Complementary roles of photoperiod and temperature in environmental sex determination in Daphnia spp.

Daphnia spp., a keystone genus in freshwater lentic habitats, are subject to environmental sex determination wherein environmental conditions dictate offspring sex and whether they reproduce asexually or sexually. The introduction of males into a population denotes the first step in the switch from asexual parthenogenetic reproduction to sexual reproduction. We tested the hypothesis that photoperiod and temperature co-regulate male sex determination and that these environmental stimuli would activate elements of the male sex determination signaling cascade. The results revealed that photoperiod was a critical cue in creating permissive conditions for male production. Further, under photoperiod-induced permissive conditions, male sex determination was temperature dependent. The two daphnid species evaluated, Daphnia pulex and Daphnia magna, exhibited different temperature dependencies. Daphnia pulex produced fewer males with increasing temperatures between 16 and 22°C, and D. magna exhibited the opposite trend. We found consistent expression patterns of key genes along the male sex-determining signaling pathway in D. pulex independent of environmental stimuli. mRNA levels for the enzyme responsible for synthesis of the male sex-determining hormone, methyl farnesoate, were elevated early in the reproductive cycle, followed by increased mRNA levels of the methyl farnesoate receptor subunits Met and SRC Environmental conditions that stimulated male offspring production significantly increased Met mRNA levels. The results indicate that male sex determination in daphnids is under the permissive control of photoperiod and the regulatory control of temperature. Further, these environmental cues may stimulate male sex determination by increasing levels of the Met subunit of the methyl farnesoate receptor.