Week 03: Why are sperm so small? Or, how did anisogamy evolve?

During the first week of this course, I jumped at the chance to lead a discussion on the evolution of anisogamy. I wanted to share research I first encountered at a conference in August — results from a lab that works in algae systems which have natural variation not only in sex determination mechanisms and but also in the presence of sexes themselves.

A large number of sexually reproducing organisms (mostly fungi, algae, and single celled organisms) have sex but not sexes. How is this possible? We define sexes (male and female) based on differential gamete size. Females produce large, sessile gametes, and males produce small, motile gametes. The term anisogamy describes the phenomenon of two dramatically different gamete sizes in a sexually reproducing species. When individuals reproduce sexually and produce similarly sized gametes, they are isogamous (same sized gametes). In isogamous populations, an individual’s genotype, or mating type, determines its partners for exchanging genetic material. Mating types prevent organisms from exchanging genetic material with similar individuals, since two individuals of the same type can not mate. Populations can have a great number of mating types, or they can simply have two. But the vector of genetic material — that is, the gamete — has the same size regardless of mating type. Many fungi and bacteria reproduce with mating type systems.

Picture1

Anisogamous gametes – a very large egg and a very small sperm

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Isogamous – identically sized –gametes

Many theoreticians posit that from anisogamy comes most, if not all, secondary sex characteristics (for more on this idea, see Hannes’ blog entry to be posted soon). However, before attending this conference in August, I’d never given much thought to how anisogamy itself evolves. And in an evolutionary sense, what comes before anisogamy?

There’s a large body of theoretical work dating to the 1930s delving into how anisogamy evolved from mating types (Kamus, 1932). Essentially, there are two successful strategies for maximizing the amount of genetic material one passes on to the next generation. An individual can either produce a huge number of gametes (which necessarily means each gamete is very small, since any one organisms has a finite amount of energy with which to make gametes) or an individual can make a small number of very large and well provisioned gametes (which increase the chances of offspring survival). Clearly, these two gamete types describe sperm and eggs, respectively. Eighty-five years ago, theoreticians proposed that a population with these distinct gamete sizes was in an evolutionary stable state. And then in the 1970s, another group of researchers proposed models describing how this stable state could evolve from mating types. In short, anisogamy evolves when selective pressures favor a reasonably large, but not huge, gamete (Parker, 1972). (If selection favored enormous gametes, all organisms would produce huge eggs.) Several years later, Charlesworth expanded on this work by positing that a locus controlling gamete size should be in tight linkage with a mating type locus. He believed that genes controlling the development of other sex specific traits would also get swept up into this locus (Fig. 1) (Charlesworth, 1978). (Put another way, Charlesworth thought that all the genes controlling differentiation of one sex would be physically close to each other in genome so that they would be inherited together.)

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Figure 1: Two contrasting models for how anisogamy and sexual dimorphism evolve

Despite the extensive theoretical interest in the evolution of anisogamy, very little empirical work has tested these predictions. And here comes in this research that caught my attention this summer. It’s work conducted in the green algae Volvox, where the Umen lab has discovered the gene responsible for male differentiation. Some algae species have mating types, while others have sexes.

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Volvox algae studied by Geng et al.

The ability to compare between species makes this group attractive for studying how anisogamy and sex evolve from mating types. Geng et al. tested the hypothesis proposed by Charlesworth, and expected to find a large locus containing genes coding for mating type, gamete size, and other sexual characteristics. And even though there are rather large male and female loci, Geng et al discovered that within that locus one transcription factor has an unduly large effect on male differentiation. It’s responsible for a whole host of male specific traits, and by knocking this transcription factor out in males, Geng et al. feminized genetically male algae. By inducing expressing of the gene in females, they could masculinize genetically female algae. It turns out that in this Volvox species, the sex determining locus doesn’t act as Charlesworth envisioned (Fig. 1). The genes controlling various aspects of male development are seemingly scattered around the genome. A Volvox algae just needs to inherit the switch – the transcription factor – to turn all those genes on to make a male.

To read more about how secondary sex characteristics are linked to anisogamy, check out Hannes’ blog post (coming soon).

-Judy

REFERENCES

Kamus, H. (1932). Biol. Zentrul. 52,716. <too old to exist on the internet>

Parker et al (1972) J. theor. Biol. 36, 529-553. PDF

Charlesworth (1978) J. theor. Biol. 73, 347-357. PDF

Geng et al (2014) PLoS Biol. 12, 7. e1001904. PDF

 

 

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