Evolution of floral display in Eichhornia paniculata (Pontederiaceae): genetic correlations between flower size and number

The evolution of floral display is thought to be constrained by trade-offs between the size and number of flowers and inflorescences. We grew in the g lasshouse 60 maternal families from each of two Brazilian populations of th e annual herb, Eichhornia paniculata. We measured flower size, daily flower number, and total flower number per inflorescence, and two indices of modu le size, leaf area and age at flowering. We also assessed the size and numb er of inflorescences produced over 6 weeks. All floral traits exhibited sig nificant heritable variation, some of which was due to genetic variation in module size. Genetic (maternal family) correlations between daily and tota l flower number did not differ from 1.0, indicating that display size (dail y flower number) cannot evolve independently from total flower number per i nflorescence. Genetic correlations between flower size and daily flower num ber ranged from negative to positive (r = -0.78 to +0.84), depending on pop ulation and inflorescence. Positive correlations occurred when variation in investment per inflorescence was high so that some families produced both larger and more flowers. These correlations became zero when we controlled for variation in module size. Families that flowered later produced fewer, larger inflorescences (r=-0.33,-0.85). These data support theoretical predi ctions regarding the combined effects of variation in resource acquisition and allocation on traits involved in trade-offs, and they emphasize the hie rarchical organization of floral displays. Our results imply that patterns of resource allocation among inflorescences influence evolutionary changes in flower size and number per inflorescence.