HETEROKARYOSIS IN FUNGI

 

HETEROKARYOSIS IN FUNGI

❖ Heterokaryosis is the main source of variation in the anamorphic (imperfect) fungi, which lack sexual reproduction. The term Heterokaryosis was proposed by Hansen and Smith in 1932, who reported it for the first time in Botrytis cinerea.

❖ The presence of genetically-different nuclei in an individual is called heterokaryosis, and the organism heterokaryon.

❖ Essentially, a heterokaryon possesses two sets of chromosomes, just like a diploid organism, but instead of being contained in a single nucleus, the two sets of chromosomes lie in separate nuclei, sharing the same cytoplasm.

❖ Heterokaryons show dominance and, thus, resemble diploids in many respects. Heterokaryosis is a major factor in natural variability and sexuality.

❖ The heterokaryotic condition can arise in a fungus by three methods, viz., (1) Mutation, (2) Anastomosis i.e., fusion between genetically-different hyphae, and (3) Diplodization-fusion between haploid nuclei to form diploid nuclei.

❖ Mutations occur frequently in fungi, and a homokaryotic mycelium is frequently converted into a heterokaryotic one. Anastomosis between spores and hyphae is a universal feature of higher fungi and certainly must be a potential source of heterokaryosis and, thus, of variability.

❖ Whether nuclei migrate from one thallus to another is a debated point but the hyphae having nuclei of both parents arise at the point of fusion.

❖ Heterokaryosis is often accompanied by parasexual cycle.

➢ Formation of Heterokaryotic Mycelium

❖ The methods of formation of heterokaryotic mycelium are described earlier under ‘heterokaryosis.

Nuclear Fusions and Multiplication of the Diploid Nuclei

❖ Nuclear fusion in somatic heterokaryotic hyphae was first noted by Roper (1952) in Aspergillus nidulans.

 ❖ Nuclear fusion may occur between genetically similar and dissimilar nuclei, resulting in the formation of homozygous and heterozygous diploid nuclei, respectively.

❖ Diploid heterozygous nuclei are formed very rarely (at a frequency of one in a million).

❖ In such hyphae, five types of nuclei are present- 2 types of haploid nuclei, their two types of homozygous diploids, and the one type of heterozygous diploids.

 

Mitotic Crossing Over

❖ Crossing over is a phenomenon which occurs during meiosis and gives rise to new linkage of genes, gene recombination. A similar mitotic crossing over occurs during the multiplication of the diploid heterozygous nuclei, though at a low frequency of 10-2 per nuclear division.

❖ However, in some other fungi e.g., Penicillium chrysogenum and Aspergillus niger, the frequency of mitotic crossing over is as high as during meiosis in sexual reproduction. (Both species lack sexual reproduction.) Mitotic crossing over is the most important, or ‘key’ event in the parasexual cycle, as it is during this step that genetic recombination occurs.

➢ Sorting Out of Diploid Strains

❖ The segregation of the diploid strains occurs when uninucleate diploid conidia are formed. The colonies that are formed by diploid conidia are recognized by various methods, e.g., higher DNA content and bigger size of the conidia and certain phenotypic characters of the colony

Haplodization:

❖ The diploid colonies show appearance of sectors on the Petri plate, which produce haploid conidia. This indicates that some diploid nuclei must have undergone haplodization, forming haploid nuclei, which later get sorted out in haploid conidia.

❖ Some of these haploids are genetically different from the original haploid parental nuclei. This is because of the recombination that occurred during the mitotic crossing over.

❖ Haplodization occurs at a constant frequency of 10-3 per nuclear division. The haplodization occurs not by a reduction division (meiosis), but by aneuploidy, a phenomenon in which chromosomes are lost during mitotic divisions.

❖ It happens in the following manner. During mitosis of the diploid nucleus, the chromatids fail to separate (non-disjunction) in the anaphase stage.

❖ One daughter nucleus gets one chromosome more (2n + 1), while the other gets one chromosome less (2n – 1) than the normal 2 sets of chromosomes (2n). Both the daughter nuclei are called aneuploidy. The deficient aneuploid nucleus (2n – 1) may lose more chromosomes in the successive mitotic division and finally reduce to haploid state (n).

❖ Mitotic crossing over and haplodization also occur with the diploid homozygous nuclei, but since the two nuclei are similar, crossing-over products or the haploid nuclei formed by haplodization, are genetically no different from the haploid parent nuclei.

❖ The parasexual cycle, thus, like the sexual cycle, involves plasmogamy, karyogamy and haplodization, but not at a specified time or place. Every step differs drastically.

 

 

 

 

KEY POINTS

v  The homothallic species were those that produced zygospores independently, while heterothallic species required the presence of the opposite mating type.

❖ In 1904 made an important observation with Mucor, which resulted in the discovery of Heterothallism.

 ❖ Blakeslee coined the terms homothallism and heterothallism to explain this phenomenon.

❖ M. hiemalis, M. mucedo, Rhizopus nigricans are examples of heterothallic species.

❖ On the basis of the distribution of sex organs, fungi can be put in the following 3 categories.

❖ Hermaphrodite - in which both male and female sex organs occur on the same thallus.

❖ Dioecious (sexually dimorphic) - The two sex organs are present on different thalli.

❖ Sexually undifferentiated- The male and female sex organs are morphologically similar and, therefore, indistinguishable.

❖ Heterothallism, according to Whitehouse (1949) can be caused by the absence of the morphological sex organs of the opposite type (morphological heterothallism) or by the absence of genetically-different nuclei.

❖ A heterothallic species may not be of only two mating types. There can be four types of thalli and one thallus can mate with only one of the rest three. This is called tetrapolar heterothallism.

❖ The hyphae produced behave as homothallic, though it involves genetically-different nuclei. This situation is termed secondary homothallism.

v  Esser (1959) suggested the use of the terms monoecious and dioecious, as done in higher plants. Call by whatever ‘term’ you please, the function of heterothallism remains unaltered.

❖ Heterothallism is a device for achieving outbreeding, which is a genetic desirability. Homothallism brings in inbreeding and provides no chance for genetic change.

❖ Heterokaryosis is the main source of variation in the anamorphic (imperfect) fungi, which lack sexual reproduction.

❖ The term Heterokaryosis was proposed by Hansen and Smith in 1932, who reported it for the first time in Botrytis cinerea.

❖ The parasexual cycle involves the following steps: Formation of heterokaryotic mycelium, Nuclear fusions and multiplication of the diploid nuclei, Mitotic crossing over during division of the diploid cells, Sorting out of the diploid strains, Haplodization.

❖ Parasexual cycle is of importance in industrial processes. Several fungi which are used in various industrial processes belong to fungi imperfecti or Deuteromycetes and in these fungi only parasexual cycle operates.

❖ New and better strains of these fungi are obtained by mutation through parasexual cycle.

❖ Parasexuality can also be applied in the analysis of genetic and physiological processes of perfect and imperfect fungi.

❖ Parasexual cycle has also been successfully employed in genetic control of pathogenicity and host-range in several species of Fusarium.