Sexual reproduction likely evolved as protection from environmental stresses, specifically, to repair DNA damage, often via homologous recombination. In higher eukaryotes, meiosis and Escision asexual and sexual reproduction production of gametes with allelic combinations different from parental type provides the side effect of increased genetic variation.
In fungi it appears that while the maintenance of meiosis is paramount for success, outcrossing is not a driving force. In the subkingdom Dikaryafungal members are characterized by existence of a dikaryon for extended stages within the life cycle.
Such fungi possess functional or, in some cases, relictual, loci that govern sexual reproduction between members of their own species. Similarly, the mating systems in the Ascomycota are bipolar, with two non-allelic idiomorphs expressed in cells of opposite mating type. Heterozygosity at both of two unlinked loci is required for cells to productively mate in tetrapolar systems, whereas in bipolar systems the two loci are tightly linked.
Finally, a trade-off exists in wild fungal populations between sexual reproduction and the associated costs, with adverse conditions leading to mating. For fungal mammal pathogens, the products of sexual reproduction can be targets for the host immune system.
The opposite appears true for phytopathogenic fungi, where mating and pathogenicity are inextricably linked. Here, we explore, compare, and contrast different strategies used among the Dikaryaboth saprophytic and pathogenic fungi, and highlight differences between pathogens of mammals and pathogens of plants, providing context for selective pressures acting on this interesting group of fungi.
Fungi, due to their ease of manipulation in a laboratory setting and short generation time, provide excellent model for studying the development of eukaryotic-specific processes, such as sexual reproduction. Unlike most higher eukaryotes, many members of this group also reproduce asexually. As such, several of these species exist as both haploid and diploid forms, whereas in higher eukaryotes such as mammals the adults are always diploid, producing haploid gametes that combine to give rise to the next generation.
The maintenance of both strategies of reproduction within a single organism allows the study of both why a mechanism such as sexual reproduction may have evolved as well as why asexual reproduction may have been maintained.
Ultimately, the broader question becomes can meiosis function as more than a means of production of reproductive cells.
In prokaryotic cells, conjugation and transformation serve as the means of generating recombinant genetic material in the absence of a meiotic pathway.
Most of the evidence suggests that prokaryotic sex has the most value as a means of response to DNA damage Michod et al. It is possible that meiosis emerged from transformation as a DNA repair mechanism. In lower eukaryotes, such as the algal species Volvox caterisexual reproduction has been found to be a means of responding to reactive oxygen species and Escision asexual and sexual reproduction resultant DNA damage Nedelcu and Michod, This alga, like many of the fungal species in this review, is a facultative sexual species, meaning that sexual reproduction is not an obligate component of their life cycle.
Similarly, depletion of the nitrogen source in the growth medium of the unicellular green alga, Chlamydomonas reinhardtiileads to differentiation of vegetative cells into gametes Sager and Granick,which can mate, form diploids and subsequently undergo meiosis. However, due to the diverse lifestyles of these fungi, it is also possible to examine subsequent steps where the products of meiosis are involved in sexual reproduction.
The timing of meiosis and the frequency with which in happens in each of these fungal populations is largely correlated with whether or not the organism is a pathogen.
Fungi have generally been classified into phyla based on the type of sexual reproduction a particular species uses as well as the amount of time spend in the sexual reproductive stage.
Molecular data have provided new tools for classifying fungi and introduced new complications since, depending on the subset of the molecular data chosen, the outcomes of phylogenetic classification may be different.
However, it is generally accepted based on available molecular data that the Dikarya are composed of two monophyletic groups, the Ascomycota and the Basidiomycota Ebersberger et al. Although the emergence of more specific taxa within each of these two groups of Dikarya is more difficult to discern Ebersberger et al.