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.
As in a lot of taxa, mating types in volvocine algae are determined by a region of the genome that has undergone rearrangements so that recombination is suppressed (the sex-determining region). Since the sex-determining region of the plus mating type doesn’t recombine with that of the minus mating type, the two are free to accumulate changes independently. The resulting divergence is taken to an extreme in species with sex chromosomes, where it can lead to dramatic differences in chromosome size.
The sex-determining region of Volvox is much larger than that of Chlamydomonas, but why? An important thing to keep in mind in this context is that multicellular volvocine algae such as Gonium and Volvox didn’t evolve from Chlamydomonas reinhardtii any more than humans evolved from chimpanzees. Rather, Chlamydomonas and Volvox shared a common ancestor 200 or so million years ago. So when we’re comparing the Chlamydomonas and Volvox genomes (the only two fully sequenced in this group), it’s hard to say whether a given difference results from a change in one lineage or the other. In the case of the sex-determining region, the difference could be because it got bigger in the Volvox lineage or because it got smaller in the Chlamydomonas lineage. Since we don’t (and probably never will) have the genome of their common ancestor, we need to look elsewhere to figure this out.
One obvious place to look for relevant information is in the genomes of other related algae. Traits that are shared between Chlamydomonas and Gonium were probably present in their most recent common ancestor; otherwise, they would have to have arisen independently in the two lineages. If something is the same between Chlamy and Gonium but different in Volvox, we can be fairly confident that the change happened in the Volvox lineage. Hamaji and colleagues sequenced the sex-determining region from Gonium pectorale and compared it to those of Chlamydomonas and Gonium to figure out how the differences among them evolved. I will only mention a couple of these changes, but the article is open access, so if you want to read about all of them, you can.
The Chlamydomonas and Gonium sex-determining regions contain three genes that have been experimentally verified to play a role in sex determination: MID and MTD1 in the minus mating type and FUS1 in the plus. Their presence in both genomes is strong evidence that they are ancestral, that is, that all three were present in the most recent common ancestor of Chlamydomonas, Gonium, and Volvox. However, the female version of the sex-determining region in Volvox does not include FUS1. We know from previous work by Hisayoshi Nozaki and colleagues that Volvox males evolved from the minus mating type and females from the plus. FUS1 must have been lost from the Volvox lineage some time after its divergence from Gonium around 190-230 million years ago, but whether it happened soon after that split or much later is unknown. Sequencing of additional genomes more closely related to Volvox, currently underway in Bradley Olson’s lab, will help to narrow this down.
MTD1, which plays a role in gamete fusion in Chlamydomonas, is present in the minus or male version of the sex-determining region in all three algae, but the copy in Volvox is a nonfunctional pseudogene. Since pseudogenes tend not to stick around over long evolutionary timescales, this change probably happened more recently than the loss of FUS1.
We are still left with some important questions. The sex-determining region in Volvox is very different from those of Chlamydomonas and Gonium: much larger, with more genes unique to each sex, and without functional copies of two genes essential to the sexual cycle in the smaller algae. Many of these changes probably have to do with the transition from isogamy to oogamy, but since Chlamy and Gonium are both isogamous, it’s hard to say which ones were essential in this change. Again, additional genomes sequences are likely to help resolve this, especially those of the anisogamous taxa Eudorina and Pleodorina. The study by Hamaji and colleagues brings us another step closer to a complete picture of the evolution of males and females, but there is still much work to be done.
[Thanks to Erica Larson for reading part of this to make sure I didn’t say anything stupid. Anything in this post that is wrong is entirely my fault.]