Abstract
According to the “temporal hybrid superiority hypothesis”, seasonal variability in environmental factors in temperate lakes gives hybrid clones within the D. longispina complex a temporary fitness advantage, thus allowing long-term, dynamic coexistence of hybrids and maternal taxa. However, the maintenance of hybrids would not require their superiority under any given set of environmental conditions if their average fitness over longer periods surpassed that of more specialized and less flexible parental clones. Phenotypic plasticity and developmental noise of several hybrid and maternal clones of Daphnia (Daphnia galeata, Daphnia hyalina, their hybrids and backcrosses) were compared in a series of laboratory experiments. Changes in depth selection and body size at first reproduction were scored in Daphnia exposed to predator (planktivorous fish) threat, to the presence of filamentous cyanobacteria (Cylindrospermopsis raciborskii), and to the presence of toxic compounds (PCB52 and PCB153). The hybrid clones were found to exhibit the broadest phenotypic plasticity of the studied traits in response to the different stress factors. Developmental noise in depth selection behaviour was the lowest in Daphnia galeata, the highest in Daphnia hyalina, and intermediate in hybrid and backcross clones. This diversity of reaction norms might permit the coexistence of closely related Daphnia clones in the variable and often unpredictable lake environment.
Highlights
Cladocerans from the Daphnia longispina complex are common in European lowland lakes (Wolf and Mort 1986; Schwenk and Spaak 1997; Thielsch et al 2017)
We show that within the D. longispina group, differences in phenotypic plasticity and developmental noise exist between parental species and hybrids, which may be the basis for the reported temporal dominance and permanent coexistence of the taxa
The hybrid Daphnia within D. longispina species complex were found to have greater phenotypic plasticity than the parental species, which may be related to their overall higher heterozygosity compared to the parent species (Griebel et al 2016)
Summary
Cladocerans from the Daphnia longispina complex are common in European lowland lakes (Wolf and Mort 1986; Schwenk and Spaak 1997; Thielsch et al 2017). The idea that seasonal variability in predation (Spaak and Hoekstra 1997; Spaak et al 2000), parasite pressure (Wolinska et al 2006), food quality (Brzezinski and Von Elert 2007) and other environmental factors gives a temporary fitness advantage to either parental species or hybrids within the D. longispina complex is a cornerstone of the ‘‘temporal hybrid superiority hypothesis’’ (Spaak and Hoekstra 1995), which explains the long-term dynamic coexistence of the taxa. The ‘‘general-purpose genotype’’ and ‘‘frozen niche-variation’’ concepts are often considered as mutually exclusive, viewing clonal lineages as either generalists or specialists. Both are well supported in ecological and phylogeographic studies.
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