Pollen tube attrition as related to breeding systems in Brassicaceae

In order to estimate pollen tube attrition and to relate it to the breeding system, numbers of microgametophytes within the stigma and at the bottom of the style were scored in species of the Brassicaceae, 17 sporophytically self-incompatible, 11 predominant inbreeders, 4 facultative outcrossers. Attrition was evident in almost all species. The mean number of tubes at the bottom of the style, and mean number of stylar tubes per ovule were significantly lower in outbreeders. The degree of attrition, measured as "attrition rate" (the proportion of tubes in the stigma that do not reach the bottom of the style) was higher in outbreeders; it was positively correlated with the pollen : ovule ratios as estimates of the degree of outbreeding in the species involved. In outbreeders, sexual selection seems to be rather effective in inhibiting certain microgametophytic genotypes from reaching the ovules. Because of the low number of tubes per ovule in most cases, we assume that, in the Brassicaceae, female choice may be more effective in sexual selection and in ensuing pollen tube attrition.