Daphnia uses its circadian clock for short-day recognition in environmental sex determination

Some organisms have developed a mechanism called environmental sex determination (ESD), which allows environmental cues, rather than sex chromosomes or genes, to determine offspring sex.1,2,3,4 ESD is advantageous to optimize sex ratios according to environmental conditions, enhancing reproductive success.5,6 However, the process by which organisms perceive and translate diverse environmental signals into offspring sex remains unclear. Here, we analyzed the environmental perception mechanism in the crustacean, Daphnia pulex, a seasonal (photoperiodic) ESD arthropod, capable of producing females under long days and males under short days.7,8,9,10 Through breeding experiments, we found that their circadian clock likely contributes to perception of day length. To explore this further, we created a genetically modified daphnid by knocking out the clock gene, period, using genome editing. Knockout disrupted the daphnid's ability to sustain diel vertical migration (DVM) under constant darkness, driven by the circadian clock, and leading them to produce females regardless of day length. Additionally, when exposed to an analog of juvenile hormone (JH), an endocrine factor synthesized in mothers during male production, or subjected to unfavorable conditions of high density and low food availability, these knockout daphnids produced males regardless of day length, like wild-type daphnids. Based on these findings, we propose that recognizing short days via the circadian clock is the initial step in sex determination. This recognition subsequently triggers male production by signaling the endocrine system, specifically via the JH signal. Establishment of a connection between these two processes may be the crucial element in evolution of ESD in Daphnia.

Sex ratio effects on reproductive success of male and female Daphnia

The intensity of mating competition and the opportunity for sexual selection are thought to depend on the operational sex ratio, the ratio of sexually active males to fertilizable females. Cyclic parthenogens, organisms that alternate between sexual reproduction and female-only parthenogenesis, show particularly high variation in sex ratios in natural populations but the effects of this variation on mating competition and reproductive success of each sex are poorly understood. In a series of experiments with Daphnia magna, we experimentally imposed five sex ratio categories, varying from one male per 81 females to an even sex ratio. We found that, in males, reproductive success strongly and monotonically decreased with decreasing number of females per male. In females, in contrast, mating success and reproductive success were reduced only at the most female-biased sex ratio (1:81), when many females remained unmated and unfertilized, and then again at equal sex ratios, probably due to negative effects of high density or stress induced by numerous males. Our results suggest that females experienced male limitation at heavily female-biased sex ratios below one male to about 50 females. As this is well within the sex ratio variation observed in natural Daphnia populations, we conclude that mating competition and the opportunity for sexual selection may exist not only in males but, at least periodically, also in females.

Allele Sharing and Evidence for Sexuality in a Mitochondrial Clade of Bdelloid Rotifers

Rotifers of Class Bdelloidea are common freshwater invertebrates of ancient origin whose apparent asexuality has posed a challenge to the view that sexual reproduction is essential for long-term evolutionary success in eukaryotes and to hypotheses for the advantage of sex. The possibility nevertheless exists that bdelloids reproduce sexually under unknown or inadequately investigated conditions. Although certain methods of population genetics offer definitive means for detecting infrequent or atypical sex, they have not previously been applied to bdelloid rotifers. We conducted such a test with bdelloids belonging to a mitochondrial clade of Macrotrachela quadricornifera. This revealed a striking pattern of allele sharing consistent with sexual reproduction and with meiosis of an atypical sort, in which segregation occurs without requiring homologous chromosome pairs.