Gametes and/or asexual single cells with flagellae > non flagellate
With sexual reproduction > asexual
Oogamy > gametangiogamy > somatogamy
The loss of flagellates may have been an adaptive response to limitations of water in land habitats. Consequently, such an evolutionary development requires either gametangiogamy or somatogamy.
Dikaryotic hyphal stages are unique characteristics of Ascomycota and Basidiomycota (Fig. 3). The delay of nuclear fusion after plasmogamy varies considerably in both groups. In ascomycetous fungi the dikaryophase is restricted to ascogenous hyphae, while in most Basidiomycota dikarya are established after plasmogamy that normally occurs very early in ontogeny and karyogamy is delayed until the formation of meiosporangia, the basidia (Kniep 1928). Because croziers in Ascomycota and clamps in Basidiomycota occur on dikaryotic hyphae they were considered as essential for maintaining the dikaryotic stages. However, many fungi are known with dikaryotic hyphae lacking such structures.
The evolutionary driving forces for establishing, maintaining and enlarging dikaryotic phases in higher fungi are not clear. In Basidiomycota, the nuclear behavior is even more complex because diploid life cycles also occur. The genomic stability of two diploid individuals of Armillaria gallica inhabiting stable environments has been shown by Hodnett & Anderson (2000). Under such conditions, phenotypic flexibility as afforded by dikaryosis, is not needed. Anderson & Kohn (2007) suggested that nuclear spacing and associated variation in gene expression are inherent to dikaryons, but not to diploids, thus allowing them improved responses to heterogeneous environments. In addition, dikaryons are capable to deliver a fertilizing nucleus to a monokaryon (Buller 1930, 1931), an ability called Buller phenomenon (Quintanilha 1937, Raper 1966). In essence, all matings are a manifestation of the Buller phenomenon (Anderson & Kohn 2007).
In many rust fungi the haplophase is comparatively long, thus indicating an old evolutionary origin (e.g. Gäumann 1964).