Abstract
Clonal isolates of Closterium ehrenbergii Meneghini ex Ralfs from a cold stream in Denmark were studied morphologically and cytogenetically and confirmed as belonging to mating group P. Variation in cell size appeared to be considerable for a single natural population, compared to the full range of morphological variation in a single mating group (biological species) known for Groups A, B, Hand P. Based on sexual compatibility, F1 viability and mating-type inheritance tests in almost all possible combinations between four strains, it was found that 2 of the 4 mating-type plus strains contained a zygotic defective factor. This suggests that Group P is a recently developed high-polyploid species. Alternatively, however, it is more likely that asynaptic mutations can be maintained rather easily in natural populations of Group P which are rarely subjected to conditions unfavourable for vegetative growth, as compared to other mating groups inhabiting temporary shallow ponds and paddy fields. The great morphological variation observed in a single natural population is discussed in the light of the unique ecological and cytotaxonomic characteristics of Group P.
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