Mother Nature’s Kama Sutra, Part 3

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.

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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?

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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.

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Mother Nature’s Kama Sutra, Part 2

KSBlogImage2Female worker bees find their food in the nectar of flowering plants. I see them flitting flower to flower sucking zealously. Most flowers have both male and female elements, leading us to think they can fertilize themselves. Mother Nature, however, blocked that mechanism in many specimens by superimposing a complicated incompatibility system to prevent the male pollen from getting to the female ovary. The only way these plants can complete the sexual act is by cross-fertilization following the matching rules of coupling between molecules with the required characteristics. The bottom line? Such plants cannot self-fertilize but can successfully fertilize most other members of the species, thus achieving outbreeding with a mix of genomes in successive generations.

Gregor Mendel, the father of inheritance—bee keeper, gardener, and monk—wanted to know how discernable traits are passed on from one generation to the next. Fortuitously, he first chose peas rather than bees for study. He cross-bred, by taking pollen from one known parent and depositing it on the tip of the tube leading to the bulbous female ovary of another. By scoring expression of certain parental traits, such as flower color and peapod shape, in the offspring of these crosses, he showed that defined characteristics of each parent were passed on from one generation to the next. Mendel tried working with bees for comparable studies. Alas the data from bees did not jibe with those of peas, due to the then unknown, unorthodox ways of sexual reproduction in bees.

Filled with the fragrance of pretty spring flowers, I turn again to that lily-padded frog pond and think of smaller creatures hidden therein—the microscopic fungi and algae. I pause to ponder the possible presence of tiny swimming spores coming from a little fresh water mold called Achlya ambisexualis, the subject of my master’s thesis many years ago. I and my mentor, John (Red) Raper fell in love while sampling small ponds in the hinterlands around Chicago to make collections of this wee critter for study back in the lab. We nurtured the spores on freshly cut hemp seeds and watched them develop into full-grown colonies of threadlike cells called hyphae. We paired the grownup colonies to look for evidence of mating.

Achlya ambisexualis is aptly named. It goes frogs, birds, fish, bees, and peas one better. It can be either male or female depending upon the partner it happens to meet. A supreme opportunist, this little mold spends no energy making fertilizing organs before they are needed. If you were one of its members, you could go either way depending upon the relative sexuality of the nearest possible partner. You could behave as male if your neighbor bears stronger female tendencies than you and vice versa.

Let’s say your male potential is stronger than that of your neighbor. She behaves as female and initiates the dalliance by sending you a signal to develop long finger-like appendages in readiness for genomic delivery. Then, and only then, do you release a message in response, telling that female to make her little nucleated egg sacks in order to receive your nuclei. She, in turn, signals your fertilizing elements hither to deliver your genes to hers within the encapsulated eggs. Voila!

My thesis aimed towards concentrating enough of a strong female’s message to determine its nature. I failed in this, but others succeeded by using better, more modern techniques. Amazingly, these later investigators defined the attracting signals of both sexes as two different steroid molecules resembling human sex hormones. The regulating genes remain unknown.

My love of the fungi persists from that time.

Moving on to the fringe of the woods, I  spot a light brown, dome-shaped mushroom hanging from a white birch tree. It must be Polyporus betulinus, a fungus studied by friend Abe Flexer, a fellow researcher in Red Raper’s lab. Abe discovered its sex life and found two sexes back in the nineteen seventies, but later investigators found more—as many as 33. How can that be?

KSBlogImage1Walking further, with mushrooms on my mind, I find a group of fan-shaped forms anchored to a log half-rotted. I kneel up close and cock my head. Each resembles a scallop shell, small as a fingernail, with irregular ridges of spore-bearing gills scoring the concave underside. It is Schizophyllum commune, my favorite mushroom-bearing fungus, with not just three dozen sexes but thousands.