High Environmentally Induced Plasticity in Spore Size and Numbers of Nuclei per Spore in Physarum albescens (Myxomycetes).

Spore size enables dispersal in plasmodial slime molds (Myxomycetes) and is an important taxonomic character. We recorded size and the number of nuclei per spore for 39 specimens (colonies of 50-1000 sporocarps) of the nivicolous myxomycete Physarum albescens, a morphologically defined taxon with several biological species. For each colony, three sporocarps were analyzed from the same spore mount under brightfield and DAPI-fluorescence, recording ca. 14,000 spores per item. Diagrams for spore size distribution showed narrow peaks of mostly uninucleate spores. Size was highly variable within morphospecies (10.6-13.5 µm, 11-13%), biospecies (3-13%), even within spatially separated colonies of one clone (ca. 8%); but fairly constant for a colony (mean variation 0.4 µm, ca. 1.5%). ANOVA explains most of this variation by the factor locality (within all colonies: 32.7%; within a region: 21.4%), less by biospecies (13.5%), whereas the contribution of intra-colony variation was negligible (<0.1%). Two rare aberrations occur: 1) multinucleate spores and 2) oversized spores with a double or triple volume of normal spores. Both are not related to each other or limited to certain biospecies. Spore size shows high phenotypic plasticity, but the low variation within a colony points to a strong genetic background.