Are Mushrooms Genetic Individuals or Genetic Mosaics?
Although most of us may not give it much thought as we stoop to liberate yet another Boletus edulis from its terrestrial confines (assuming, that is, you were at the Mendocino Woodlands Foray last month!), the mushroom we are picking represents the union of two mating type compatible strains of the species—a fungal “Mom and Dad,” in essence. In the life cycle of a typical mushroom, these strains are present in the soil or other substrates in the form of long, filamentous cells termed hyphae (and mycelium collectively), which arise from germinating spores produced by mushrooms of previous generation(s). It is the mycelium that represent the business end of the fungus, absorbing nutrients and water from the substrate, forming associations with plants, etc. The mushrooms that we so diligently hunt for are simply the reproductive structures of these organisms. It is within specialized sexual cells of the mushroom, called basidia, that mushroom sex finally takes place and from which spores are produced. Like other organisms that reproduce sexually, the sterile (i.e., somatic) cells in mushrooms and the mycelium that give rise to them, are genetically identical, each containing a contribution from both the parent strains. Or are they? Recent studies indicate that this condition may not be true for all mushrooms, and that divergence from this common pattern may have profound effects regarding the ecology and longevity of these fungi.
Within the typical mushroom life cycle, spores and the primary mycelium they give rise to, are haploid in their genetic makeup, meaning that the nucleus present in each cell contains only a single copy of each chromosome (think sperm and eggs here, folks). Depending on their genetic compatibility, when hyphae of the same species encounter one another in the environment, they will fuse and give rise to a new secondary mycelium. The hyphal cells of this secondary mycelium each contain two un-fused, haploid nuclei—one from each strain (again, think eggs and sperm). This condition of two haploid nuclei per cell is termed “dikaryotic.” Mushrooms and other higher fungi are unique in that this dikaryotic phase is believed to persist for an extended portion of the life cycle. In most other organisms, compatible haploid nuclei (usually in the form of gametes) fuse soon after they encounter each other.
When environmental conditions are suitable, the secondary mycelium will form primordia that soon develop into mushrooms. As mentioned above, it is within the basidia that reproduction finally occurs. In these sexual cells, fusion of the two haploid nuclei occurs, creating a diploid nucleus that has a full complement of chromosomes (one copy from each strain). Shortly after this fusion, meiosis occurs, returning the resulting four daughter nuclei to the haploid condition. These resulting nuclei eventually migrate into the developing spores, which are soon dispersed to start the life cycle over again. During meiosis, recombination of DNA may occur between sister chromosomes, shuffling together new combinations of genes and resulting in novel genetic variation differing from that found in the nuclei of the parent strains. It is this resulting variation that is the evolutionary advantage of sexual reproduction, for it is these new combinations of genes that natural selection may act upon. Although the majority of genetic variation resulting from sexual reproduction undoubtedly has little or no effect, it will on occasion be beneficial in some form; this thereby confers a selective advantage upon the mutated population. As you can imagine, this novel variation is paramount in helping a species adapt over time to the challenges of an ever-changing environment.
Interestingly, the life cycle of Armillaria species are quite different from the typical cycle described above. In Armillaria the haploid nuclei present in newly produced secondary mycelium quickly fuse, forming diploid nuclei. Unlike other mushroom species, in which the individual cells are typically thought to be dikaryotic (i.e. contain two genetically distinct haploid nuclei) throughout most stages of the life cycle, the somatic cells of Armillaria appear to each contain a single diploid nucleus. The mycelium of Armillaria, and presumably the mushrooms it produces, therefore lack the extended dikaryotic stage believed to be characteristic of most higher fungi. This, however, is by no means the end of the story. Recent work done by several teams of investigators (see sources below) indicates that in at least two species of Armillaria, A. gallica and A. tabescens, the individual nuclei within the cells of both the mycelia and non-basidia portions of the mushrooms are in the haploid condition, rather than diploid as they have long been assumed to be.
Using various methods of nuclear staining and fluorescence microspectrophotometric measurements, investigators were able to determine the quantity of DNA within the nuclei of cells constituting the various stages of the life cycle. As would be expected, haploid cells had on average half of the DNA present in diploid cells, or cells caught in the act of replicating their DNA in preparation for mitotic or meiotic division. Knowing that mushrooms are developed from secondary mycelium (i.e. two compatible strains that have fused), the implications of the above findings are that prior to mushroom formation, there must have been an event similar to what occurs in the basidia during a typical mushroom life cycle. The two haploid nuclei initially present in the secondary mycelium must fuse to form diploid nuclei (diploidization), and then go through meiosis to produce haploid daughter nuclei (haploidization). In essence, what these investigators propose is the occurrence of one or more extra-basidial diploidization-haploidization events prior to mushroom formation. This haploid, secondary mycelium eventually produces mushrooms, the somatic cells of which also contain a single, haploid nucleus.
