The evolution of sex is one of the big outstanding problems in evolutionary biology. The origin of sex is one of Maynard Smith and Szathmáry’s “Major Transitions,” on which I’m currently teaching a course here at the University of Montana. Our discussion of sex luckily coincided with the visit of the grad-invited Distinguished Speaker, Sally Otto, an important theorist on this topic (among others). Dr. Otto generously agreed to join us for the discussion, which turned out to be one of the best we’ve had.

A related problem to the origin of sex is the origin of males and females. Sexual reproduction doesn’t always involve males and females: lots of species that don’t even have males and females have sex. There are lots of traits that we associate with males and females — fancy plumage, differences in body size and type of genitalia, presence and absence of exaggerated weapons — but what actually defines males and females is differences in gamete size. Animals, plants, and other organisms with males and females are oogamous: males have small, swimming sperm, and females have large, immotile eggs. But lots of single-celled eukaryotes have only one size of gamete. We call these isogamous (‘equal gametes’).

Some volvocine algae are isogamous (such as Chlamydomonas), some are oogamous (such as Volvox), and some (such as Pleodorina) are anisogamous (‘unequal gametes’), meaning that the gametes come in two sizes but both can swim. In spite of not having sexes per se, Chlamydomonas, like a lot of isogamous organisms, comes in two ‘mating types’, which are arbitrarily called ‘plus’ and ‘minus.’ The mating types are self-incompatible, in other words plus can only mate with minus and vice versa.

All this variation in mating systems makes the volvocine algae a great model system for understanding the evolution of sex and the sexes (see ‘Volvox 2015: all about sex‘). We know from previous work that males evolved from the minus mating type and females from the plus in this lineage. But males and females have evolved from isogamous ancestors many times, and to my knowledge we don’t know which came from which for any other group.

Takashi Hamaji and colleagues have just published an analysis of the genomic region that determines mating type in Gonium pectorale, an isogamous alga more closely related to Volvox than to Chlamydomonas.

Figure 1 from Hamaji et al 2016. A schematic diagram for phylogenetic relationships of selected volvocine species based on Nozaki et al. (2000); Herron and Michod (2008). The top row illustrates gamete type and structure. Tubular mating structures in isogamous gametes are indicated with red bars at the flagellar base. The possession of a MID gene is shown next to the minus mating type or male gametes. The lower row of cartoons depicts vegetative morphology (not to scale) for the indicated species.

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