The evolution of flower size and flowering behaviour in plants: The role of pollination and pre-dispersal seed predation
University of Southampton, School of Biological Sciences,
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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