Mating Types

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In many species, male and female sexes do no exist; instead of producing sperm and egg gametes (anisogamy) these species produce ​mating type gametes of equal size and behaviour (isogamy), as illustrated in the figure above. Sex is (as with the sexes) restricted to occurring between gametes of distinct types. However, while anisogamous species have just two sexes, isogamous species can potentially have many thousand mating types. This is exemplified by the fungus Schizophyllum commune, which has over 23,000 mating types.

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An unresolved question has been what governs the number of mating types in isogamous species. Naively, we might expect that many thousands of mating types is the norm; any new mating type that evolves will initially reproduce more quickly than its ancestors as it has more opportunities for reproduction with potential non-self partners. However most isogamous species, such as the green alga Chlamydomonas reinhardtii and the yeast Saccharomyces cerevisiae

have just two.

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Recently, Hanna Kokko and I proposed that the number of mating types might simply be a result of a balance between mutations and extinctions. We constructed a mathematical model that accounted for the fact that many isogamous species reproduce asexually as well as sexually. Our model predicted that when sexual reproduction was rare, fewer mating types could be maintained. On evaluating the empirical literature, we found that this insight was well supported:

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The rate of facultative sex governs the number of expected mating types in isogamous species - Nature Ecology and Evolution.

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For more information about this paper, there's a wonderfully written commentary by Sujal Phadke: 

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Sex begets sexes - Nature News and Views.

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And a blog-post by myself on how the paper came to be:

 

Why do most species have so few mating types, yet some have so many? - Behind the Paper, Nature Community.

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Follow up work with Peter Czuppon has demonstrated that although the number of mating types observed in natural populations may result from a mutation-extinction balance, this does not imply a high turnover rate of mating type alleles: in fact, low numbers of mating types can be very stable over long, trans-specific, evolutionary periods:

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Invasion and extinction dynamics of mating types under facultative sexual reproduction - Genetics

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Meanwhile work with Yvonne Krumbeck and Tim Rogers has shown that small fitness differences between mating type alleles may also play a key role in limiting mating type diversity:

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Fitness differences suppress the number of mating types in evolving isogamous species, Royal Society Open Science

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Sex chromosome Evolution

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In most animals, sex is determined genetically by a sex-specific chromosome: the Y chromosome in species with male heterogamety (XX females, XY males) an the W chromosome in species with female heterogamety (ZW females, ZZ males). However transitions between these states occur frequently in fish (see right, Xiphophorus maculatus) amphibians (see right, Rana rugosa) reptiles and invertebrates (see right, Musca domestica). That these transitions are common is surprising given that the sex-specific chromosome should degrade over time, leading theorists to speculate that direct selective forces such as sex-specific selection may be responsible.

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Working with Carl Veller, Pavitra Muralidhar and Martin Nowak, we were able to show in fact that such transitions could be observed in a simple null model that accounted for genetic drift:

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Drift-induced selection between male and female heterogamety

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Here we proved mathematically that there is a drift-induced bias favoring dominant mutations (see Figure, left). In an extremely minimal way our model predicts that dominant sex determining mutations are more likely than recessive ones, qualitatively recapitulating the empirical observation that that sex determining cascades were built from the bottom up by a series of dominant mutations. 

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You can read more about the paper, and its context in the literature, in a review by Prof. Deborah Charlesworth:

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F1000 Prime Recommendations

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