The picture below may give you a hint about how the bird's nest fungus got its name. But what it doesn't show you is the rather fascinating love life that they have, and what this might tell us about where our own sexual preferences come from.
Bird's nest fungi live in places like rotting trees, dung piles, mulched woodpiles, nursery pots, and various other places; they've done quite well in human habitats, and so several species are thriving. When it first sets up shop, a fungus will grow out long filaments all through the body of whatever it's growing on, gradually digesting it with enzymes that transform wood (or whatever) into simple sugars. The fungus keeps growing until it touches a prospective mate: at this point, the two fungi will grow into each other, exchanging not just DNA but entire cell nuclei. The resulting "dikaryotic" ("two-nuclear") fungus then grows the fruiting bodies that give it its name: little cups with spores in them that look like eggs in a bird's nest.
These spores aren't firmly attached: in fact, they're designed to fly. When a raindrop hits a cup, it will propel the spores outwards (using the cup as a ramp) in all directions. The spores trail long, sticky filaments behind them, which get caught on branches; the (very lightweight) spores then wind around the branch grappling-hook style, leaving them firmly attached and ready to start their new life. The parent, meanwhile, will keep manufacturing more bird's nests for as long as it has the food and water to keep going.
There's just one catch: because the spores get distributed by rain, they don't fly very far, and that means that children of the same parents will end up close by. This means that the fungus has to have some way to avoid inbreeding. (Inbreeding causes bad mutations to build up, in the sort of way that dubious X-Files episodes parodied, and that makes the fungus less able to survive. The non-silly version of this is called "inbreeding depression," and you can get a good overview of it at http://en.wikipedia.org/wiki/Inbreeding_depression)
The fungi achieve this by being very picky about their mates. Humans come in two genders, and these are roughly our "mating compatibility groups." These fungi, on the other hand, use what's called a "tetrapolar mating system." What it means is this: instead of their being one category of gender, each fungus has two kinds of gender, with the poetic names "MAT-A" and "MAT-B." Two fungi can only mate if both their MAT-A and MAT-B genders are different. And each of these doesn't just come in two varieties – they can have dozens, or even hundreds.
(For what comes next, if you want to know the details I highly recommend this paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3607108/)
Take Cyathus stercoreus, the "dung-loving bird's nest" (don't you love fungus names?), which is one of the most widespread of the bird's nest fungi. It has 39 different possible MAT-A's, and 24 MAT-B's. This means that there are a total of 936 (39×24) different genders, and an arbitrary fungus will be able to mate with 874 (38×23) of them. The children of this mating will be one of four possible genders (getting their MAT-A's and MAT-B's independently from each parent), and each child would only be physically able to mate with one in four of its siblings – the ones which have both a different MAT-A and MAT-B. That means that there's a 25% chance of successful mating with a relative, compared to a 94% chance with a random fungus it meets in the street. (Or rather, "in a pile of dung," but that seems a little less romantic) (Unless you're a fungus)
But to maintain 936 different genders, you need a lot of fungi, and in species that don't have as many individuals around, we indeed find that the number of distinct genders goes down in time, as various MAT-A and -B variations are no longer present. Cyathus striatus, the fluted bird's nest, only has 3 MAT-A's and 11 MAT-B's – giving strangers only a 61% chance (2×10/3×11) of being able to mate, with siblings still having that 25% chance. And in fact, C. striatus has been showing increased trouble breeding.
There's one other important difference between fungi and people: these hundreds of different genders (the technical term is "mating compatibility groups") don't have any differences in their large-scale physical shape. To tell the genders apart, you need genetic testing.
This may give us a hint as to how gender started out in the first place. At the simplest end, we have asexual reproduction: creatures that divide via mitosis and leave it at that. Next, we have creatures that can penetrate each other's cell walls and exchange nuclei, like these fungi do; that gives them the advantages of cross-breeding. Compared to them, every asexual species is suffering from permanent inbreeding depression, as each creature only "mates" with itself. Then you see the development of things that quickly kill off any attempt to mate with excessively similar creatures, like this system of genders. You could easily imagine the next stage: the genetic variation between the genders starts to get used in building the physical structure of the creature. This opens up the possibility of different genders specializing in various ways, including in parts of the reproductive process – and the rest, as they say, is (pre)history.
But even we mammals haven't given up on the old systems of genders! Studies in a wide range of species have shown that everything from butterflies to rats will actively avoid mating with anything that smells too much like them. Scents come from a variety of sources, but significantly, many of these scent components are inherited. What we have is a collection of genetic variants that make people who are too closely related to us not smell like prospective mates. This doesn't physically prevent mating, but as you'll have noticed above, even the fungi's rather elaborate system only reduces the inbreeding rate to 25%; an imperfect system is a lot better than no system at all.
So the next time you smell your relatives, think about the mating habits of fungi, and how your pattern of scents may well be the evolutionary remnant of a system of thousands of different genders that let our earliest ancestors know their kin.
Many thanks to for the original article (shared below) which sparked my curiosity with its talk of "mating compatibility groups." Who would have known that fungi could do that? Well, apart from mycologists, I guess.
Paris will begin removing the “cadenas d’amour.” Almost a million padlocks, weighing up to a staggering 45 tons, will be taken away. Last year, a part of the railing at the Pont des Arts collapsed under the weight of the locks
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