The generic differences between Acaulospora and Entrophospora, much like those for Glomus and Sclerocystis, may not be as great as the current classification would suggest. The main property that separates the two genera is the position of the spore originating from the neck of the sporiferous saccule (a structure common to all members of the family Acaulosporaceae). Developmental studies of fungal isolates in INVAM indicate that mycorrhizal development and saccule formation is similar in both genera and that differences arise only as the spore bud begins to form laterally on (Acaulospora) or within (Entrophospora) the neck of a fully expanded saccule (see the illustration on this page). Once the spore is positioned and has expanded to its full size, then discrete successional stages in spore differentiation are identical in species of both genera. All spores differentiate a spore wall, one or two hyaline semi-flexible inner walls, and an additional bilayered innermost wall with a beaded surface. Species-level differences are the the properties of spore wall layers (color, ornamentation, reaction to Melzer’s reagent, etc.). Because of identical patterns of spore ontogenesis, we would predict convergent patterns of evolutionary change. In other words, spores of any given species in Acaulospora is likely to have an identical counterpart in spores of an Entrophospora spores in subcellular structure. With the exception of E. infrequens and A. gerdemannii, both of which possess glomus-like properties in spore formation and differentiation, such a pattern is emerging. For example, A. mellea (= A. dilatata) and E. colombiana are indistinguishable without saccules or without observing scars where the saccule neck is detached. The same applies to an undescribed Acaulospora (AU103, or WUM18 contributed by L. K. Abbott) and Entrophospora kentinensis (Wu et al., 1995). The latter is not documented photographically, because it has yet to be obtained and cultured in the collection. Acaulospora trappei and E. schenckii also may be "sister" species, but similarities have yet to be studied.

The absence of any developmental differences, except for location of the spore in relation to the neck of the saccule, suggests a small positional shift in spore formation that had no effect on all other events in spore subcellular differentiation. The relevant evolutionary (and thus systematic) question becomes: Did this shift occur once and thus lead to the evolution of the Entrophospora lineage as a monophyletic group (a group with a unique common ancestor different from that of Acaulospora) OR did this shift occur more than once, with each species evolving independently (a polyphyletic group in which each Entrophospora arose from a developmental mutation from a different Acaulospora ancestor)?

The proportion of Acaulospora to Entrophospora species is 5:1 at this time. Inadequate sampling may be partly responsible for this proportional difference, a possibility if Entrophospora species have limited (or localized) distributions due to recent evolution and concomitant smaller range of dispersal. Alternatively, the low number of Entrophospora species may be the result of a small genetic or regulatory change that converted each Acaulospora to an Entrophospora under rare circumstances (either recently or over several hundred million years). The few DNA sequences of rDNA published thus far (Simon et al., 1993) suggest a monophyletic origin, but these sequences are so highly conserved that the low number of taxa and absence of relevant comparisons could be easily misleading. Sequence comparisons are needed between "sister" taxa to determine the degree of matching between them relative to other taxa. Biogeographic data also is needed to assess distribution and range of related Acaulospora and Entrophospora species. It will be especially important to be vigilance in testing whether "sister" species can be discovered at the same location in surveys or more empirical comparative studies. Since the genus of "sister" species can be determined only from information on spore position relative to the saccule (see photos above), sporulation in pot cultures are essential (saccules are rare in field specimens) to obtain accurate identifications.

Finally, the answer to the question posed in the title of this page is a conditional yes. From an evolutionary perspective, all organismal evidence indicates that Entrophospora is an artificial genus erected because of a very small developmental difference in position of spore formation that appears bigger than it really is when viewed statically as a morphological difference. If we wish to establish a classification that is strictly phylogenetic, then species of Entrophospora should be renamed as Acaulospora species with just a descriptor explaining how the spores are formed differently. Operationally, however, this move would lead to all kinds of confusion, especially to new investigators and anyone not completely familiar with these fungi and their taxonomy. We take the view that operational clarity should supercede evolutionary pattern in this group and that anyone working with these fungi should be aware that Entrophospora species may function and behave similarly (within the boundaries of natural contigent variation) biologically and ecologically to "sister" Acaulospora species as a result of common genealogy.

Wu, Chi-Guang et. al. 1995. Mycotaxon 53: 283-294.

Simon, L., J. Bousquet, R. C. Levesque, and M. Lalonde. 1993. Nature: 67-69.