Biosystematics of the Didymium squamulosum complex

Thirty-three isolates of the Didymium squamulosum morphospecies, from several geographic areas, were subjected to a biosystematic investigation that considered reproductive systems, plasmodial incompatibility, morphology and isozymes. Six of the isolates were heterothallic and the remaining 27 were nonheterothallic and presumably apomictic. The six heterothallic isolates represented three sexually incompatible biological species. Two of the biological species were represented by a single isolate, but the third biological species contained four isolates and displayed a multiple allelic mating series. A dendrogram derived from the isozyme difference data separated the isolates into two related groups. The first of these containing five heterothallic isolates, representing all three biological species, and a number of nonheterothallic isolates. The other containing a single heterothallic isolate, of the A2 biological species, and a number of nonheterothallic isolates. A second dendrogram, based upon morphological variations, did not correlate with the isozyme dendrogram, although several characters (peridial calcium carbonate, stalk color and plasmodial form) did conform to the two isozyme-defined groups. Thus, the D. squamulosum morphospecies is apparently a widespread species complex with large, but overlapping, morphological and genetic variations.