In this article we will discuss about homothallism and heterothallism in fungi
- Fungi can reproduce through asexual and sexual methods
- In fungi, the sexual reproduction is classified into two types: homothallism and heterothallism
I. Homothallism
- The single individual is capable of sexual reproduction without mating partner.
- Because homothallic fungi have both mating types in the same individual.
- In homothallic species, the mycelium is bisexual.
II. Heterothallism
- Two different mating types are required for sexual reproduction.
- Because heterothallic fungi have only one mating type in each individual.
- In heterothallic species is unisexual, (+) and (-) strains represent the two different sexes.
Heterothallism in fungi
- A. F. Blakeslee (1904) an American Geneticist, made an important observation with Mucor, which resulted in the discovery of Heterothallism.
- Blakeslee observed, that while some isolates of Mucor formed sporangia as well as zygospores (e.g., M. tenuis), some others failed to form the zygospores and reproduced only by sporangiospores.
- When he grew these non-sexually reproducing isolate with other similar isolates, zygospores appeared in the region where the hyphae of the different isolates came in contact with each other.
- Blakeslee coined the terms homothallism and heterothallism to explain this phenomenon.
- The homothallic species were those that produced zygospores independently, while heterothallic species required the presence of the opposite mating type.
- Heterothallism defined as the condition in which Zygospore formation takes place only when mycelia of two genetically different mating types (+) and (-), are allowed to interact.
- Heterothallism has certain advantages over homothallism as it promotes outbreeding, reduced inbreeding depression and increases genetic diversity in the offspring.
- Heterothallism occurs in all major groups of fungi such as Hymenomycetes, Mucorales, Uredinales, Saprolegniales, Euascomycetes and Ustilaginales etc.
- Whitehouse (1949) distinguished two types of heterothallism: Morphological heterothallism and Physiological heterothallism
Types of Heterothallism in Fungi
I. Morphological heterothallism in fungi
- Morphological heterothallism defined as two interacting thalli have morphologically different sex organs or gametes.
- The two sex organs or gametes are morphologically different that it is easier to term one of them as male (+) and the other as female (-).
- Whitehouse (1949) also used the term haplodioecious for morphologically heterothallic species of fungi.
- E.g., Achlya ambisexualis, A. bisexualis, Blastocladiella variabilis, Dictyuchus monosporus, Phytophthora palmivora and Peronospora parasitica
II. Physiological Heterothallism in fungi
- Sexual reproduction takes place by two morphologically similar but physiologically different hyphae is called Physiological Heterothallism.
- Physiological heterothallism is two types: Two-Allele Heterothallism and Multiple Allele Heterothallism
A. Two-Allele Heterothallism:
- Heterothallism is determined by two alleles at a single locus.
- Fungal species having this type of heterothallism are divided into two strains commonly called as (+) strain and (-) strain or ‘A’ and ‘a’.
- Plus (+) and minus (-) strain are essential for mating, whereas the (+) and (+), and (–) and (–) strains are self-sterile or self-incompatible.
- E.g. Mucorales (Rhizopus, Mucor, Phycomyces), Pyrenomycete (Neurospora), Discomycetes (Ascobolus), rust fungus (Puccinia) and smut fungus (Ustilago).
B. Multiple Allele Heterothallism:
- In this type sexual compatibility is controlled by more than two alleles.
- The larger number of alleles involved in this type of heterothallism, chances of mating of compatible strains increase.
- Multiple Allele Heterothallism is two types: Bipolar and Tetrapolar multiple-allele heterothallism
Bipolar multiple-allele heterothallism
- This type of heterothallism is controlled by multiple alleles at a single locus, instead of a pair of alleles.
- Single gene involved
- For example, if the locus is named as (L), the multiple alleles will be (L1,L2,L3,L4) & these are present on the single locus.
Tetrapolar multiple-allele heterothallism
- This type of heterothallism is controlled by multiple alleles at a different locus.
- Two genes involved
- This type of heterothallism is characteristic of Basidiomycetes except rusts.
- Multiple allele the compatible factor is present on two loci (L1) & (L2) of two chromatids of chromosome. It can be represented as (A1B1) & (A2B2).
Reference
S, K. (2016, November 28). Biology Notes on Heterothallism in Fungi (With Diagram) | Fungi. Biology Discussion. https://www.biologydiscussion.com/fungi/biology-notes-on-heterothallism-with-diagram-fungi/63802
G, R. (2018, August 1). Fungi: Heterothallism in fungi, heterokaryosis and parasexuality | Biology. Biology Discussion. https://www.biologydiscussion.com/fungi/fungi-heterothallism-heterokaryosis-and-parasexuality-biology/86041
Kadambini Das, (n.d.). HOMOTHALLISM AND HETEROTHALLISM IN FUNGI. https://www.rlsycollegebettiah.ac.in/wp-content/uploads/2023/02/file_63e39af70e978.pdf
Choudhary, R. (2021, January 4). Heterothalism in fungi [Slide show]. SlideShare. https://www.slideshare.net/slideshow/heterothalism-in-fungi/240900743
