Local adaptation of sex induction in a facultative sexual crustacean: insights from QTL mapping and natural populations of Daphnia magna

Abstract

Dormancy is a common adaptation in invertebrates to survive harsh conditions. Triggered by environmental cues, populations produce resting eggs that allow them to survive temporally unsuitable conditions. Daphnia magna is a crustacean that reproduces by cyclical parthenogenesis, alternating between the production of asexual offspring and the sexual reproduction of diapausing eggs (ephippia). Prior to ephippia production, males (necessary to ensure ephippia fertilization) are produced parthenogenetically. Both the production of ephippia and the parthenogenetic production of males are induced by environmental factors. Here, we test the hypothesis that the induction of D. magna resting egg production shows a signature of local adaptation. We postulated that Daphnia from permanent ponds would produce fewer ephippia and males than Daphnia from intermittent ponds and that the frequency and season of habitat deterioration would correlate with the timing and amount of male and ephippia production. To test this, we quantified the production of males and ephippia in clonal D. magna populations in several different controlled environments. We found that the production of both ephippia and males varies strongly among populations in a way that suggests local adaptation. By performing quantitative trait locus mapping with parent clones from contrasting pond environments, we identified nonoverlapping genomic regions associated with male and ephippia production. As the traits are influenced by two different genomic regions, and both are necessary for successful resting egg production, we suggest that the genes for their induction co-evolve.

Genetic architecture of resistance in Daphnia hosts against two species of host-specific parasites

“…This F2 panel is described in full detail in earlier publications (Routtu et al, 2010;Roulin et al, 2013) (summary at http://evolution.unibas.ch/ebert/research/qtl/index.htm). In short, two parental strains, Iinb1 and Xinb3 from Germany and Finland, respectively, were crossed to form an F1 hybrid clone.…”

“…The presence of spores, in these animals, indicates successful vertical transmission. the SNP array linkage map used can be found in Routtu et al (2010), Roulin et al (2013) and Routtu et al, submitted. For the QTL analysis, genome-wide significance level was established using 10 000 permutation tests with significant (α = 0.05) and suggestive (α = 0.1) LOD scores of 3.78 and 3.44, respectively. Analysis of variance was used to estimate the proportion of total variance explained by the fitted models.…”

“…The current study used 195 F2 clones. The clones of the F2 panel were previously genotyped for 109 VNTR markers (Routtu et al, 2010) and later for 1324 SNP markers (Routtu et al, submitted), which were used to construct genetic maps for the phenotypic traits (Routtu et al, 2010;Roulin et al, 2013).…”

“…The QTL F2 panel used in this study originates with the crossing of a Finnish D. magna clone, susceptible to both P. ramosa and H. tvaerminnensis, and a German D. magna clone resistant to both parasites. This F2 panel is kept clonally in the laboratory and has previously been used to map other D. magna traits, such as recessive deleterious mutants, induction of males and production of resting eggs (Routtu et al, 2010;Roulin et al, 2013). Here, our aim was to use this panel to elucidate the number of QTL in the host genome that contribute to resistance against P. ramosa and H. tvaerminnensis, test for interactions among QTLs (epistasis) and test for colocalization of QTLs.…”

Salinity and depth as structuring factors of cryptic divergence in Moina brachiata (Crustacea: Cladocera)

“…In addition, hydroperiod also selects for differences in the timing of sexual reproduction: species that are adapted to ephemeral habitats may be much more sensitive to switch to sexual reproduction when the volume of the habitat is decreasing (Roulin et al 2013). While we picked up depth and salinity as important factors influencing the occurrence of different M. brachiata clades, part of this impact may be mediated by biotic interactions like predation, which are associated with hydroperiod (Wellborn et al 1996).…”