Phylogenetic Relationship of Basidiomycetes

In this article we will discuss about the phylogenetic relationship of basidiomycetes. 

It is almost agreed among the mycologists that the Basidiomycetes have evolved from the Ascomycetes. Both these groups of fungi have similarities in their hyphal structure, in having an elaborate dikaryophase and in the development of asci and basidia.

It is also accepted that a basidium has evolved from an ascus, during which the number of spores has been reduced from eight to four and the spores have taken external position on the sterigmata of a basidium.

The Ustilaginales and the Uredinales are considered to have been derived from some Ascomycetes possibly before the Pezizales, Sphaeriales, and Dothideales. Linder (1940) believed that the Uredinales have evolved from the fungi close to the Dothi­deales and the Ustilaginales again from the Uredinales by reducing life cycles. He traced the origin of the teleutospore and promycelium from the ascus with a two- layered wall.

In such ascus, the endoascus containing ascospores passes out thruogh the softened apex of the ascus and projects some distance beyond the outer wall of the ascus and eventually ruptures and the spores are set free. Linder also assumed that such an ascus becomes more thick-walled to become a resting spore and by further evolution has given rise to resting spores of the Uredinales and the Ustilaginales.

According to Linder, with the gradual loss of obligate parasitism and simplification of the life cycle the Uredinales led to the origin of the Auriculariales. Linder also suggested that all other Basidiomycetous fungi arose from the Uredinales.

According to another school of thought it is held that the Heterobasidiomycetes other than the Uredinales and the Ustilaginales may have arisen from some Ascomycetes somewhat like Ascocorticium.

Rogers (1932, 1934) suggested that the genus Tulasnella has been derived from an Ascocorticium-like ancestor. He further stated that two lines have been derived from Tulasnella, Tremella and Auriculariales; one being the Uredinales and the other the Homobasidiomycetes.

A tenable scheme has been proposed by Saville (1955). According to him, the phylogenetic age of the host plant may be used as an index of the approximate age of the parasite. The most primitive existent Basidiomycete genus is assumed to be Uredi- nopsis, a rust parasitic on the phylogenetically oldest rust-parasitized genus, the fern Osmunda.

Presumably, the hypothetical ancestral Protobasidiomycete was a rust that shared some resemblance to Uredinopsis. This rust has no pycnia, aecia, nor uredia, and it had simple teliospores which may have merely been rounded up hyphal cells. The mycelium inhabited fern fronds and was self-sterile, clampless, and dikaryotic.

The hyphal cells rounded up to form teliospores, which produced a basidium bearing four subspherical basidiospores upon sterigmata. Presumably the spores germinated upon the fern frond, and the hyphae from two compatible spores fused either imme­diately upon or after infection.

This hypothetical rust ancestor is remarkably similar to the Ascomycete genus Taphrina, which also forms a dikaryotic mycelium in its host and rounded up cells from which the ascus forms. Under abnormal conditions, Taphrina may bud externally, producing subspherical spores upon sterigma-like projections.

Both the rusts and Taphrina parasitize plants of ancient linkage, the ferns and woody dicots, indicating that they existed during the same geological age span; of greatest importance, a fungus similar to Taphrina would be unspecialized and plastic enough to be ancestral to phylogenetic lines which must adapt to different environments.

A Ttphrina-like fungus, or a Prototaphrina ancestor, is assumed to have given rise to both present-day Taphrina and the Basidiomycetes. Both the Prototaphrina and Protobasidiomycete ancestors are extinct. The Proto- basidiomycetes gave rise to the Uredinales, beginning with the Uredinopsis-like forms.

Life cycles of the rusts lengthened with the introduction of more spore forms. The first heteroecious rusts originated when part of the life cycle transferred from the telial to the aecial host.

The Protobasidiomycete ancestor also gave rise to a primitive parasitic fungus in the Heterobasidiomycetidae. The unstable and highly variable heterobasidium is assumed to be primitive and not likely to be derived from the undivided basidium which has become a stable feature in the Homobasidiomycetidae.

This primitive fungus was the type found in the family Auriculariaceae in the Tremellales, which has a trans­versely septate basidium. This basidium type is considered to be primitive because it resembles a mycelium and because the fungi in which it appears are morphologically simple parasitic forms on phylogenetically primitive hosts and, most importantly, because clamps appear sporadically in this group.

This primitive, Auricularia-type ancestor gave rise to: 

(i) The Septobasidiaceae (parasitic and with no clamps),

(ii) The Ustilaginales (parasitic with clamps)

(iii) The remainder of the Tremellales (saprobic with clamps), and

(iv) The Homobasidiomycetidae.

The early members of the Homobasidiomycetidae line were parasitic fungi like some Polyporales (the family Thelephoraceae) which form a flat hymenium on a resupinate basidiocarp, either with or without clamps.

Through evolutionary pressure, the mymenium developed a greater spore-producing surface through the formation of ridges, teeth, gills, and pores. In addition, there was a tendency for the spore-producing surface to project downward as this increased the efficiency of spore discharge.

This evolutionary line gave rise to the more advanced Hymenomycetes. The Gasteromycetes represent a polyphyletic group in which convergent evolution under the pressures of the same habitat produced superficial similarities. Both the Gasteromycetes and the Glavariaceae represent polyphyletic groups originating from the Hymenomycetes.

Teixeira (1962), Gaumann (1964), and Olive (1965) have suggested that the clamp connection of Basidiomycetes as homologous to the crozier at the tip of asco- genous hyphae in Ascomycetes, and have argued that the Basidiomycetes evolved from Ascomycete ancestors.