Tests of sib diversification theories of outcrossing in Impatiens capensis: Effects of inbreeding and neighbour relatedness on production and infestation

Several models of the evolution of genetic systems posit very strong frequency-dependent selection acting on small spatial scales; in such circumstances a genetically diverse sibship outperforms a genetically uniform sibship, and genes for mixis may spread in a population. Such selection regimes may derive from resource limitation and/or parasite transmission. We describe a greenhouse experiment designed to test these ideas, using the annual herb Impatiens capensis. Plants were potted in pairs; the genetic variance within pots was manipulated by using progeny from either inbred or outcrossed parents and by using either full sibs or unrelated individuals. Treatment combinations designed to increase genetic diversity resulted in greater phenotypic variance in both morphology and production, though not in the density of spider mites or whiteflies. Despite evidence of resource limitation, there was no effect of genetic diversity on productivity, nor was there an effect on infestation. These results fail to support either the sib competition or the sib contagion theory of outcrossing.     

An acarine predator-prey metapopulation system inhabiting greenhouse cucumbers

The two-spotted spider mite ( Tetranychus urticae ) is a serious pest on greenhouse cucumbers, but can be controlled by the phytoseiid predator Phytoseiulus persimilis. The two mite species exhibit considerable fluctuations in overall population densities but within acceptable limits. The system appears to be persistent at a regional (greenhouse) scale in spite of frequent local extinctions (e.g. at individual plants). Experimental evidence indicates that the mites form a metapopulation system characterized by 'shifting mosaic' dynamics. A stochastic simulation model is used to analyse the role played by dispersal in the dynamics and persistence of the system. It shows that demographic stochasticity generates sufficient endogenous 'noise' to counteract the synchronizing effect of density-dependent dispersal, provided dispersal rates are not too high and the system is not too small. Low dispersal rates, on the other hand, increase the risk of local outbreaks of spider mites that may cause destruction of plants.