JPR Advance Access originally published online on September 30, 2004
Journal of Plankton Research 2005 27(1):37-45; doi:10.1093/plankt/fbh148
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Journal of Plankton Research Vol. 27 No. 1 © Oxford University Press 2004; all rights reserved
Delayed mixis in rotifers: an adaptive response to the effects of density-dependent sex on population growth
1 Institut Cavanilles de Biodiversitat I Biologia Evolutiva, Universitat de Valencia, A.O. 22085, Valencia 46071, Spain, 2 School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA and 3 Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
* Corresponding Author: manuel.serra{at}uv.es
Received January 22, 2004; accepted in principle August 11, 2004; accepted for publication September 14, 2004; published online September 30, 2004
In most cyclically parthenogenetic life cycles, sex is needed to produce resting stages. In several species of cyclically parthenogenetic rotifers, some generations of clones are not responsive to a density-dependent signal that triggers sexual female production. These unresponsive rotifers hatch from resting eggs and typically pass 812 generations of female parthenogenesis before becoming receptive to the mixis signal. We addressed the selection for mixis delay using a simulation model. A delay of sexual reproduction could increase population growth through parthenogenesis and thus the number of resting eggs ultimately produced. In a monomorphic population without mixis delay, we determined the optimal ratio of mictic to amictic females (mixis ratio) to be 45%, and the optimal population density threshold for induction of mictic females (mixis threshold) to be 82 rotifers L1. This mixis pattern, however, was not an evolutionarily stable strategy. A mixis ratio of 14% and threshold of 70 rotifers L1 proved to be resistant to invasion by other mixis patterns. When we gave this phenotype a mixis delay of 810 days, it could invade a population with the same mixis pattern, but lacking a mixis delay. The advantage of delaying mixis was relatively small, suggesting that a polymorphism is possible.
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