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The family Gigasporaceae consisted of the two genera Gigaspora and Scutellospora when first erected. In a recent revision of this classification, Scutellospora was divided into three families and four genera based on two main lines of evidence: (1) phylogenetic patterns of coevolving small and large rRNA genes and (2) morphology of spore germination shields. The rRNA trees were assumed to accurately reflect species evolution, and shield characters were selected because they correlated with gene trees. These characters then were used selectively to support gene trees and validate the classification. To test this new classification, a phylogenetic tree was reconstructed from concatenated 25S rRNA and β-tubulin gene sequences using 35% of known species in Gigasporaceae. A tree also was reconstructed from 23 morphological characters represented in 71% of known species. Results from both datasets showed that the revised classification was untenable. The classification also failed to accurately represent sister group relationships amongst higher taxa. Only two clades were fully resolved and congruent among datasets: Gigaspora and Racocetra (a clade consisting of species with spores having one inner germinal wall). Other clades were unresolved, which was attributed in part to undersampling of species. Topology of the morphology-based phylogeny was incongruent with gene evolution. Five shield characters were reduced to three, of which two were phylogenetically uninformative because they were homoplastic. Therefore, most taxa erected in the new classification are rejected. The classification is revised to restore the family Gigasporaceae, within which are the three genera Gigaspora, Racocetra, and Scutellospora. This classification does not reflect strict topology of either gene or morphological evolution. Further revisions must await sampling of additional characters and taxa to better ascertain congruence between datasets and infer a more accurate phylogeny of this important group of fungi.

A putative mutant of Scutellospora heterogama has been maintained for 19 pot culture generations over 15y. The mutant differed phenotypically from the wild-type parental lineage in characteristics of the spore wall: albino versus pigmented red-brown color, high plasticity in acidic mutants versus rigid and finely laminate, deep red-purple reaction versus no reaction in Melzer's reagent, respectively. This variation was equivalent to or greater than that between any two species in Scutellospora or any other genus in Glomeromycota. Comparison of spore ontogenesis revealed that the terminal (mature) state of the albino mutant was similar to a transient intermediate stage in the wild-type. The albino phenotype, therefore, did not result from emergence of a unique morphological innovation. Rather, it arose from a mutation that led to premature termination of spore ontogenesis so that a unique transient juvenile stage became permanent in mature spores. Because this mutation was homogeneous in all progeny populations, it is hypothesized to be a recessive trait expressed only after the allele was distributed in all nuclei of the fungal thallus. Sampling of the genomes of the putative mutant and wild-type isolates by microsatellite-primed PCR suggested a local mutation. The profile of the mutant was identical to that of the wild-type parent and was 60-97 % similar to those of four other S. heterogama isolates. Phylogenetic analysis of the D1-D2 domains of the 25S rRNA gene and a β-tubulin gene with and without three variable introns placed the albino mutant solidly within the S. heterogama clade. These results suggest that stability of morphological traits is not a suitable criterion by itself to recognize species. The albino phenotype was a discrete and heritable mutation that became fixed in a population and was stable through time and space. In the absence of negative selection, this mutation could persist, disperse and then be misinterpreted as a new species in nature. Genetic markers expose this mutation as a population-level variant and therefore of no macroevolutionary significance. Assessment of genetic divergence amongst multiple isolates is important in ascertaining the contribution of morphological characters toward recognition of species in glomeromycotan clades., (Copyright © 2010 The British Mycological Society. Published by Elsevier Ltd. All rights reserved.)

The 3' end of the beta-tubulin gene was amplified from 50 isolates of 45 species in Glomeromycota. The analyses included a representative selection of all families except Pacisporaceae and Geosiphonaceae. Phylogenetic analyses excluded three intron regions at the same relative positions in all species due to sequence and length polymorphisms. The beta-tubulin gene phylogeny was similar to the 18S rRNA gene phylogeny at the family and species level, but it was not concordant at the order level. Species in Gigasporaceae and Glomeraceae grouped together but without statistical support. Paralogous sequences in Glomus species likely contributed to phylogenetic ambiguity. Trees generated using different fungal phyla as out-groups yielded a concordant topology. Family relationships within the Glomeromycota did not change regardless if the third codon position was included or excluded from the analysis. Multiple clones from three isolates of Scutellospora heterogama yielded divergent sequences. However, phylogenetic patterns suggested that only a single copy of the beta-tubulin gene was present, with variation attributed to intraspecific sequence divergence.