Low crowding threshold for induction of sexual reproduction and diapause in a Patagonian rotifer

Abstract

This study investigates the basis and ecological significance ofthe extremely highpropensity for mixis (sexual reproduction) observed in laboratory populations of Brachionus calyciflorus from a temporary pond in Patagonia.2. Experiments with stem females hatched from resting eggs showed that these females were exclusively amictic but produced mictic daughters even at very low population densities. In six experiments, newly hatched stem females were cultured individually in differentvolumes (1.5, 12, 40 and 150 mL). The percentage of mictic daughters (mixis ratio) was high in the smaller volumes (e.g. 44-83% in 1.5 mL) and lower in the largest volume (6-21% in 150 mL).A regression analysis combining the data from these experiments showed a highly significant decrease in mixis ratio with volume and indicated that the lowest population density at which mixis still occurs (mixis threshold) would be 3.4females L-1 (95% CL 2.9 - 4.0 females L-1).This value is considerably lower than mixis thresholds for other rotifers (25-250 females L-1 for many species and 9,000-477,000 females L-1 for some Spanish Brachionus plicatilis).3. In three additional experiments, stem females and their amictic daughters were culturedindividually in 150 mL. The percentage of mictic daughters produced by these two generations of females was not significantly different, showing that the mixis response to crowding is not inhibited in thestem-female generation.4. Laboratory experiments showed that two common predators of the Patagonian B.calyciflorus (the calanoid copepod Parabroteas sarsi and the backswimmer Notonectavereertbruggheni) each ate 60-70 B. calyciflorus predator-1 d-1 and cleared all rotifers from c. 250 mL d-1. Thus, a very low mixis threshold and high maximal mixis ratio may ensure production of some resting eggs soon after colonization of the pond and before complete removal from the plankton.5. Two laboratory experiments showed that resting eggs of the Patagonian B. calyciflorushatched at variable rates (28% and 81%) after a brief diapause when kept in the conditionsunder which they were produced and oviposited (20-21° C; L: D 16: 8). Early hatching ofresting eggs from pond sediment may allow multiple periods of colonization and resting-egg production in a season. This may offset the fitness cost of limited population growth due to female parthenogenesis in the face of unpredictable and abrupt risk of extinction due to predators.