Intraspecific Variation in Responsiveness to Juvenile Hormone May Underlie Differences in Diapause Strategy Among Daphnia cf. pulex Clones

Daphnids survive unsuitable conditions by producing resting eggs. Resting eggs contribute to population persistence and coexistence by enabling temporal dispersal strategies. Two genotypes of asexual <i>Daphnia</i> cf. <i>pulex</i> (JPN1 and JPN2) differ in the frequency with which they produce resting eggs, despite occurring simultaneously in the same lake. To clarify molecular mechanisms underlying differences in diapause frequency, we hypothesized that juvenile hormone (JH) is involved in resting egg production in <i>Daphnia</i> and that variation in resting egg production between genotypes may reflect differences in responsiveness to JH. We exposed these two <i>D</i>. cf. <i>pulex</i> genotypes to JH analogs and methyl farnesoate (MF), which act on the JH receptor. We found that JH analogs and MF exposure increased resting egg production and did not induce male offspring in the JPN2 genotype. However, in the JPN1 genotype, JH exposure induced male offspring, despite slightly increased resting egg production. Our findings suggest that variation in resting egg production between these two genotypes is partly explained by differences in responsiveness to endogenous JH, potentially influencing population coexistence and dynamics. Finally, we determined that the JH-sensitive period which induces resting egg production occurs about two instars before egg holding. This is slightly earlier than the JH-sensitive period for male production.