POLLEN-TUBE ATTRITION AS RELATED TO BREEDING SYSTEMS IN BRASSICACEAE

In order to estimate pollen tube attrition and to relate it to the bre eding system, numbers of microgametophytes within the stigma and at th e bottom of the style were scored in species of the Brassicaceae, 17 s porophytically self-incompatible, 11 predominant inbreeders, 4 faculta tive outcrossers. Attrition was evident in almost all species. The mea n number of tubes at the bottom of the style, and mean number of styla r 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 rati os as estimates of the degree of outbreeding in the species involved. In outbreeders, sexual selection seems to be rather effective in inhib iting certain microgametophytic genotypes from reaching the ovules. Be cause of the low number of tubes per ovule in most cases, we assume th at, in the Brassicaceae, female choice may be more effective in sexual selection and in ensuing pollen tube attrition.