Nonrandom sexual reproduction and diapausing egg production in a Daphnia hybrid species complex

Sexual reproduction in Daphnia results in the production of diapausing eggs, which are enclosed in a structure called ephippium. Ephippia, accumulated by sedimentation, can be preserved for decades and offer the opportunity for microevolutionary studies as well as the study of former pelagic populations. In a Swiss subalpine lake (Greifensee), we studied the genetic structure of the pelagic Daphnia galeata X hyalina hybrid species complex. We examined sexual females, males, and ephippia production. Eggs from ephippia were hatched, and the genotypes and taxa composition of all daphnids were determined using four polymorphic allozyme loci, two of which are each diagnostic for D. galeata and D. hyalina. We found significant differences between the genetic composition and the backcross level of pelagic asexual females, sexual females, males, and ephippial eggs (Daphnia hatchlings). The asexual daphnids were dominated by hybrids. In contrast, sexual females, especially Daphnia hatched from ephippial eggs, are dominated by D. galeata. We conclude that hybrid Daphnia have a lower sexual reproductive success than the parental D. galeata. The recent hybrid dominance suggests that D. galeata that hatch from diapausing eggs are not able to alter the pelagic population. The genotypic class composition of the diapausing eggs does not reflect the extant pelagic population; therefore, Daphnia diapausing egg banks do not always represent the past lake taxa structure.