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 stress and low environmental heterogeneity. Here, we present the results from an investigation of two localities where both sexual and parthenogenetic forms of the species occur, a sandy beach at the seaside, and a copper polluted field, in Jutland, Denmark. The beach was studied over a transect of 50 meters, stretching from the edge of the water straight inland. The contaminated field was studied over a 140 meter long transect across a copper gradient spanning 20 to 3000 mg Cu/kg dry soil. At both transects we found a fundamental change in species composition of the collembolan community. In the unstressed areas M. macrochaeta constituted a minor part of the community but it was totally dominant in the stressed habitats. Also, the sex ratio changed radically at both sites. We found about 40 % males in populations from the areas characterized by severe stress and low environmental heterogeneity, while populations from the unstressed areas showed an all-female composition. The observations conflict with current hypotheses for geographic parthenogenesis and the relationship between reproductive mode and environmental heterogeneity Additionally, the genetic diversity, measured by RAPDs, was higher among the parthenogens than in the sexual population at the beach site. These findings denote that environmental heterogeneity is not a crucial factor for reproductive strategy in this case, indicating either a species-specific phenomenon, or illustrating a special form of environmental stress promoting sexual reproduction.