Taxonomic and phylogenetic divergence among five Scutellospora species based on comparative developmental sequences

Abstract

Morphological characters synthesized from differentiation of mycorrhizae and spores were defined developmentally and their stability examined among eight organisms of five Scutellospora species (Glomales, Zygomycetes). Morphology and architecture of fungal hyphae at entry points in Sudan-grass roots were similar to that observed in other Scutellospora species. Surface topology of extraradical auxiliary cell were similar among the taxa studied, and intermediate between that of Gigaspora and other Scutellospora species. Spore differentiation was partitioned into four discrete stages: differentiation of layers in the spore wall (stages 1 and 2), subsequent formation of a bilayered flexible inner wall (stage 3), and lastly, the synthesis of a germination shield (stage 4). Spore growth (expansion) was coupled only with stages 1 and 2. Stages of differentiation could be homologized among taxa because of their division and stability in ontogenesis. Stage 3 was shared by all five Scutellospora species, thus uniting them, together with S. castanea, in a monophyletic group. Species-level divergence in this group was expressed only in properties of spore wall layers. Stages 1–3 were homologous with the first three of five stages in differentiation of S. heterogama spores. These shared stages suggest a pattern of Haecklian recapitulation in the evolution of flexible inner walls and both a historical and a contemporaneous link between all flexible inner walls and germination events. Thus, developmentally defined morphological characters provide a causal linkage between the taxonomic hierarchy and a hierarchy in evolution of spore subcellular structure and also suggest causal relationships between form and function.

Key Words:

• arbuscular mycorrhizal fungi
• Scutellospora coralloidea
• S. fulgida
• S. gregaria
• S. persica
• S. verrucosa
• systematics