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

Abstract

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 ϫ 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.

Do parasites lower Daphnia hybrid fitness?

“…In Greifensee, only a small part of the population switches to sexual reproduction; however, taxa-specific differences have been observed. Mostly D. galeata clones hatch from ephippia, whereas F 1 hybrid clones seem to be maintained by asexual reproduction (Keller and Spaak 2004).…”

“…D. galeata ϫ hyalina hybrids dominate the Daphnia population of Greifensee (Spaak et al 2001), with an average proportion of 75% during 1998-2002 (Keller and Spaak 2004). Moreover, it cannot be excluded that D. hyalina clones are backcrossed hybrids, whereas pure D. galeata occur in the lake (Spaak et al 2001).…”

Identifying hybridizing taxa within the Daphnia longispina species complex: a comparison of genetic methods and phenotypic approaches

“…Early studies used electrophoresis of one or two species-specific allozyme markers (Wolf & Mort, 1986;Gießler, 1997). Despite a small number of fixed diagnostic loci and sampling limitations imposed by the need to deep-freeze samples, this method has become widespread in studies on Daphnia hybridization (e.g., Spaak, 1996;Spaak, 1997;Gießler, 2001;Winder et al, 2001;Keller & Spaak, 2004). It still remains in use and continues to provide valuable results Keller et al, 2008;Petrusek et al, 2008b).…”

The impact of human-made ecological changes on the genetic architecture of Daphnia species

“…The induction of sexual females and males is determined by environmental factors such as the change in food level, crowding (high densities of conspecifics), and photoperiod (10). Therefore, abundances of resting eggs over time do not necessarily represent the short-term success or fitness of taxa or clonal lineages at a given time period (11,12). Instead, relative abundances of taxa or genotypes derived from resting egg banks reveal information about lineages that successfully contributed genes to the next generation (13).…”