Read the below given notes to learn about heterothallism with the help of diagram.
The term Heterothallism was first used by an American geneticist A.F. Blakeslee in 1904 when he observed that zygospores could develop in some spp. only when two mycelia of different strains were allowed to come in contact with each other.
Blakeslee made these observations as a result of his studies on zygospore formation in Mucorales. He found that in some species of Rhizopus, zygospores were formed freely while in others like R. nigricans zygospores were formed rarely.
In Mucor hiemalis, too zygospores were formed rarely. On the basis of his studies, he divided the various species of Mucorales into two groups: Heterothallic and Homothallic.
Thus heterothallic species are those which require mycelia of two different strains to interact to enable the zygospores to be formed while the homothallic species are those which require myceha of only one strain to interact for the formation of Zygospores.
According to Blakeslee (1904) Heterothallic condition is “essentially similar to that in dioecious plants and animals and although in this case the two complimentary individuals which are needed for sexual reproduction are in general not so conspicuously differentiated morphologically as in higher forms, such a morphological difference is often distinctly visible.”
He concluded that the Zygospore formation is a sexual process. In homothallic species, the mycelium is bisexual while the mycelium in heterothallic species is unisexual, (+) and (-) strains represent the two different sexes.
Heterothallism may therefore be defined as the condition in which Zygospore formation takes place only when mycelia arising from asexual spores of two genetically different mating types (+) and (-), are allowed to interact.
On the other hand the condition, in which one individual originating from a single asexual spore is capable of forming zygospores independently, is known as Homothallism.
Blakeslee proved the phenomenon of heterothallism on the basis of the experiments he conducted using several species of Mucorales. He inoculated spores of two different strains of Mucor hiemalis on a petridish containing synthetic agar medium.
After a few days it was observed that the zygospores were formed along the zone of contact of two myceha. In another experiment he inoculated spores of only one strain on a petridish containing synthetic agar medium and after some time it was observed that absolutely no zygospores were produced in this experiment.
From these experiments, he concluded that zygospores could be formed only when mycelia of two different strains were allowed to come in contact.
Blakeslee and his coworkers (1928) examined different genera of Mucorales to test whether these were homothallic or heterothallic. Most of the genera tested were homothallic while only a few genera were found to be heterothallic.
In heterothallic species of Mucor mucedo, the zygospores upon germination produced germsporangia which contain spores of only one strain (either + or -).
In this case, zygospores could be formed only when mycelia formed from spores of (-) strain formed in germsporangia produced on germination of zygospores are allowed to come in contact with the mycelia produced from the spores of (+) strain.
But in the heterothallic species of Phycomyces nitens Spores of (+) and (-) strains are produced in the same germsporangium. Blakeslee thus regarded the (+) and (-) strains of heterothallic species as differing in sex and the term heterothallism may therefore be treated as equivalent to dioecism in haploid organisms.
Since the discovery of heterothallism in Mucorales by Blakeslee, the phenomenon has been reported in several groups of fungi. Though variations may occur, but all heterothallic species share one common feature of intermycelial contact.
While in Dictyuchus monosporous it is dioecism, in Ascobolus magnificus, it is expressed as self- sterility or self-incompatibility. Both the fungi resemble the heterothallic Mucorales in that sex-organs or gametangia are formed only when opposite strains come in contact.
In Dictyuchus monosporous the two sex-organs are formed by different strains but in Ascobolus magnificus, each strain produces sexorgans but these are self-sterile.
On the basis of these observations, the heterothallism may be of two types: Morphological heterothallism and Physiological heterothallism.
Morphological heterothallism may be defined as the condition when morphologically different male and female sex organs are produced in two closely associated mycelia.
The two sex organs or gametes are so morphologically different that it is easier to term one of them as male and the other as female-examples of such type of morphological heterothallic fungi are: Achlya ambisexualis, A. bisexualis, Blastocladiella variabilis, Dictyuchus monosporus, Phytophthora palmivora and Peronospora parasitica (photographs given below).
However, in Blastocladiella variabilis the male and female gametangia are morphologically distinct, the male being smaller than the female.
