Responses of floral traits to selection on primary sexual investment in Spergularia marina: The battle between the sexes

Two widespread assumptions underlie theoretical models of the evolution of sex allocation in hermaphroditic species: (1) resource allocations to male and female function are heritable; and (2) there is an intrinsic, genetical ly based negative correlation between male and female reproductive function . These assumptions have not been adequately tested in wild species, althou gh a few studies have detected either genetic variation in pollen and ovule production per flower or evidence of trade-offs between male and female in vestment at the whole plant level. It may also be argued, however, that in highly autogamous, perfect-flowered plant taxa that exhibit genetic variati on in gamete production, strong stabilizing selection for an efficient poll en:ovule ratio should result in a positive correlation among genotypes with respect to mean ovule and mean pollen production per flower. Here we repor t the results of a three-generation artificial selection experiment conduct ed on a greenhouse population of the autogamous annual plant Spergularia ma rina. Starting with a base population of 1200 individuals, we conducted int ense mass selection for two generations, creating four selected lines (high and low ovule production per flower; high and low anther production per fl ower) and a control line. By examining the direct and correlated responses of several floral traits to selection on gamete production per flower, we e valuated the expectations that primary sexual investment would exhibit heri table variation and that resource-sharing, variation in resource-garnering ability, or developmental constraints mold the genetic correlations express ed among floral organs. The observed direct and correlated responses to sel ection on male and female gamete production revealed significant heritabili ties of both ovule and anther production per flower and a significant negat ive genetic correlation between them. When plants were selected for increas ed ovules per flower over two generations, ovule production increased and a nther production declined relative to the control line. Among plants select ed for decreased anthers per flower, we observed a decline in anther produc tion and an increase in ovule production relative to the control line. In c ontrast, the lines selected for low ovules per flower and for high anthers per flower exhibited no evidence for significant genetic correlations betwe en male and female primary investment. Correlated responses to selection al so indicate a genetically based negative correlation between the production of normal versus developmentally abnormal anthers (staminoid organs); a po sitive correlation between the production of ovules versus staminoid organs ; and a positive correlation between the production of anthers and petals. The negative relationship between male versus female primary investment sup ports classical sex allocation theory, although the asymmetrical correlated responses to selection indicate that this relationship is not always expre ssed.