Seasonal variation as a determinant of population structure in rotifers reproducing by cyclical parthenogenesis

Abstract

Monogonont rotifers live in habitats that display extensively variation in both biotic and abiotic components. Much of this variation is seasonal and therefore predictable for a given pond or lake. In 1972, King proposed one physiological and two genetic models presenting alternative modes of adaptation to this temporal variation. Our purpose in the present paper is to review and evaluate how our knowledge of the seasonal structure of rotifer populations has changed in the past 25 years. Seasonal changes in clone frequencies have been reported from three studies of natural populations using electrophoretic analysis of isozymes. In one of these studies there was evidence for substantial temporal overlap of multilocus genotypes suggesting that these clones were broad-niched generalists. By contrast, both the genetic and ecological analyses in the other two studies support a non-overlap model in which clonal groups are composed of narrow-niched specialists that undergo seasonal succession. In both of these studies the clonal groups appear to have achieved the status of sibling species, a phenomenon that we conclude is probably common in monogonont rotifers. Strong competition promotes reproductive isolation between successive groups of seasonal specialists. The existence of this competition has been inferred from natural populations and demonstrated by studies in the laboratory. Also required, and also supported by field observations, is a temporal separation of periods of mictic (sexual) reproduction. A final requirement of the nonoverlap model is seasonal variation in the timing of resting egg hatching. That is, clones established from hatching of resting eggs must enter a physiologically appropriate habitat if they are to increase in number and achieve a competitive advantage. Unfortunately, we still have little information on this topic. Finally, we present the results of a study analyzing the effects of variation in the mictic ratio (i.e., the relative frequency of mictic females) on the adaptive structure of rotifer populations. Mixis may shift the balance between costs and benefits of specialization thereby producing seasonally specialized populations that overlap in space but not time. Life history patterns may therefore provide fundamental insights on the adaptation of rotifers to the extensive temporal variation in their environments.