Environmental stress and reproductive mode in Mesaphorura macrochaeta (Tullbergiinae, Collembola)

Sexual populations of the normally parthenogenetic collembolan Mesaphorura macrochaeta have been reported from localities associated with abiotic stre ss and low environmental heterogeneity. Here, we present the results from a n investigation of two localities where both sexual and parthenogenetic for ms of the species occur, a sandy beach at the seaside, and a copper pollute d field, in Jutland, Denmark. The beach was studied over a transect of 50 m eters, stretching from the edge of the water straight inland. The contamina ted field was studied over a 140 meter long transect across a copper gradie nt spanning 20 to 3000 mg Cu/kg dry soil. At both transects we found a fund amental change in species composition of the collembolan community. In the unstressed areas M. macrochaeta constituted a minor part of the community b ut it was totally dominant in the stressed habitats. Also, the sex ratio ch anged radically at both sites. We found about 40 % males in populations fro m the areas characterized by severe stress and low environmental heterogene ity, while populations from the unstressed areas showed an all-female compo sition. The observations conflict with current hypotheses for geographic pa rthenogenesis and the relationship between reproductive mode and environmen tal heterogeneity Additionally, the genetic diversity, measured by RAPDs, w as higher among the parthenogens than in the sexual population at the beach site. These findings denote that environmental heterogeneity is not a cruc ial factor for reproductive strategy in this case, indicating either a spec ies-specific phenomenon, or illustrating a special form of environmental st ress promoting sexual reproduction.