Intraspecific variation in two cosmopolitan myxomycetes, didymium squamulosum and didymium difforme (Physarales: Didymiaceae)

The myxomycetes (plasmodial slime molds or myxogastrids) are one of three groups considered to be true slime molds (class Eumycetozoa sensu Olive 1975). Two vegetative states--amoebae and plasmodia--along with a spore-producing fruiting body characterize the life cycle of the myxomycetes. These organisms are associated with decaying plant material and are found in all terrestrial habitats worldwide. A number of species are considered cosmopolitan, being found worldwide, where they are associated with a diversity of microhabitats and substrates. A review of the literature, including molecular investigations in all three groups of slime molds, is presented, and this is followed by four original studies of the intraspecific variation that exists in two cosmopolitan species of myxomycetes. Molecular intraspecific variation in these two species, Didymium squamulosum (Alb. & Schwein.) Fr. and Didymium difforme (Pers.) S.F. Gray, was investigated using DNA sequence analysis. Initially, 14 specimens of Didymium squamulosum from widely distributed localities were examined, using the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA). Although this genetic marker was found to be too variable for continued analysis, it did offer the first evidence that significant intraspecific variation exists within cosmopolitan species of myxomycetes. An additional genetic marker located within the mitochondrial small subunit (mtSSU) was investigated for 96 collections of Didymium squamulosum from worldwide localities and 56 collections of Didymium difforme distributed among three widely separated regions. For both species, conclusions were derived from molecular analyses using Bayesian methods and a haplotype network from TCS. It was concluded in both species that for this genetic marker no clear geographical assemblages emerged. While some sequences formed groups based on biogeography, there were a number of instances in which sequences from specimens that originated from distant geographical localities were more closely related to each other than to sequences from specimens obtained in nearby localities. In Didymium squamulosum, four morphological characters were observed for each collection and mapped onto the gene tree produced using Bayesian methods. While this species is known to have great diversity in morphology, no patterns emerged which would suggest that observed morphological diversity was related to molecular variation. This is the first molecular evidence that morphological diversity in a cosmopolitan species of myxomycete is the result of phenotypic plasticity rather than genetic divergence. Further evidence for phenotypic plasticity was obtained from an effort to culture each specimen of Didymium squamulosum spore-to-spore on agar, which resulted in only two successful cultures. In both cases, the fruiting bodies exhibited a degree of variation in morphological diversity that was different from the original specimen that had developed under natural conditions in the field.