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

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 adapta tion to this temporal variation. Our purpose in the present paper is to rev iew and evaluate how our knowledge of the seasonal structure of rotifer pop ulations has changed in the past 25 years. Seasonal changes in clone freque ncies have been reported from three studies of natural populations using el ectrophoretic analysis of isozymes. In one of these studies there was evide nce for substantial temporal overlap of multilocus genotypes suggesting tha t these clones were broad-niched generalists. By contrast, both the genetic and ecological analyses in the other two studies support a non-overlap mod el in which clonal groups are composed of narrow-niched specialists that un dergo seasonal succession. In both of these studies the clonal groups appea r to have achieved the status of sibling species, a phenomenon that we conc lude 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 population s and demonstrated by studies in the laboratory. Also required, and also su pported by field observations, is a temporal separation of periods of micti c (sexual) reproduction. A final requirement of the nonoverlap model is sea sonal variation in the timing of resting egg hatching. That is, clones esta blished from hatching of resting eggs must enter a physiologically appropri ate habitat if they are to increase in number and achieve a competitive adv antage. Unfortunately, we still have little information on this topic. Fina lly, we present the results of a study analyzing the effects of variation i n the mictic ratio (i.e., the relative frequency of mictic females) on the adaptive structure of rotifer populations. Mixis may shift the balance betw een costs and benefits of specialization thereby producing seasonally speci alized populations that overlap in space but not time. Life history pattern s may therefore provide fundamental insights on the adaptation of rotifers to the extensive temporal variation in their environments.