Formation of Dikaryons
- Overview of life cycle of the Ascomycota and Basidiomycota:
- The somatic (vegetative) mycelia of Ascomycota and Basidiomycota species are HAPLOID.
- At some stage during the life cycle of the fungus two compatible nuclei are brought together by various methods, but they don't usually fuse immediately.
- Instead, they continue to divide as separate nuclei - although their daughter nuclei remain in closely associated compatible pairs.
- Cells or compartments containing such a pair of closely associated compatible nuclei are desribed as DIKARYOTIC (a DIKARYON).
- Sooner or later a pair of nuclei in a cell or compartment will FUSE to form a DIPLOID NUCLEUS.
- The diploid nucleus will undergo MEIOSIS as a preliminary to the formation of HAPLOID SPORES (ascospores or basidiospores).
- The DIKARYOTIC STATE IS UNIQUE TO FUNGI - it is neither a diploid state, nor is it strictly haploid since both nuclei of the pair influence metabolism and development of the hypha.
Compatible nuclei are united by one of the following four methdods, depending upon the group and species of fungus.
- 1. Gametangial conjugation:
- This method is confined to the Ascomycota.
- Two similar gametangia come into contact with one another and fuse.
- There is no prolonged dikaryotic phase in this instance, since nuclear fusion occurs immediately after fusion of the gametangia.
- In the example illustrated, two yeast cells of different but compatible mating types behave like gametangia and fuse - resulting in a diploid zygote, which is transformed directly into an ascus (containing eight ascospores).
- 2. Gametangial contact:
- This method is also confined to the Ascomycota.
- In this case morphologically distinct gametangia are formed - called ANTHERIDIA (male) and ASCOGONIA (female).
- The TRICHOGYNE (receptive neck of the ascogonium) receives the male nuclei from the antheridium.
- Upon passing along the trichogyne into the ascogonium each male nucleus pairs with a female nucleus in the ascogonium, but the pairs of nuclei don't fuse.
- ASCOGENOUS HYPHAE emerge from the ascogonium and the nuclei, still in their compatible pairs, pass into these hyphae - which are now dikaryotic.
- The ascogenous hyphae are destined to develop into asci.
- 3. Spermatization:
- Occurs in some species in both the Ascomycota and Basidiomycota.
- Uninucleate spore-like structures (known as SPERMATIA in the Ascomycota and OIDIA in the Basidiomycota) are carried by air currents, water or insects to the sides of somatic receptive hyphae.
- A pore develops at the point of contact between the hypha and the 'spore'.
- The contents of the 'spore' (including its nucleus) pass into the hyphal compartment, which as a result becomes dikaryotic.
- Some conidia have the potential to behave like spermatia, but unlike true spermatia, conidia are asexual spores and capable of germinating to produce germ-tubes. .
- 4. Somatogamy:
- Occurs in both the Ascomycota and Basidiomycota.
- The fusion of somatic hyphae of two compatible mycelia results in a dikaryon from which a dikaryotic mycelium may develop.
- In the Ascomycota, the dikaryotic phase is limited to mycelium within the fruiting body (ascocarp).
- But in the Basidiomycota the mycelium continues to grow in the dikaryotic state for some time and fruiting bodies (basidiocarps) form only at a much later stage.
We are now at a stage where in the Ascomyciota we have a number of dikaryotic compartments (i.e. ascogenous hyphae) in a fruiting body (ascocarp), and where in the Basidiomycota we have a dikaryotic mycelium and dikaryotic hyphae forming the fruiting body (basidiocarp).