Abstract
Many plant species abort a large number of seeds, often producing an increase in the fitness of progeny, and a decrease in the proportion of inbred offspring. Where genetic load is caused by deleterious recessive mutations in mutation-selection balance, the removal of inbred offspring may prevent the expression of mutations as homozygotes. This would shelter the mutations from selection, and permit an increase in the genetic load relative to the case with no competition among embryos. This study employs existing models of genetic load to assess the effects of embryo competition on load. Mutations which are not expressed in the embryo (e.g. chlorophyll deficiency mutations) are sheltered from selection by the removal of inbred seed, and increase in frequency relative to the case without embryo competition. Moreover, if embryo competition is discontinued after several generations, expression of the accumulated load causes a temporary, but pronounced drop in mean fitness. By contrast, mutations which are expressed in the embryo may affect the outcome of competition, and thus experience added selection relative to the case with no competition. Genetic load at these loci may thus be reduced, although the exact outcome depends upon the expression of the mutations in heterozygotes. The mechanism of embryo competition must be known before the long-term effects of management practices which enhance embryo competition can be predicted. Even where selective abortion increases the fitness of stocks over the long term, the accumulation of additional mutations can produce a dramatic loss of fitness if selective seed abortion is discontinued. Thus, once a programme to enhance selective abortion is begun, it may be impossible to cease this programme without irreparable damage to the stocks.
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Acknowledgements
I would like to thank Ned Friedman for motivating this study, and for many useful discussions. Robin Bush and Brian Husband gave kind permission to cite unpublished results and made many helpful suggestions. This work was funded by a Postgraduate Scholarship from the Natural Sciences and Engineering Research Council of Canada.
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Latta, R. The effects of embryo competition with mixed mating on the genetic load in plants. Heredity 75, 637–643 (1995). https://doi.org/10.1038/hdy.1995.183
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DOI: https://doi.org/10.1038/hdy.1995.183