Mycology Notes on Heterothallism (With Diagram) | Fungi

Read the below given notes to learn about heterothallism with the help of diagram.

Ehrenbergh (1829), for the first time studied zygospores in the order Mucorales. The American mycologist Blakeslee (1904), reported that in the several genera of the order Mucorales, the zygospores are not formed at all.

He also supported his view with facts and reasons, and also investigated that in the same order two types of species are found which may be named as homothallic and heterothallic species. When the two hyphae of the same mycelium produced by a single spore fuse with each other and a zygospore is developed, the species is said to be homothallic, e.g., Mucor hiemalis.

Mucor mucedo and Mucor stolonifer are the typical heterothallic species. In heterothallic species the fusion can take place only among the different strained hyphae, which develop on different mycelia of different (+ and -) strains. In these species the zygospores cannot be produced by the fusion of two hyphae of the same strain. In 1904 Blakeslee reported that in heterothallic species whenever the mycelia of + and – strains remain apart from each other, the zygospores are not produced and only the sporangia are formed.

On the other hand, when + and – mycelia grow together, the fusion takes place and the zygospores are produced. Morphologically the + and – strained mycelia are quite similar in structure, but different in physiological behaviour. In other words, they are morphologically identical and physiologically different.

Sometimes, it has also been observed that the growth of the + mycelium is comparatively faster, and the gametangia of + mycelium are bigger than that of the – mycelium, and they can be distinguished as female and male gametangia. Many mycologists do not support this view and advocate that this behaviour is only because of nutrition.

Blakeslee proved the phenomenon of heterothallism on the basis of experiments. He inoculated many + and – strained spores on the agar medium in a petri dish and observed that the fusion occurs only at those points where the hyphae of different strains (i.e., + and -) come in contact. The zygospores were only produced at the points of contact of different strained hyphae. Absolutely no zygospores were produced at the points of contact of same strained hyphae.

This experiment may be clearly understood by the figure given in the text. Five spores of different strains are inoculated in a sterilized petri dish on agar medium at different five points designated as A, B, C, D and E. The zygospores are produced only at those points where the mycelia are opposite strained and produced from + and – spores designated as A, C and E are + strained, B and D are – strained.

The zygospores are produced at the junctures of AB, BC, DE and AD where the opposite strained mycelia take part in fusion. The zygospores are not produced at the junctures of AC and AE because of the mycelia of same strains.

Blakeslee also investigated that if the + strained hyphae of one heterothallic species fuse with the – strained hyphae of another heterothallic species the imperfect zygospores are resulted. In such conditions the zygospores do not mature. He also investigated that in the heterothallic species Mucor mucedo the zygospores produced germsporangia on germination, which contain single strained spores, and therefore, the fusion is possible only when the mycelia are produced from the spores formed in different strained germsporangia. In the heterothallic species of Phycomyces nitens the + and – strained spores are produced in the same germsporangium.

The phenomenon of heterothallism was first reported in the order Mucorales, but now this has been established in rusts, smuts, Homobasidiomycetidae and other fungi.