Considering what we know about meiosis and recombination, it is apparent that the extra round(s) of diploidizationhaploidization have the potential to generate additional genetic variation beyond what we see in the typical mushroom life cycle. In fact, as researchers have demonstrated, the genetic variation generated by these extra events can become incorporated into a single mushroom, such that the individual cells making up the tissues of this structure contain nuclei that harbor different combinations of genes. Unlike the typical mushroom life cycle in which the cells of the secondary mycelium and mushrooms are genetically identical, the cells of these tissues in A. gallica and A. tabescens likely represent a mosaic of genetically distinct nuclei. As this genetic mosaic of nuclei are incorporated into basidia (where they experience another diploidization haploidization event), there is the potential to generate even more genetic variation.
As discussed above, the generation of genetic variation is beneficial to a species as a whole, as it may provide the means over generations to adapt to a changing environment. However, in the case of these Armillaria species, the production of genetic variation may even be beneficial at the individual level. As most of us are aware, Armillaria species are believed to represent some of the largest and longest-living organisms on Earth, the so-called “humongous fungus” of Michigan and more recently Oregon. In these fungi, individuals are estimated to span an area of 15 to 900 hectares, and range in age up to 8,500 years, persisting in the environment in the form of mycelium and rhizomorphs (i.e. thick mycelial cords). Is it possible that the longevity and capacity for growth these fungi display could be linked to the genetic mosaicism present in their cells? To address this question, researchers have tested the ability of genetically distinct cell lines, isolated from a single individual, to grow under diverse environmental conditions, including water availability, temperature, and substrate pH. Not surprisingly, these genetically distinct cell lines display variation in their growth ability under variable conditions.
Such results support the idea that genetic mosaicism may even be beneficial at the level of the individual, and may indeed play a role in the longevity and growth rates these fungi display. It is easy to imagine the magnitude of environmental challenges and stresses that a fungus would have to overcome during a life that may span hundreds of years. The ability of various regions of these large somatic structures to differentially react to a variable environment, due to differences in their genetic makeup, would undoubtedly be selectively advantageous at some level. The question remains, however, how widespread is genetic mosaicism within the fungi? So far the presence of an extra-basidial diploidization-haploidization event has only been documented in two species of Armillaria. Additionally, what I have presented here is a very simplified explanation of a very complex process. As you can imagine, there is much about genetic mosaics that is yet to be understood, and even those who research this process do not agree on all the mechanisms by which it occurs. Genetic mosaicism is undoubtedly a very important process that has played a large role in the evolution of at least some species of fungi, and shows great promise for exciting research in the years to come.
Sources and additional information:
- Grillo, R., Korhonen, K., Hantula, J. and Hietala, A.M. 2000. Genetic evidence for somatic haploidization in developing fruit bodies of Armillaria tabescens. Fungal Genetics and Biology 30: 135-145.
- Peabody, D.C and Peabody, R.P. 1985. Widespread haploidy in monokaryotic cells of mature basidiocarps of Armillaria bulbosa, a member of the Armillaria complex. Experimental Mycology 9: 212-220.
- Peabody, D.C., Peabody, R.B., Tyrrell, M.G. and Towle, M.J. 2003. Phenotypic plasticity and evolutionary potential in somatic cells of Armillaria gallica. Mycological Research 107: 408-412.
- Peabody, R.B. and Peabody, D.C. 1987. Haploid monokaryotic basidiocarp tissues in species of Armillaria. Canadian Journal of Botany 65: 69-71.
- Peabody, R.B., Peabody, D.C. and Sicard, K.M. 2000. A genetic mosaic in the fruiting stage of Armillaria gallica. Fungal Genetics and Biology 29: 72-80.
- Peabody, R.B., Peabody, D.C., Tyrrell, M.G., Edenburn-MacQueen, E., Howdy, R.P. and Semelrath, K.M. 2005. Haploid vegetative mycelia of Armillaria gallica show among-cell-line variation for growth and phenotypic plasticity. Mycologia 97: 777-787.