Distance-Dependent Regulation of Stamen Number in Crosses of Scleranthus annuus (Caryophyllaceae) from a Discontinuous Population

Abstract

Scleranthus annuus is a highly inbreeding annual that has varying numbers of fertile stamens per flower. Two stamen-positions always have fully fertile stamens, whereas the other eight carry staminoids or stamens to varying degrees. I measured male expression in progeny produced by crossing individuals growing in a discontinuous population. Four types of progeny were analyzed: from self-pollinations, from cross-pollinations within a patch, from cross-pollinations between patches, and from cross-pollinations between populations. Selfed progeny showed the lowest total male fertility (25.8), followed by between-population crosses (26.7), between-patch crosses (27.4), and within-patch crosses (27.8). The effect of crossing, as measured by the relative increase in frequency of fully fertile stamens compared to selfed progeny, is highest for within-patch crosses and declines with increasing spatial separation between parents. The increase was strongest for one of the antipetalous stamen positions in progeny produced by between-patch crosses (490%). The response to crossing measured as an increase in stamen fertility was not the same for all ten stamen positions. A strong increase of fertile stamens is noted in all types of crossed progeny for the five stamen positions in the outer whorl (antipetalous stamens), positions that in selfed progeny carry staminoids. The three positions in the inner whorl that are not occupied by fully fertile stamens show varying responses to crossing.