If you've been following the series, then you should know about some pretty weird forms of sexual reproduction. I want to take a step back from the specific mechanisms and talk about one of the key components of sexual reproduction: mating types. What are mating types? That’s a good question. The simplest answer I can give is that they are mechanisms that work at a genetic or chromosome level to restrict mating in some way. That may seem like a pretty vague answer, but that’s because the answer is far from simple. I’m not going to try to cover everything in this post, but by the end of it you should have a good idea of mating types, how they are regulated, and just how much variety is out there.
A note about terminology.
Some of the terms I use have different meanings for different people. To avoid confusion, these are the definitions that I am working with.
Sex is both a noun and a verb. As a verb it refers to the act of mating, or sexual reproduction. As a noun it refers to an organism’s status as either male or female. More specifically it refers to the type of reproductive cells (gametes) that an organism produces.
Sexuality and gender, though related to biological sex, are specific to humans, and are outside of my expertise.
The most familiar system for mating types (and indeed the one used by humans) involves two sexes (see note) that are controlled by sex chromosomes (we call them X and Y). An individual’s sex determines what type of gametes (reproductive cells) that it produces. Males produce smaller, more mobile gametes (sperm), whereas females produce larger gametes that contain the organelles and other structures needed for cellular function (eggs). Eggs always contain an X chromosome, while sperm can contain either an X or a Y. Upon fertilization, if the sperm carries another X chromosome, the resulting embryo will develop into a female. If it has a Y chromosome, the embryo will become male. Thus it’s the sperm that determines the sex of the offspring. This is called chromosomal regulation of sexes.
But that’s just the beginning. There are some organisms where sex is determined either partially or entirely by environmental factors. For example, in many reptiles (especially turtles and crocodilians) the ambient temperature during a specific development period of egg development (and not chromosomes) determines whether the animal will become male or female. In some species of flounder, high or low temperatures can cause individuals that are genetically female to turn into males (Luckenbach et al. 2003). For some amphipods, the photoperiod (day length) during development will determine the sex of the offspring. Echiuran worms will develop as females, unless swallowed by an adult female.
If you think that’s complicated, let’s add in hermaphrodites (that’s where individuals produce both eggs and sperm). Such organisms are broken up by whether they produce sperm and eggs at the same time (simultaneous hermaphrodites) or at different points throughout their lives (sequential hermaphrodites). Most species of slugs and snails (both terrestrial and aquatic) are simultaneous hermaphrodites, meaning that they have both male and female reproductive organs at the same time. When they mate, it’s usually reciprocal and both organisms will go on to lay eggs. Most have some mechanism that prevents an individual from fertilizing its own eggs (a process called selfing) though some (notably Banana slugs, Ariolimax spp.) will do just that when a mate cannot be found. The clown fish of the genus Amphiprion are sequential hermaphrodites that start out as males, and may develop into females. A colony will consist of one female, one mature male, and several immature males. If the female dies, then the mature male becomes a female and one of the immature males matures. The slipper limpet, Crepidula fornicata (yes the name was intentional) is another sequential hermaphrodite that is, for a time, simultaneous. This gastropod forms mating chains of up to a dozen individuals (see picture), where the large animals at the bottom of the chain are females, the small ones at the top are males, and the ones in-between are.... well, in-between. The larger and older animals eventually die and new ones are added; the sequence continues. Plants and algae also show a variety of forms of hermaphrodism, but I won’t get into those in this post. If you want to know more about them, I encourage you to read about “alternation of generations” (or come back for a future post).
Okay, having male and female gametes (anisogamy) clearly makes things complicated. What about organisms that display isogamy (gametes that are identical)? Surely they must be less complicated. Not in the least. It turns out that when gametes differ in size and ability to move, then evolution greatly favors only two mating types. When gametes are identical, you can end up with a lot more mating types. In many species of fungus, mating types are controlled by individual genes, rather than entire chromosomes. A species may have multiple genes that control mating type and multiple alleles (specific forms of a gene) for each one. Theoretically there is no limit to the number of genes and alleles that determine mating type. So far the winner is the mushroom Schizophyllum commune with over 28,000 known mating types (Kothe 1996).
After learning about all of the strange and complex systems of mating types, I found myself wondering “If sexual reproduction is so important to the evolution of an organism, why did so many mechanisms evolve to limit it? Then I’m reminded of my high-school biology teacher’s tongue-in-cheek answer: “It evolved because it worked.” That’s certainly true here. Schizophyllum commune is possibly the most widespread fungus on earth. Nearly every sexually reproducing organism on Earth is the product of reproduction that was restricted by mating types. The better question (and the one my biology teacher was trying to draw out) is what forces caused the evolution of mating types? That’s a good question. If you know the answer, please submit it for publication.
In all seriousness, that is a question that has baffled scientists for a long time. There are currently a number of competing theories out there, but that is a subject for another post.
Some other posts from "Sex is Weird"
- Beukeboom, L. and N. Perrin. 2014. The Evolution of Sex Determination. Oxford University Press, Oxford, UK.
- Kothe, E. 1996. Tetrapolar fungal mating types: Sexes by the thousands. FEMS Microbiology Reviews 18:65-87.
- Luckenbach, J. A., J. Godwin, H. V. Daniels, and R. J. Borski. 2003. Gonadal differentiation and effects of temperature on sex determination in southern flounder (Paralichthys lethostigma). Aquaculture 216:315-327.