Evolution of floral display in Eichhornia paniculata (Pontederiaceae): Direct and correlated responses to selection on flower size and number

Trade-offs between Rower size and number seem likely to influence the evolu tion of floral display and are an important assumption of several theoretic al models. We assessed floral trade-offs by imposing two generations of sel ection on Rower size and number in a greenhouse population of bee-pollinate d Eichhornia paniculata. We established a control line and two replicate se lection lines of 100 plants each for large flowers (S+), small flowers (S-) , and many flowers per inflorescence (N+). We compared realized heritabilit ies and genetic correlations with estimates based on restricted-maximum-lik elihood (REML) analysis of pedigrees. Responses to selection confirmed REML heritability estimates (flower size, h(2) = 0.48; daily flower number, h(2 ) = 0.10; total flower number, h(2) = 0.23). Differences in nectar, pollen, and ovule production between S+ and S- lines supported an overall divergen ce in investment per flower. Both realized and REML estimates of the generi c correlation between daily and total flower number were r = 1.0. However, correlated responses to selection were inconsistent in their support of a t rade-off. In both S- lines, correlated increases in flower number indicated a genetic correlation of r = -0.6 between flower size and number. In contr ast, correlated responses in N+ and Sf lines were not significant, although flower size decreased in one N+ line. In addition, REML estimates of genet ic correlations between flower size and number were positive, and did not d iffer from zero when variation in leaf area and age at first flowering were taken into account. These results likely reflect the combined effects of v ariation in genes controlling the resources available for flowering and gen es with opposing effects on flower size and number. Our results suggest tha t the short-term evolution of floral display is not: necessarily constraine d by trade-offs between Rower size and number, as is often assumed.