Evolution of Floral Display in Eichhornia Paniculata (Pontederiaceae): Direct and Correlated Responses to Selection on Flower Size and Number

Abstract

Trade-offs between flower size and number seem likely to influence the evolution of floral display and are an important assumption of several theoretical models. We assessed floral trade-offs by imposing two generations of selection on flower size and number in a greenhouse population of bee-pollinated Eichhornia paniculata. We established a control line and two replicate selection lines of 100 plants each for large flowers (S+), small flowers (S-), and many flowers per inflorescence (N+). We compared realized heritabilities and genetic correlations with estimates based on restricted-maximum-likelihood (REML) analysis of pedigrees. Responses to selection confirmed REML heritability estimates (flower size, h2 = 0.48; daily flower number, h2 = 0.10; total flower number, h2 = 0.23). Differences in nectar, pollen, and ovule production between S+ and S- lines supported an overall divergence in investment per flower. Both realized and REML estimates of the genetic correlation between daily and total flower number were r = 1.0. However, correlated responses to selection were inconsistent in their support of a trade-off. In both S- lines, correlated increases in flower number indicated a genetic correlation of r = -0.6 between flower size and number. In contrast, correlated responses in N+ and S+ lines were not significant, although flower size decreased in one N+ line. In addition, REML estimates of genetic correlations between flower size and number were positive, and did not differ from zero when variation in leaf area and age at first flowering were taken into account. These results likely reflect the combined effects of variation in genes controlling the resources available for flowering and genes with opposing effects on flower size and number. Our results suggest that the short-term evolution of floral display is not necessarily constrained by trade-offs between flower size and number, as is often assumed.

Rapid response to artificial selection on flower size in Phlox

“…For example, if the magnitude of response were maintained over generations, this would have taken about eight generations for P. cuspidata, because the average difference in flower diameter between P. drummondii and P. cuspidata is about 9 mm. Responses of similar magnitude were reported by Worley and Barrett (2000) who selected on flower area in Eichhornia paniculata. After two generations of bi-directional selection, flower size increased by 18%, and decreased by 13%.…”

“…The first such selection program was conducted by Worley and Barrett (2000). Closely related floral characters other than corolla diameter, such as calyx diameter (Meagher, 1994) and petal area were also shown to respond to selection for more than two generations.…”

Artificial selection shifts flowering phenology and other correlated traits in an autotetraploid herb

“…To produce the G 1 generation, each plant was crossed to three other members of its selected group acting either as a male (pollen donor) in two crosses and a female (pollen recipient) in one cross or as a male once and a female twice. This design resulted in 30 families per selection line (see Worley and Barrett, 2000 for a similar design). Because C. americanum is self-compatible and protandrous, flowers used as pollen recipients were emasculated while in the male phase.…”

“…It has been used to elucidate patterns of heritability for numerous traits (Falconer and Mackay, 1996;Roff, 1997;Brakefield, 2003;Conner, 2003;Fry, 2003) and their correlated responses (Worley and Barrett, 2000;Sarkissian and Harder, 2001;Geber and Griffen, 2003;Delph et al, 2004). However, response to selection in C. americanum may differ from these studies because it is an autotetraploid.…”

Adaptive plasticity of floral display size in animal-pollinated plants

“…Nevertheless, the morphology of individual flowers is remarkably constant (e.g. MacDonald et al 1988;Worley & Barrett 2000;Herrera 2001), perhaps reflecting the advantages of maintaining the mechanical fit between flower and pollinator (e.g. Cresswell 2000).…”