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NucleAR Behaviour in the Thallus

Introduction

Initiation of contact between compatible isolates is followed by the dikaryotisation (two haploid nuclei in one compartment) of the thallus or a portion of it. In brief, at least some haploid compartments of the thallus become dikaryotic following plasmogamy. Karyogamy is followed by meiosis, thus completing the sexual cycle. The phases of haploidy, dikaryosis and diploidy differ markedly within the fungi, and the differences are associated with the control of the movement of nuclei.

Fungi control the movement of nuclei in various ways. The simplest pattern is found in yeasts, and this pattern is close to the common patterns of plants and animals. However, some fungi have extensive periods as haploid organisms, others as dikaryotic fungi. The nuclear phases may also be spatially separated in some fungi. Others commence existence as haploid fungi and then become extensively dikaryotic following the acquisition of a single haploid nucleus. LINK These latter cycles differentiate fungi from other well known diploid organisms.

Study of the movement of nuclei is particularly difficult. Fungal nuclei are small and their movement is difficult to track in living hyphae. Early experiments using auxotrophic mutants provided indirect evidence. More recently, direct observations of nuclei have also been used. [see the web site of the BMS which has a link to some wonderful video clips of the migration of nuclei] Several patterns have emerged from these studies. The patterns appear to be consistent within major taxa.

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Mucorales

In the Mucorales, the process seems relatively simple. LINK The sexual arena is delimited from the vegetative part of the thallus of each mating type by a secondary septum formed at the base of the progametangium. Hyphal tips of two compatible fungi fuse. Two different haploid nuclei may then divide by mitosis. The resultant nuclei are retained in the progametangia. Single nuclei from each mating type then fuse to form the diploid, and a zygospore then forms around the diploid nucleus. The suspensors collapse. The zygospore passes through meiosis, usually at germination, and emergent hyphae are again haploid.

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Ascomycota

In Neurospora, haploid cultures bear asexual spores and form properithecia. Spores from a compatible mating type will join with the properithecia and form a sexual stage. The nuclei from the spore pass into the trichogyne, and immediately migrate into the ascogonium. The ascogenous hyphae develop and an ascus is formed. LINK

In principle, a similar pattern takes place in all filamentous Ascomycota. One spore of one mating type fuses with the receptive cell of the second mating type. Only the nucleus passes into the receptive cell. The nuclei of the second mating type may divide (Crozier formation). Nuclei of the second mating type are limited to the sexual apparatus by septa.

The yeast Saccharomyces cerevisiae has a different pattern of movement of nuclei. Cells of the haploid phase of one mating type fuse with cells of the opposite mating type. The cytoplasm of both parent cells fuse and the nuclei move into karyogamy immediately. The fungus also differs from filamentous Ascomycota in that isolates can have extensive periods as haploid and diploid organisms. The switch to commence meiosis is induced by environmental conditions. The entire cell is converted into an ascus and the resultant ascospores initiate the haploid phase again. The dikaryotic phase appears to be extremely limited.

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Basidiomycota

In Basidiomycota LINK such as Schizophyllum and Coprinus, germination of the basidiospore results in the formation of a thallus with single haploid nucleus in each compartment, and the thallus is called a monokaryon. Following fusion between compatible hyphae of different mating types, the nucleus of one migrates into the other LINK.The single haploid donor nucleus then passes into mitosis, and one of the resultant haploid nuclei passes into the adjacent cell. This nucleus then divides mitotically and one of the resultant haploid nuclei passes into the next adjacent cell. The mitotic division and migration of this nucleus continues, and eventually, the entire thallus becomes dikaryotic.

The migration of the nucleus might be prevented by the narrow pore between compartments. However, it appears that the septum can degenerate enabling movement of the nucleus into the adjacent compartment, after which the septum reforms. The migration of nuclei is also extremely fast. Migration at rates between 1 and 40 mm per hour have been recorded. An entire thallus in culture conditions may become dikaryotised within a short period.

The migration of nuclei through the thallus differentiates the Basidiomycota from other filamentous fungi. However, as with Ascomycota and Zygomycota, the nucleus is the only component from the compatible compartment that is transferred. Cytoplasm is not exchanged between fungi and cytoplasmic elements remain with their parents.

Oidia are formed on monokaryons of some Basidiomycota. Oidia are small haploid conidia which are commonly released in mucilage. The oidia can initiate dikaryons in some fungi. They either fuse directly with hyphae, or they germinate and the resultant hyphae fuse with compatible hyphae.

The regulation of dikaryotisation appears to be under genetic control. For instance, one gene locus in the tetrapolar compatibility system of Coprinus is associated with dissolution of the septum, and the other with formation of the join of the clamp with the penultimate compartment. Thus if the fungi which mated have similar alleles at one locus but dissimilar at the other, then they may form a dikaryon cell from which either the nucleus may not migrate or the clamp formation remains incomplete. The consequent compartment of an incompatile interaction is unlikely to survive.

Haploid mycelia may also fuse with dikaryotic mycelia. Following fusion, the nucleus of the compatible mating type will migrate through the haploid mycelium from the dikaryotic thallus. Thus dikaryotic mycelia may donate nuclei to a range of thalli, different nuclei to each.

Most basidiospores are haploid at release, though five variations have been documented:

  • The nucleus may divide in the basidium, one daughter nucleus migrates to the sterigma and passes into the spore, the other degenerates.
  • The nucleus may divide in the sterigma and one daughter nucleus passes into the spore, the other degenerates.
  • The nucleus may divide in the spore, and one nucleus passes back into the basidium where it degenerates.
  • The nucleus may divide in the basidium or spore and both daughter nuclei remain in the spore.
  • The nuclei formed from meiosis may pass through a mitotic division and all nuclei pass into the spores, each with two nuclei which may be of the same or different mating type. If different they commonly germinate to give fertile dikaryotic mycelia.

While mycelia are commonly haploid monokaryons or dikaryons with haploid nuclei from two compatible parents, other patterns are found. The fungus may have multiple copies of a single haploid nucleus in each compartment, and is called a homokaryon. As well, multiple copies of different nuclei may be present, and these may randomly segregate in different parts of the thallus. The result of the latter process is a thallus which has a varying genetic construction.

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Conclusion

Dikaryotisation of haploid compartments is an essential step in the sexual process. Mitotic divisions of the invading nucleus then follow with quite specific genetic controls regulating the process of sexual reproduction.

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References

Burnett J. (2003) Fungal Populations and Species. OUP.

Davis R.H. (2000) Neurospora. QUP

Moore D & Novak Frazer LA (2002) Essential Fungal Genetics. Springer.

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Vegetative Compatibility

The fusion between hyphae of different isolates of the same species of fungus takes place between sexually compatible isolates. A single nucleus is transferred resulting in sexual spores. In vegetative fusion, between vegetatively compatible isolates, the result is vegetative heterokaryons. LINK

Study of the formation of conidia in Aspergillus nidulans has led to a better understanding of the basis of control of compatible interactions within asexual fungi. The formation of vegetative heterokaryons requires identical alleles at 8 loci. Thus the resultant mycelium following fusion will have a mixture of nuclei and cytoplasmic elements of both parents. Generally, these groups do not undergo sexual recombination, though the route is via formation of the cleistothecia when it occurs.

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