| Morphological
characters synthesized from differentiation of mycorrhizae and spores were
defined developmentally and their stability examined among eight organisms
of five Scutellospora species. Morphology and architecture of fungal
hyphae at entry points in sudangrass roots were similar to that observed
in other Scutellospora species. Surface topology of extraradical
auxiliary cells 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. |
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