| Sequences
in the development of subcellular organization in soil-borne spores of Acaulospora
laevis, A. spinosa and Entrophospora colombiana were analyzed
to define discrete morphological characters informative in taxonomic and
phylogenetic analyses of arbuscular mycorrhizal fungi in the family Acaulosporaceae,
Glomales. A sporiferous saccule initially forms terminally on a sporogenous
hypha, followed by spore formation. Spore ontogenesis proceeds in a linear
series of discontinuous stages that are identical in all three species studied,
with each stage recognized by addition of a new subcellular structure from
the spore cytoplasm. The juvenile spore wall is a single hyaline layer continuous
with the wall of the sporiferous saccule hypha, followed by differentiation
of a pigmented spore wall layer with thin adherent sublayers (or laminae)
which varies phenotypically among species. A third spore wall layer then
is differentiated in spores of Acaulospora, which varies phenotypically
between species. Once the spore wall is completely differentiated and spores
have ceased expansion, species in both genera sequentially form two discrete
and separate bilayered flexible hyaline inner walls. Layers of the first
inner wall are thin (< 1 µm) and adherent. The inner layer of the second
flexible wall increasingly reacts in Melzer's reagent as it matures in E.
colombiana, indicating the layers are formed sequentially rather than
concurrently. After both inner walls are fully differentiated, a "germination
orb" is synthesized, although this structure rarely is observed in
newly harvested spores from pot cultures. Structures formed in each stage
of spore differentiation vary in the extent to which they are conserved
among species, indicating different levels of taxonomic resolution. All
layers of the spore wall and process-related features such as spore size
define species-level variation, and flexible inner walls resolve taxa not
yet recognized in glomalean classification. Ontogenesis in Acaulosporaceae
consists of both shared and unique stages relative to developmental stages
in other glomalean families, Glomaceae and Gigasporaceae. However, early
stages are more similar to those of Glomaceae, suggesting closer relatedness
to that family. |
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