Whitehouse (1949) also used the term haplodioecious for morphologically heterothallic species of fungi.
In physiological heterothallism, the interacting thalli differ in mating type or incompatibility, irrespective of the presence or absence of the sex organs or gametes. This means that sexual reproduction takes place by two morphologically similar but physiologically different hyphae in physiological heterothallism.
The gametangia as well as gametes do not show morphological differentiation but physiologically they behave differently.
Physiological heterothallism may be of two types:
(i) Two Allelomorphs or Two-Allele Heterothallism:
When nuclei of both the mating types are different in genetic characters, this type of Heterothallism is known as Two-Allele heterothallism. In these types compatibility is governed by a pair of Alleles represented by A and a located at single same locus of the chromosome.
Due to the dominance of A over a, A is represented by (+) and a by (-). At the time of meiosis, separation of the chromatids take place. Half of the haploid spores thus have (+) and the other half (-) allele.
The spores bearing (+) allele will produce (+) mycelia and the spores with (-) allele will give rise to (-) mycelia. The mycelia of (+) and (+) and (-) and (-) are self-sterile or self-incompatible. Thus, two complimentary mating types (+) and (-) are essential for sexual reproduction.
Two-Allele heterothallism has been reported in several fungi of like Ascobolus magnificus, Puccinia graminis, Neurospora sitophila Mu mucedo, Ustilago kolleri etc. (photographs given below)
(ii) Multiple Allelomorph or Multiple Allele Heterothallism:
In this type of heterothallism, more than two (multiple) alleles determine the sexual compatibility. These may be located at one (bipolar) or two (tetrapolar) loci.
Because of the larger number of alleles involved in this type of heterothallism, chances of mating of compatible strains increase.
As stated above, the multiple allele heterothallism may be of two types:
(a) Bipolar Multiple-allele heterothallism
(b) Tetrapolar multiple-allele heterothallism.
(a) Bipolar Multiple-Allele Heterothallism:
This type of heterothallism is controlled by multiple alleles at a single locus, instead of a pair of Alleles. For example, if the locus is named as L, the multiples alleles will be designated as L1, L2, L3, L4—Ln and these are present on the single locus L.
The meiotic division will give rise .to thalli which may be of several mating types, generally equal to the number of alleles. The thallus containing the allele L1 can mate with a thallus of any mating type except L1.
Similarly L2 can mate with any thallus except that containing L2 allele and so on. In this type of heterothallism, incompatibility factors are more commonly involved. Bipolar multiple allele heterothallism is characteristic of Basidiomycetes except rusts and smuts (Fig. 17.3)
(b) Tetrapolar Multiple Allele heterothallism:
This type of heterothallism is characteristic of Basidiomycetes except rusts. In this type of heterothallism, which is very similar to bipolar multiple allele heterothallism, compatibility is determined by two loci.
Multiple allele—the compatible factor is present on two loci L1 and L2 of two Chromatids of a chromosome. At the time of meiotic division, both the loci are separated with chromatids.
It is estimated that at least 100 alleles are present on each locus. In Schizophyllum commune, 122, alleles of factor A and 61 of B have been identified in the laboratory.
According to rough estimates, the number of alleles may be even more, about 350-450 of L1, and 65 of L2. Any two mating types, which differ in allele present on L1 and L2 are compatible.
If the allele composition of mating type is A1 B1, it would be compatible with any other type of allele composition except A1 B1. But the mating type with allele composition is not fully compatible with allele composition A2 B2 or A2 B2.
Figure 17.4 fully explains type of heterothallism which has been reported in Ustilago maydis and Comprinus firmaterius.
This type of heterothallism encourages out-breeding. Whereas in bipolar multiple Allele heterothallism, the out-breeding is 25%, in tetrapolar, it is 100%. This may be due to enormous increase in the number of possible mating types of thalli.
According to Garrett (1963), “heterothallism promotes the out-breeding and therefore subserves the same end as the sexual process, which it renders most efficient. Hetrothallism is not the same as sex, it is refinement super imposed upon it.”