XVI International Botanical Congess
Trade-offs between flower size and number seem likely to influence the evolution of floral display, but are supported by limited empirical evidence. We imposed two generations of selection on flower size and number in Eichhornia paniculata. We established a control line, and two replicate selection lines of 100 plants each for large (S+), small (S-), and many (N+) flowers. Direct responses to selection confirmed earlier heritability estimates. In S- lines, correlated increases in flower number were consistent with a genetic correlation between flower size and number of -0.78. No correlated decreases in flower number or size occurred in either S+ or N+ lines. Asymmetrical correlated responses are common and theoretical work suggests they may result from negative pleiotropy. Our results are therefore consistent with the assumption that genes with negative pleiotropic effects (trade-offs) influence floral evolution. However, asymmetrical responses and earlier evidence that genes enhancing resource acquisition increase both flower size and number suggest that trade-offs may not always constrain floral evolution.