The evolution of flower size and flowering behaviour in plants: The role of pollination and pre-dispersal seed predation

Abstract

This thesis describes research into the effects of insect mediated pollination, and pre-dispersal seed predation, in common species of Asteraceae, varying both between and throughout growing seasons, and suggests that these exert selective pressures influencing the evolutuion of both inflorescence size and flowering phenology.
Changes in the level of infestation of capitula (by tephritid fly larvae) and in capitulum size were monitored by twice-weekly collections from populations of common species of Asteraceae over the course of five years’ flowering, in 2002 to 2006. Seed counts of drying flower heads were made over the course of the research, identifying levels of seed set in both uninfested and infested flower heads. A mathematical model was created to investigate the extended influence of the parameters measured in the field, and predict their influence on longer term evolution of Asteraceae.
Two flower species exhibited both significant levels of infestation, and significant differences in fecundity. In Leucanthemum vulgare, infestation (by Tephritis neesii) peaked mid-season at all sites. In Centaurea nigra, infestation (by gall-forming Urophora spp) peaked mid-season, infestation (by non-gall forming Chaetostomella cylindrica) was lowest in mid-season. In both species the probability of attack increased with capitulum size, despite a consistent decline in capitulum size through the season. This suggests that the insects choose larger capitula from those available at each stage, regardless of absolute size. The presence of infestation showed a consistent reduction in the level of fecundity in both species. These findings suggest a selective effect.
The mathematical model provides evidence of the long term effects, up to 1000 generations, of variations in both pollination success, and pre-dispersal seed predation, and suggests that the values measured in the field are consistent with selective pressures that contribute to the evolution of both flower size and phenology.

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