Developmental patterns defining morphological characters in spores of four species in Glomus

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Abstract
Spore differentiation sequences are defined and compared in different geographic isolates of four Glomus species to better define species-level morphological characters and to integrate these sequences into a model of spore ontogenesis in Glomales. Phenotypically discrete characters in spores of all four species originated as component parts of a spore wall. This spore wall differentiated in a linear sequence of new layers that formed concurrently in the wall of the subtending hypha. Species were separated into two groups that differed in structure of the spore and hyphal wall in the most juvenile stage: (i) a single mucilaginous layer in G. etunicatum and G. clarum and (ii) a mucilaginous layer together with a semiflexible hyaline layer in G. intraradices and G. claroideum. Differentiation in all species then is followed by synthesis of a laminate layer in the spore wall. It begins as a thin sublayer that increases in thickness with synthesis of additional sublayers. Spores of G. clarum diverge by forming a second pigmented laminate layer in the spore wall. Spores of G. claroideum form an additional thin flexible layer resembling those found in spores of Acaulospora, Entrophospora, and Scutellospora species except that it originates as an attachment of the subtending hyphal wall and thus is interpreted as another component of the spore wall. Stages of differentiation in Glomus are similar to those documented for Gigaspora (Gigasporaceae), except that number and diversity of layers are greater in both spore and subtending hyphal walls of the former. Character differences in development support the separation of Glomaceae and Gigasporaceae into separate suborders and further suggest the order Glomales may be polyphyletic.

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