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Nuclear States

Introduction

Each fungus can be found in a variety of nuclear states. That is, the number of nuclei (ploidy) and the degree of duplication (haploid/diploid) can vary throughout the life cycle of the fungus. Though most commonly haploid, each fungus may have significant periods in each part of the life cycle.

Cell Cycle

A haploid organism contains a single set of chromosomes in each of its nuclei. LINK Following plasmogamy (fusion of cytoplasm) in filamentous fungi, the number of nuclei doubles, though these need not necessarily go through karyogamy (fusion of the nuclei) immediately. The state where two genetically distinct haploid nuclei coexist in one cell is called a dikaryon (adj. dikaryotic). When the nuclei fuse in karyogamy, the nucleus becomes diploid. In this cycle, fungi are similar to most organisms. However, diploid fungi almost always then pass immediately into meiosis, resulting in the formation of spores with a single haploid nucleus. LINK

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Variations

The fungi have a number of variations on this simple theme and these variations provide some interesting potential advantages. Some fungi lack septa in their hyphae. Thus, the fungus is multinucleate, and the nuclei are usually haploid. The nuclei may be of the same, or compatible and different types. For instance, the thallus of fungi in Zygomycota and Glomeromycota is non-septate, and multinucleate. The diploid state is only found in the sexual zygospore, such as in members of the Mucorales.

Most fungi have significant periods as haploid thalli. The vegetative states of most Ascomycota are haploid. The fungus goes through plasmogamy and passes immediately to karyogamy, followed by meiosis and the formation of ascospores. These fungi are only diploid for very short periods of time, and only in very limited parts of the thallus.

A variety of nuclear states in compartments has been reported among the Basidiomycota. Spores germinate to form a haploid mycelium. Plasmogamy with a compatible haploid mycelium results in a dikaryotic mycelium, where both nuclei may be functional. The dikaryotic mycelium may never pass into the diploid state. However, if it does, then the sexual phase may be reached, where karyogamy might be followed by meiosis, though these states may be separated in time and space.   Reports of multinucleate mycelia indicate that more than two haploid nuclei may coexist in a cell. Indeed, haploid and diploid nuclei may coexist in one compartment. LINK

The nuclear state of most fungi is regulated. One species is commonly found with one nuclear state. This is particularly evident in fungi with secondary homothallism (the fungus can mate with itself in the absence of a compatible mating type). The variation from common behaviour is possible because nuclei may move from compartment to compartment, and the expression of mating type need not necessarily be found at one locus. LINK Indeed, nuclei can function independently of one another. Following formation of a compatible dikaryon, the incoming nucleus passes through mitosis independently of the second nucleus, with one of the resultant nuclei passing through to the next compartment. LINK

The variation in ploidy through the life cycle is enormously important. If fungi exist as haploids for significant periods, then selection against specific alleles will ensure thalli with deleterious alleles will be removed from the population. LINK Thus subsequent diploids will have only neutral or beneficial alleles for that specific environment.

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Conclusion

The nucleus contains most of the genetic material of the cell. The nucleus controls the function of the cell. In fungi, the thallus is commonly haploid for a significant part of the life cycle, and thus selection against undesirable mutations is immediate. However, a wide range of variations on the basic theme in a cell cycle indicate that more than one strategy of nuclear behaviour can lead to successful outcomes.

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References

Burnett J. 2003. Fungal Populations and Species. OUP

Elliott CG 1994 Reproduction in Fungi. Chapman Hall

Markham P in Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall

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