Macrogeographic patterns of breeding system diversity in the Daphnia pulexgroup from the United States and Mexico

Studies on the biogeographical patterning of reproductive systems promise t o extend understanding of the factors which modulate breeding system transi tions. Two closely allied cladoceran crustaceans, Daphnia pulex and D. puli caria, show varied modes of reproduction, with populations reproducing by e ither cyclic or obligate parthenogenesis. Prior studies have provided a det ailed understanding of their breeding system diversity in the polar and col d temperate regions of North America. The present investigation extends thi s analysis, characterizing breeding systems and clonal diversity at sites t hroughout the United States and Mexico. Genotypic diversity in these southe rn areas was high, but only diploids were detected, indicating that polyplo ids are restricted to the north. F-1 hybrids and their two parental species were present in most areas, although their frequencies varied geographical ly. Hybrids invariably reproduced by obligate asexuality, but both parental taxa showed regional shifts in their breeding system. The complexity of th ese latter patterns suggests that they reflect the interplay of historical factors and selection.