Biosystematics of the Physarum compressum morphospecies

Thirty-three isolates of the Physarum compressum morphospecies were investigated comparing their morphology with genetic variation in reproduction, isozymes, and plasmodial incompatibility. Only one isolate was heterothallic, while all the rest were nonheterothallic clones. A phenogram derived from isozyme banding patterns indicated that the majority of the isolates were closely related to each other. Only two isolates were separated from the rest and clustered with the outgroup isolate. These two isolates, which produced aberrant sporangia, were probably misidentified. In the 528 possible pairwise plasmodial fusion tests only three fusions occurred: one between isolates derived from the same banana hand, and the other two between isolates from adjacent regions. These last two fusions were between isolates which had somewhat different isozyme patterns. Therefore, this cosmopolitan morphospecies is probably composed of a large number of related clonal lines which are apparently derived from a relatively small number of sexual lines. Also, a morphological phenogram derived from the sporangial characteristics did not correlate with the isozyme phenogram. Apparently the morphological differences found in these isolates are mostly random genetic or developmental variations which,at the scale examined, provide little taxonomic information. However the variations found in these isolates are such that Physarum niccaraguense, which is considered to be a closely related species, can be considered to be a morphological variant of an expanded I? compressum circumscription.