Fungi (a group that includes molds, mushrooms, yeasts, and perhaps some other organisms that don’t neatly fit into any particular category) have evolved various reproductive strategies. The majority of fungi produce reproductive cells called spores. Spores are specialized for dispersal and often possess some sort of dormancy to delay germination until conditions are favorable for growth.
Reproduction in Fungi: The Basics
Like many eukaryotes, (organisms that have nuclei in their cells) some fungi reproduce sexually when two compatible mating types find each other, fuse together, and undergo a cycle of meiosis to produce new genetic combinations. Other species (especially those we call molds) can produce spores asexually without the need for a partner. Many fungal species can undergo both sexual and asexual reproduction.
Then there are the renegades that have dispensed with any kind of spore production whatsoever. A few species (perhaps 300-400) when isolated and cultured in the laboratory have never been observed to produce spores of any kind. These are the mycelia sterilia (literally sterile mycelia).
How to Succeed Without Doing It
The fungal life style has proven successful for a number of reasons, one of which is that they are prolific reproducers. A single individual can produce millions, billions, or even trillions of spores in a short time under the right conditions.
Spores are light and easily dispersed by wind, water, and in or on animals, so they are quite efficient at finding new sources of food (usually formerly living organic matter). So, how can a fungal species make it in this world if it cannot produce spores?
Fragmentation. That is how mycelia sterilia reproduce and access new sources of food. When cultured in the lab mycelia sterilia are often found to grow very quickly and produce a loose fluffy colony. The hyphae (the basic vegetative cell type of fungi are aerial), which means they stick up into the air to a greater degree than is usually seen in other fungi. This growth habit increases the chance that the cells will be disturbed, break loose, and get carried away to start a new colony somewhere else.
Classification and Evolutionary Implications
Scientists classify fungi (or at least try to) based on the morphology of fungal reproductive structures and through analysis and comparison of macromolecular sequences (DNA, RNA, and protein). Since there are no reproductive structures observable in mycelia sterilia, molecular methods provide the only insight into their origins and relationships.
No matter their true relationships, most mycologists are content to place them with the Deuterotomycetes, which contains fungi having no known sexual phase. However, based on molecular data, it is believed that the majority of mycelia sterilia are allied with the Basidiomycetes (which includes the rusts, smuts, and many mushrooms).
One advantage of sexual reproduction is that it results in new genetic combinations to try in the environment, which can produce individuals that are more fit for survival than their predecessors. Alternatively, random mutations introduced during the process of generating large numbers of asexual spores can accomplish the same thing.
So how is it that those species that never developed sporulation, or lost it along the way somewhere, remain viable? Apparently, the rapid growth and easy break-up of the hyphae in mycelia sterilia is enough to spread them around and generate just enough change (through random mutation during cell division) to keep them one step ahead of Mother Nature’s sword.
References
1995, Pearson, L. C., The Diversity and Evolution of Plants, CRC Press, Boca Raton, FL, p. 173.
(Date unknown), Vinnere, O., Fatehi, J., and Gerhardson, B., The fungal group Mycelia sterilia – its interactions with crop plants and their pathogens, (accessed Dec 8, 2008)
Philip McIntosh - The author holds a B.Sc. in Botany and Chemistry and an M.A in Biology and he has thirty + years of experience in science and industry.
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