Paper Trails by Jay H

Week #2: Sex determination

14 Jan 2018

Hi everyone! This week’s paper is about sex determination, or how different species go about determining whether an individual ends up male, female, or otherwise. (The “otherwise” category is more common than you might think.) Because this is a scientific essay and not a research article, it’s actually pretty readable, so I recommend checking it out if you have the time. But if you take nothing else away from this week’s email, make sure you read about the Wolbachia parasite.

Paper: Sex Determination: Why So Many Ways of Doing It?, by Doris Bachtrog et al. (PLoS biology, 2014) [pdf].

Why do we have sex(es)?

Before talking about how sex is determined, we should ask ourselves: Why do we even have different sexes?

(For the sake of clarity, when I say the word “sex” you can assume I’m referring to male, female, etc. If I ever mean the sexy kind of sex, I’ll call that ~sex~.)

From an evolutionary perspective, the sort of catch-all answer to why organisms have ~sex~ with other individuals is that it promotes genetic diversity; by swapping genetic material, mutating, etc., our offspring have a chance to get the best of both worlds, so to speak.

But having ~sex~ doesn’t necessarily imply that we should have different sexes, does it? Hermaphrodites, which have both male and female sex organs, can reproduce alone or with others. (94% of plants are hermaphrodites, by the way, while only 5% of animals are.) So why should we have multiple sexes?

Obviously, reproducing with yourself is a lot easier. But separate sexes have evolved independently in so many different species, so we can assume there must be a cost to self-breeding. One hypothesis about the function of having multiple sexes, then, is that it prevents an organism from having sex with itself (or reproducing with itself, at least). Let’s say you’re a little multicellular organism, and you’re ready to make little multicellular offspring. If your genitals are incompatible with themselves, then you can’t just have ~sex~ with yourself, so you’ll have to seek out another multicellular organism with genitals compatible with yours in order to reproduce. So in order to ensure that no organism can reproduce with itself, you need two different sexes.

And in case you’re wondering how scientists can look at a multicellular organism like fungus or yeast and say “Oh yeah, this one’s male,” it turns out this all comes down to whether the gametes are different sizes. For example, sperm are smaller than eggs, so the sperm-makers are called males and the egg-droppers are called females. In some species, there are clearly different sexes, but the gametes are the same size. In that case, they are just called “+” and “-“. (Not very creative names.)

Sex determination myths

So we now know you need multiple sexes in order to ensure organisms can’t reproduce with themselves. When you think about life on Earth, you probably assume that most species have males and females, and that this has something to do with the X and Y chromosomes. The main point of this paper, however, is that many of the things we think are true about sex determination are not even that common once you start considering species other than placental mammals and fruit flies. Because of this, our understanding of the evolution of sex determination is quite limited.

In this paper, the authors focus on dispelling three key “myths” of sex determination:

  1. Sex is typically determined by X and Y chromosomes
  2. Sex is controlled by one master-switch gene
  3. Sex chromosome differentiation and degeneration is inevitable

(I didn’t know about those last two points, but I guess now I know they’re myths.)

In all placental mammals, only one chromosome pair is sex-determining. In females, the chromosome pair is called XX, while in males it’s called XY. In fact, development as a male comes down to only a tiny portion of the Y chromosome, a “master-switch” gene called Sry.

However, while X and Y chromosomes and master-switch genes may be common in many mammals, they are not at all the norm in other species. In fact, sex-determination has evolved independently in so many different species that it’s hard to call anything “normal” at all. For example, in birds and some insects, fish, and reptiles, it’s the male who has a matching chromosome pair (called ZZ), while females are (ZW). And in many species, sex-determination isn’t set off by genetics at all, but by the environment. I’ll give you a few of the stranger examples.

A non-exhaustive list of bizarre sex-determination mechanisms

About that parasite that transforms male roly polies into females: It’s called Wolbachia, and it is probably the craziest parasite I’ve ever heard of. In other arthropods, it just straight up kills the males, or it causes virgin females to have babies.

There are so many more interesting things mentioned in this paper that I don’t have the space to get into. Overall, this paper does a really great job of making the case that, if we only study sex determination in organisms such as mammals and fruit flies, we’re probably going to miss some things. In many species, the mechanisms that determine sex evolve rapidly, and finding good model organisms for these cases is going to be key for finding out more about the evolutionary fitness of being male, female, or something else entirely.

Summary

  • Subject: Even though many species have two different sexes of individuals, the mechanisms that determine sex are varied and evolve rapidly
  • Key observation: Our understanding of sex determination based on a few model organisms masks the diversity apparent across species as a whole
  • Conclusion: Using genomics and different model organisms will enable a better understanding of how and why different sexes evolve

If you ever want to read an academic paper for free, try entering the url at sci-hub.tw.

Coming up next week: x-rays and crystals

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