, we’re also aware ofmore exotic variations like hermaphrodit-ism and Tiresias-like midlife gender-bend-ing. Indeed, the further we get from thebirds and the bees, the more it becomesunclear what exactly we mean by ‘‘male’’and ‘‘female.’’ Are these just tags, or arethey fundamentally different? And if thelatter, how did this fundamental differencecome to be?Sex arose as a way of allowing the mixingand shuffling of genotypes between twoindividuals, generating a blended range ofgenotypes in the offspring that mightmaximize the chances of thriving in achanging environment. However, to getthe most out of this you need to discouragematingwithyournearestanddearest.Manyunicellular organisms have solved thisproblem by evolving a mating type sys-tem—this typically involves a genetic switchthat determines which of two or moreoptional ‘‘mating types’’ a given individualbelongs to. It’s essentially a compatibilitycheck—mating can only occur betweenindividuals of different mating type.As life becomes more complex, howev-er, so does sex. Multicellular organismscan have cells whose shape and functionare tailored to a particular task, and canafford to delegate sex to a specializedsubset of their cells—the gametes. Butbecause the sexually active cells no longerhave to perform as an independentunicellular organism, they’re free to beinfluenced by complex evolutionary forcesthat further specialize them. So ratherthan remaining equal, a recurrent patternemerges in gametes across the tree of life.The gametes of one mating type becomesmall, mobile, and numerous to enablemating at a distance. And the gametes ofthe other mating type become large,immobile, and few to allow for effectiveprovisioning of the developing multicellu-lar offspring. This state of having sexuallydimorphic gametes is called anisogamy (asopposed to isogamy), and the mating typesare now true sexes—male and femalerespectively.But what’s the mechanism behind thisstriking phenomenon? Theorists havesuggested that it could happen by theinsertion of a gene that determines cell sizenext to the gene that determines matingtype, but this hasn’t been shown experi-mentally. Sa Geng, Peter DeHoff, andJames Umen, the authors of a paper justpublished in PLOS Biology, have exploitedtwo species of volvocine algae to probehow anisogamy arises and to test thistheory. These two species, separated by200 million years of evolution, couldn’t bemore different in their appearance orsexual proclivities. Chlamydomonas reinhardtiiis a 10-

What Are Little Boys and Girls Made of? The Origins of Sexual Dimorphism