Allochronic divergence and clonal succession: two microevolutionary processes sculpturing population structure of Brachionus rotifers

The life cycle of cyclical parthenogens provides a unique model for the study of spatiotemporal microevolutionary processes at ecologically relevant time scales. Considerable effort has been invested to understand spatial population structure, but critical information is still missing on temporal population divergence. This study aims to provide insights toward the latter direction by investigating the year-round species and clonal variation patterns of the cyclical parthenogenetic rotifers of the genus Brachionus. For this, a nuclear SSR genetic assay with environmental measurements in sequential population samples from an inland lake (Lake Koronia, Greece) was used. Species assignment was based on a combination of mitochondrial RFLPs (COI, 16S rRNA) and phenotypic characters. Two species pairs succeeded each other, namely Brachionus plicatilis sensu stricto (s.s.)/B. sp. Austria and B. ibericus/B. dimidiatus. In B. plicatilis s.s., a strong positive and significant correlation between genetic divergence of temporal samples and time (r = 0.85, P < 0.05) was observed, resulting in high levels of population differentiation (F (ST) > 0.3). We also observed a progressive succession between two clonal groups of B. plicatilis s.s. There was further a marked discrimination of the abundance of cryptic species by temperature and an association between temperature and the abundance of the two B. plicatilis s.s. clonal groups was also noticed. Our results provide evidence for ecological specialization at both the inter- and the intraspecific levels. High allochronic population divergence within relatively short periods of time underlines the microevolutionary role of clonal selection behind these observations.