Responses to predation pressure involve similar sets of genes in two divergent species of Daphnia.

Species that are not closely related can express similar inducible traits, but molecular mechanisms underlying the observed responses are often unknown, nor is it known if these mechanisms are shared between such species. Here, we compared transcriptional profiles of two Daphnia species (D. mitsukuri and D. sinensis) from different subgenera, at both juvenile and adult developmental stages. Both species were exposed to the same predation threat (fish kairomones), and both showed similar induced morphological changes (reduced body length). At the early developmental stage, response to predation risk resulted in similar changes in expression levels of 23 orthologues in both species. These orthologues, involved in 107 GO categories, changed in the same direction in both species (over- or underexpressed), in comparison to non-exposed controls. Several of these orthologues were associated with DNA replication, structural constituents of cuticle or innate immune response. In both species, the differentially expressed (DE) genes on average had higher ω (d /d ) values than non-DE genes, suggesting that these genes had experienced greater positive selection or lower purifying selection than non-DE genes. Overall, our results suggest that similar suites of genes, responding in similar ways to predation pressure, have been retained in Daphnia for many millions of years.