Followers of this blog will recall that one of its recent posts discussed reproduction. I have chosen to pursue the phenomenon further by describing the life cycle of the Bear Lentinus, a Bascidiomyete fungus, shown below.
When human reproduction is concerned I believe that we tend think in terms of bodies making more bodies. Male and female contribute sperm and egg which fuse to form a zygote. Development then is the process whereby the zygote forms an embryo which undergoes a series of structural changes collectively called gestation. It is gestation which culminates in parturition. So two bodies produce a third. Now, some terminology. We say that gametes, sperm and egg, are haploid because they contain a single set of chromosomes (in humans this number is 23). As a result of fertilization, the cells which make up our bodies are diploid because they are derived from that very first cell which is the result of the union of egg and sperm. Bodies are diploid because each component cell carries two sets of chromosomes; one ultimately from each parent. Diploid bodies produce haploid gametes which participate in reproduction to produce diploid bodies once again. And, of course, the diploid body is what we think of when we think of people, or jellyfish, or cats, or starfish, or elephants, or oak trees. The diploid stage is what we know while the haploid stages, the sperm and egg, are hidden from view, they are cryptic. But what do you think of when you think of a fungus? My guess is that something shaped like a mushroom comes to mind. An edible, morsel you see wrapped in plastic at the grocery. Or perhaps you think of toadstools, puffballs, bracket fungi, or smuts. And, of course, these would all be correct. But these are all highly visible structures called fruiting bodies which are haploid and only one of many stages of a typical fungus life cycle.
For most of its life a basidiomycete fungus assumes a cryptic form called the hypha which is haploid (or possibly diploid, see below), mostly subterranean, and filamentous. A number of hyphae collectively form a mycelium. [This stage of the life cycle can be extensive indeed. The most humongous fungus is known to be one of the largest organisms on Earth; its hyphae are thought to occupy an area equivalent to 1000 football fields and the individual may be nearly 8,000 years old.] Among hyphae there are no males and females but instead monokaryotic hyphae comprise different mating types (+ and – if you like). These fuse in a process called plasmogamy which results in dikaryotic hyphae (containing nuclei of the two mating types but which have not yet fused). Dikaryotic hyphae coalesce as a mycelium which forms the familiar fruiting body which we know as a mushroom. Under appropriate conditions the nuclei within specific tissues of the fruiting body fuse in a process called karyogamy to produce diploid tissue for the first time. Meiosis, the very same process which produces eggs and sperm in humans, follows to form basidiospores which are liberated and then germinate to form monokaryotic hyphae once more, thereby completing the life cycle. As in all sexual life cycles variation among monokaryotic hyphae results because of the union of mating strains and the exchange of genetic information … the traditional (Darwinian) hedge against environmental change.
Boy, this seems unnecessarily complicated. Why go to such trouble? Perhaps some of the difficulty we may have in coming to terms with this life cycle stems from a comparison with our own pattern of reproduction. Animal bodies are large, visible, diploid and long lived while animal gametes are small, cryptic, haploid and short lived. In contrast, fungal hyphae are small, cryptic, haploid and long lived … while fruiting bodies (mushrooms) are large, visible, haploid (and then very briefly diploid), and short lived. Perhaps, but what’s my point? My point is if you distill the essence of life … what it is to be living, alive … it is reproduction; and what’s more, there is no one best way of doing it. Reproduction involves passing chromosomes, genes, DNA, into the bodies of those that follow. And they pass DNA into more bodies and so on. And it isn’t necessarily the chromosomes themselves that pass into future generations … but rather the messages encoded by them. But maybe this final thought is a topic for another day?