DISCUSSION OF HOW FUNGI REPRODUCE
Fungal reproduction can be sexual, asexual or both. While sexuality in humans and other mammals is easy to understand we must examine the subject in a little more detail to understand how the term can apply to fungi. As in humans, sexuality in fungi involves combining genetic information from two parents in one or more offspring. Although the offspring may resemble one or both parents they are each unique in that they possess some traits from both. Read the section below for a more detailed discussion of the concepts of sexuality and life histories.
Asexual reproduction is a simple cloning process, passing on the traits of a single parent to the offspring. All offspring are identical to each other and to the parent.
The Biology of Sexuality
First of all we must deal with mitosis and meiosis. These two terms describe how the nucleus of a cell divides and distributes its DNA. The DNA (genetic material) contained in the nucleus of a cell is divided up into a number of ribbon-like units called chromosomes. The chromosome number of most organisms is constant. During mitosis the chromosomes of a nucleus are all duplicated so that the two nuclei resulting from its division will be genetically identical. Mitosis can occur in a diploid nucleus, that is, one containing pairs of chromosomes representing each of its parents or in a haploid one containing a single set of chromosomes.
Meiosis, often called "reduction division", yields four daughter nuclei with the number of chromosomes reduced to half the original number. The overall result is that one diploid nucleus becomes four haploid nuclei. Although each daughter nucleus contains a full set of chromosomes, events during meiosis ensure that each of these nuclei contains genetic material derived from both parents and that each daughter nucleus is genetically different from the others.
The are several good illustrated discussions of meiosis and mitosis on the Internet. Use your browser to locate one of these if you wish to examine this subject in detail.
Sexual reproduction always involves meiosis at some point in the life cycle of an organism. When and where meiosis occurs is crucial to an understanding of the life histories of these organisms. Three types of meiosis are generally recognized. These are 1) gametangial meiosis, 2) zygotic meiosis, and 3) sporic meiosis. Each type characterizes a particular kind of life history.
Organisms having zygotic meiosis are fundamentally haploid. Growth and mitotic cell divisions always involve haploid cells. These organisms produce gametes (sex cells equivalent to sperm and egg) via mitosis, since they themselves are already haploid. Gametes may be produced in a great variety of structures, but ultimately the gametes must come together, fuse and form a diploid zygote. The zygote never divides mitotically; instead it undergoes meiosis and the resulting cells are again haploid, thereby completing a full life cycle. The following are characteristic of zygotic meiosis:
Organisms having gametangial meiosis are fundamentally diploid. Growth and mitotic cell divisions always involve diploid cells. These organisms produce gametes via meiosis, since, unlike their gametes, they themselves are diploid. Gametes are produced in structures called gametangia. Gametangia contain diploid cells (at least initially) but some or all of these undergo meiosis to form the gametes. The gametes ultimately fuse to form a zygote. The zygote, which is already diploid like its parents, never undergoes meiosis, but instead develops by mitotic divisions, thereby completing the full life cycle. Humans have this type of life cycle. The following are characteristic of gametangial meiosis:
Sporic meiosis is more complicated. As in life histories with zygotic meiosis the organisms are initially haploid. Growth and mitotic cell divisions all involve haploid cells. These organisms produce gametes (sex cells) via mitosis, since they themselves are already haploid. Gametes may be produced in a great variety of structures, but ultimately the gametes must come together, fuse and form a diploid zygote. Here the plot thickens: instead of undergoing meiosis the zygote divides mitotically to form a diploid individual. This diploid individual may live attached to its haploid parent or it may live completely on its own. Ultimately certain cells of the diploid individual undergo meiosis and give rise to haploid spores. These haploid spores develop into a new haploid generation, thus completing the life cycle. This kind of life cycle is often referred to as "Alternation of Generations". The following are characteristic of sporic meiosis:
Haplobiontic organisms have either gametangial or zygotic meiosis. They never have sporic meiosis. These organisms are always either diploid or haploid, never alternating.
Diplobiontic organisms have sporic meiosis. They always have alternation of generations.
