Capitulum characters in a seed heteromorphic plant, Crepis sancta (Asteraceae): variance partitioning and inference for the evolution of dispersal rate.

Abstract

In Crepis sancta (Asteraceae), achenes produced in the periphery of the flower head have reduced dispersal ability and are larger than achenes produced in the centre of the head, which disperse farther. The proportion of central achenes produced by a single individual represents the potential dispersal rate of its progeny. Seed variation in dispersal ability may be important where there is spatio-temporal variability of habitats, but its evolutionary significance mainly depends on the heritability of the relative proportions of each achene morph. However, the number of peripheral achenes in a capitulum, and that of involucral bracts are suggested to depend on the number of parastichies, a canalized character. From a diallel cross design, phenotypic variance for several capitulum traits was partitioned among six variance components, including the additive variance. The phenotypic values of some head traits reflected the expected frequency due to ontogeny, in particular the number of involucral bracts. Yet, this character also had a significant heritability, suggesting that variation around the mode of the distribution was not only due to developmental noise. The additive variance for number of peripheral and central achenes was not significantly different from zero. In contrast, their respective proportion had a narrow sense heritability greater than 0.20. The present results suggest that the percentage of central achenes per individual, and thus the potential dispersal rate in Crepis sancta, is under quantitative genetic control, and could undergo microevolutionary changes in natural populations.