MICTIC PATTERNS OF THE ROTIFER BRACHIONUS-PLICATILIS MULLER IN SMALL PONDS

Populations of the rotifer Brachionus plicatilis were monitored in thr ee small ponds in a marsh on the Mediterranean coast. Samples were tak en approximately every three weeks from July 1992 to November 1993. Sa linity, temperature, conductivity, pH and oxygen concentration were me asured in the field. Population density was determined from preserved quantitative samples. Individuals were classified as mictic females, a mictic females, non-ovigerous females, and males, differentiating betw een two morphotypes ('S' and 'L'). From these counts, a level of mixis was calculated. We also determined the proportion of mictic females i n natural populations by culturing females isolated from fresh samples . From these data, mictic patterns over time and correlation between l evels of mixis and environmental and population parameters were analyz ed. From a previous study 'S' and 'L' morphotypes were known to corres pond to genetically different clonal groups. Our data showed that repr oduction was predominantly parthenogenetic in these clonal groups, but mictic females were found in most samples, the proportion of mictic f emales ranging from 0 to 29%. The clonal groups showed different patte rns of mixis. L clonal group presented a continuous sexual reproductiv e pattern. In contrast, S clones showed a rather punctuated mictic pat tern. A positive correlation between levels of sexual reproduction and population density was found for S and L groups. However, they differ ed in their density threshold for mictic reproduction. The adaptive me aning of these patterns and their implications in maintaining genetic diversity within and between populations are discussed.