DIKARYOTIC LIFE HISTORIES
Members of the subphylum Dikarya of the kingdom Fungi have an odd variation on these patterns. Here the organisms begin as haploid individuals. At some stage of their development they undergo cell fusion and pairing of gametes, but these gametes do not fuse immediately, and instead undergo synchronized mitotic divisions (conjugate division) for extended periods. These binuceate (dikaryotic) cells may develop into extensive thalli. Ultimately, in certain specialized cells, the gametes fuse to form a zygote. The zygote undergoes immediate meiosis without intervening mitotic stages. The products of meiosis reestablish the haploid generation. Thus the Dikarya have a sort of alternation of generations but with zygotic meiosis. No other group of organisms does this. The following are characteristic of dikaryotic life cycles:
Mating Systems in Fungi
Many fungi are termed homothallic, meaning that they are self-fertile and do not need to find a compatible mate to accomplish sexual reproduction. This is much the same situation as in many plants where cross-pollination is not required. You might wonder why sexuality is needed at all if no partner is involved. The reason for partner-less sexuality may be that although it involves no genetic exchange it does lead to the production of spores that may have a specific function in the life cycle of the fungus. To return to the self-pollinated plant, if it did not have the structural features of the sexual process it might not produce seeds. Without seeds the plant would have to depend upon vegetative reproduction alone and would lack a highly specialized means of dispersal. We should also remember that homothallic fungi (and self-pollinated plants) are fully able to mate if a compatible partner is available, they are just able to carry on if they find themselves alone.
In heterothallic Zygomycota and Ascomycota there are two mating types. We don't usually call these types male and female as we do in animals and plants, but only as plus and minus (or A and B), since the two mating types are identical in appearance. Genetically these mating types possess one or the other of two mating sequences at a single genetic locus. These loci may occur as alleles (alternate but similar forms of a gene or group of genes) or may be different enough that one consists of a single gene while the other may contain two or three. During mating these sequences each control such events as the release of pheramones, hyphal fusions, nuclear behavior and other phenomena. Self-mating is impossible because the full set of genes is not present and many of the events simply can't take place. Because of the great dissimilarity between the mating sequences in the Ascomycota, many fungal geneticists prefer the term "idiomorph" to "allele" when discussing mating systems in this phylum. The two ascomycete idiomorphs, designated MAT1-1 and MAT1-2, have been extensively studied by geneticists.
The situation in the Basidiomycota is more complicated because the mating systems involve more than just two mating alleles. In fact there are often more than a hundred. Here any given mating type will be unable to mate with itself but is compatible with any other mating type. The result of this is that the hyphae arising from spores of one individual may have some self-compatibility (either 25% or 50%) but as high as 100% compatibility with hyphae from another individual. Compare this with the two-allele system where any potential mating is held at a 50% level of success. Of course campatibility may be lower withing a limited area such as a small patch of forest because certain alleles may be shared by adjacent populations. Spores blowing in from further away may have a greater chance of finding a mate than one of more local origin. A system of multiple mating alleles is a very sophisticated and stable way to balance the absolute need for reproduction against the desirability of outbreeding.
Uni- and bifactorial mating systems
In a unifactorial mating system there is a single mating locus containing one of many potential mating alleles, often labelled A1, A2, A3, A4, etc. A basidiomycete with a unifactorial mating system will produce basidiospores bearing either of two of these alleles at its mating locus. Which two of the many potential alleles will depend upon those it received from its parents. When the basidiospore germinates it will not be able to form a successful mating with the hyphae from any spore carrying the same mating allele. This means that within one fruiting body a spore will be compatible with only half of the other spores, since half carry one of the alleles and the other half carry the other allele. A fruiting body from another part of the forest may carry a different pair of alleles and thus spores of the first fruting body will have 100% compatibility with those of the second. Although the number of mating alleles may be large, as pointed out already these alleles may be shared among adjacent populations, leading to mating success rates between 50% and 100%.
Bifactorial mating systems add yet another level of sophistication. Here we still have a multi-allelic system but with mating alleles at two loci (usually called Locus A and Locus B) instead of one. The same situation exists as in the bifactorial system except that the alleles must be compatible at both Locus A and Locus B. Thus a mating with alleles A1A2B1B1 will not be successful because the B alleles are identical and therefore not compatible. A mating with alleles A1A21B1B5, on the other hand, would be successful because both the A and B loci have compatible alleles, that is, anything but self. Fruiting bodies with bifactorial mating systems (most mushrooms for example) will produce spores with a 25% chance of mating with others from the same fruiting body because ¾ of the other spores will be incompatible with it at one or both of the mating loci.
It must be pointed out that uni- and multifactorial mating systems are considerabley more complex than presented here. For example, the A and B loci are each known to be themselves linked loci with complex interactions. If you need further information, there is a rich literature on the subject ready for your attention.