My favorite mushroom-bearing fungus, Schizophyllum commune, has not just a few, but thousands of sexes. This little wood-rotter caught my attention back in the fifties when my mentor (later, my husband) Red Raper began to study it at the University of Chicago. After a few year’s time-out from science, raising our new born offspring, I went back to the lab and collaborated with Red on this research and have not lost interest since.
Paired collections of specimens from all over the world revealed thousands of different compatible mating-types. We and associates located two genetic loci involved, each of which were later shown to contain multiple genes.
Let’s say you’re a member of this little critter whose job it is to rot fallen logs in the forest and recycle carbon. Then you would be one of over 20,000 different sexes. If you traveled all over the world and mingled with others of the species, you would be able to mate and reproduce with up to 98% of your kinfolk.
Unlike a human, you could not perceive possible mates just by looking at them. To find attraction, you would have to get up close and sense their pheromones to see if any of them fit your receptor. Compatibility would lead to merger. But that’s not all. After union, you must then couple your special molecules (called homeodomain proteins) in just the right way with another set of different but similar molecules from your chosen mate. Those bound proteins then turn on a cascade of genes leading to sexual reproduction within both partners. In this system not only you make mushrooms bearing the babies but so does your mate—fertilization is a two-way street.
How can this be? Well, nature devised a way to produce a large variety of pheromones, pheromone receptors, and different types of special homeodomain proteins to fit together as locks and keys opening or closing the gates to sexual procreation. Everyone carries them and most of these molecules couple properly with those of other individuals, but not in self. The encoding genes evolved in such a way as to keep the gates closed within each individual , thus preventing inbreeding, but to open them in others, thus promoting promiscuous outbreeding.
Now that you’ve imagined yourself with such procreative abilities, what do you think?
Might it be fun or just utterly unmanageable? Schizophyllum seems to manage admirably—its offspring travel by jet stream all over the planet and do their life-sustaining jobs wherever wood is found. Many other mushroom-bearing fungi function similarly.
Too fanciful for you? Ah well, let’s make it simpler like sex in the edible button-mushroom, Agaricus bisporus, found in the supermarket as well as in the woods and meadows. Now you can do it all on your own as an hermaphrodite but without any need for sex organs. You can make your own gametes of two different types and fertilize yourself. How less complicated can it be? But sexually reproducing your own kind over and over in the absence of a partner might be just a bit too boring. It borders on reproduction without any sex at all as in some other members of the fungal kingdom. Recent discoveries, however, have revealed worn out remnants of sex genes in some so called asexuals. They must have been sexy sometime in the past. Now without those formally functional genes, these fungi reproduce by cloning.
Nevertheless, sex is generally compelling and ubiquitous.
Other strategies exist in fungi such as that in Cyphellopsis anomala where one can have it both ways—mate with a suitable partner should one be available or, if not, have sex all by yourself.
In more prosaic fashion, you could function as does the pink bread mold Neurospora crassa with just two sexes, and one-way fertilization—rather like we humans but without all the same accouterments. This fungus grows not only on bread, but in nature—preferably in burned-out woods.
The fungi seem a major testing ground for all ways possible to reproduce sexually.
Alas, we humans have only two distinct sexes capable of coupling for procreative purposes. Sure, we know many ways of enjoying that process, but reproductively we’re somewhat limited compared to many other living creatures.
How does this all sort out?
Way back at the beginning of life on earth, some 3.5 billion years ago, those teeny tiny critters, the bacteria and viruses, cloned themselves. They reproduced by making more of the same with an occasional merger between two individuals in exchange of some particular ingredients, but that was a rather random process. Sure, mutations happened because of spontaneous alterations in the blue-printing molecules, but most of those changes did not survive. The rare good ones did, but that kind of evolution took a very long time. After about 2.3 billion years of trials and errors, Mother Nature worked out the complicated process of sexual reproduction as a better, more controlled way for living creatures to evolve in harmony with ever changing environments. Needing two times the energy of fission by cloning, sexual procreation comes at a cost, but its ubiquity indicates worth.
Sexual attractants, the molecules of passion and pleasure, provide motivation for the fertilizing act. Little is known of their nature, yet they play a strong role in the kind of procreation that provides a vast variety of offspring capable of fitting into Nature’s many niches.