Weird sex: the underappreciated diversity of sexual reproduction

Abstract

The idea for this project started over lunch in an Italian restaurant in Copenhagen in 2010. We were not planning to talk about sex at that time, but to discuss the evolution of uniparental inheritance. But as they say, one thing leads to another, and we started to talk about sex and its underappreciated weirdness. And the need for a project like this. Along the way, we were continuously reminded of the importance of such a project: the most recent example was a comment in an undergraduate 'learning diary', a teaching tool in which students are asked to explain hard to understand concepts, or concepts that are surprising to them. The student expressed great surprise to hear that vertebrate sex determination does not always follow the 'normal' XY pattern; in birds, sex is instead determined by ZW chromosomes and females are the heterogametic sex. Well, we thought, if ZW sex determination seems 'weird', then you ain't seen nothing yet!The student of course was not stupid, merely rather uninformed-or better put: biased in the same way we all tend to begin our careers as biologists. Our academic forays in sex and sexual selection start by learning that there are primary and secondary sexual characteristics, that these differences are caused by sex chromosomes, packaged in different kinds of gametes that need to come together by one type of gamete (sperm) finding the other (egg) via mostly internal fertilization. Only later might we realize that sex determination mechanisms are so diverse they deserve an entire book [1]. If we then retain our interest, at some point we learn, to our surprise, that monogamy is a really unusual reproductive system, and that sex itself isn't a self-evident feature of all reproduction. Even more so, sexual reproduction is a contradictio in terminis, since sex, in its very essence, involves fusion of gametes, thus reducing numbers instead of increasing numbers. The very existence and persistence of sexual reproduction remains an evolutionary puzzle.The list goes on. Selfing is common in plants, while more than a few animals also appear to have no objection to self-fertilization. Similarly, most evolutionary biologists are surprised when they learn that hermaphroditism in animals is actually rather widespread: close to 30% of all species are hermaphrodites [2]. Most plants do not have separate sexes, while the notion of 'maleness' and 'femaleness' becomes something rather different when one includes fungi and unicellular life. Slowly then, as we learn more, our minds open up.One could argue that whatever field one chooses, biology is so diverse that it is impossible to expose students to the entire bewildering array of biological phenomena; let us then at least try to understand the best-studied model systems and hope that they are in some sense representative examples of what goes on in nature. Fair point. But when it comes to sex at least, it is easy to become intrigued by its wonderful diversity. Why, then, do we still so narrow-mindedly stick to the familiar?In psychology, WEIRD is an a...

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