Sex-allocation trade-offs have long been invoked as a primary factor underlying the evolution of separate sexes and the reduction of pollen production accompanying the evolution of selfing. In the present study, I conducted stamen and style removal experiments to explore the existence of such trade-offs in Nigella sativa, a hermaphroditic plant species whose flower structure allows early manipulation of both male and female function. Plants on which all stamens were removed at the bud stage had a higher rate of flower initiation and produced significantly heavier seeds than did plants whose flowers remained intact, apparently by using resources that were released when the stamens were removed. However, there was no effect of stamen removal on the number of flowers that reached anthesis, the total biomass allocated to seed production, or the vigour of plants in the progeny generation. In contrast, prevention of fruit production ( style removal) increased the amount of biomass invested in stamen by 57% relative to plants whose flowers were allowed to set fruit. These observations verify the existence of a sexual trade-off in N. sativa but also raise the possibility that stamen-suppressing mutations sometimes lack the pleiotropic consequences of increasing female function, at least in species with large, expensive fruits.
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