Taxonomic reinterpretation of morphological characters in Acaulosporaceae based on developmental patterns

Abstract

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.

Key Words:

• Acaulospora
• arbuscular mycorrhizae
• Entrophospora
• Glomales
• ontogeny
• phylogeny
• taxonomy
• vesicular-arbuscular mycorrhizal fungi
• Zygomycetes