Print PageTo rotifers and other microscopic aquatic invertebrates, permanently wet systems allow for constant population growth, whereas the short hydroperiod in ephemeral systems interrupts activity with a forced dormancy. While conditions are favorable monogonont rotifers typically reproduce asexually allowing for rapid population growth, but they can transition to sexual reproduction. The product of sex (mixis) is a diapausing embryo that can remain dormant during pond desiccation or otherwise harsh conditions, then hatch and resume activity when water returns. As cyclically parthenogenetic zooplankton capable of varying allocation to sexual or asexual reproduction, monogonont rotifers are well positioned for examining the adaptation of reproduction and other life history traits to varying hydroperiod. Populations of the Brachionus plicatilis species complex in Spain occupy ponds known to experience different frequencies of desiccation, or pond permanence. Here we examine the hypothesis that pond permanence affects life history evolution of these monogonont rotifers by testing for correlations of hydroperiod with several aspects of fitness and reproduction.
Results/Conclusions
Permanence negatively correlated with hatching frequency of B. plicatilis diapausing embryos collected from 11 ponds of varying hydroperiod in Eastern Spain (Spearman’s rho= -0.795, P=0.006). There was no significant correlation of hydroperiod with fecundity or lifespan of asexual (amictic) females, but there were correlations with traits related to sexual reproduction. In the Brachionus plicatilis species complex, sex induction occurs when a chemical signal produced by the rotifers accumulates to threshold concentrations in the water. We found a positive correlation between the population density required to induce sex and pond permanence (Spearman’s rho= 0.236, P=0.039); thus populations from more permanent systems must reach higher densities for sex induction and are less prone to sexual reproduction. In the first quantification of neutral lipids in rotifer diapausing embryos, we report a negative correlation between lipid content and permanence (Spearman’s rho= -0.265, P=0.005). We also demonstrate a positive correlation of hydroperiod with total diapausing embryo production (Spearman’s rho= 0.291, P=0.004). Our findings provide insight for understanding how animals adapt to their environment, and suggest that hydroperiod has influenced rotifer evolution, promoting higher investment in sexual reproduction in temporary waters.
