Population Ecology of Heterostyle and Homostyle Primula Vulgaris: Growth, Survival and Reproduction in Field Populations

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Abstract
(1) Homostyle primroses are self-fertile and are also able to fertilize heterostyles with the pin morph. In theory these characteristics should give homostyles an advantage over heterostyles and homostyly should spread. In fact primrose populations with significant numbers of homostyles are rare in Britain and the frequency of homostyles in them has not noticeably increased during forty years of observation. (2) In search of an ecological explanation for the homostyle handicap, growth, survival and reproduction of heterostyle (pin and thrum) and homostyle primroses were compared in natural field populations, and the pollination of heterostyles was tested in a glasshouse experiment. (3) No significant difference between morphs was found in the size of flowering plants in the field, or in their rate of clonal expansion when grown in pots. In two successive years in the field homostyles survived significantly better than thrums but were similar to pins. (4) In the glasshouse, heterostyle primroses only set seed when caged with moths. Seed set was significantly lower in thrums than in pins when the pollinators were micromoths. Pins set seed equally in the presence of large or small moths, and homostyles equally in the presence or absence of any moths. (5) Levels of flower damage in the field were low and did not differ between morphs. In two out of three years thrums had significantly more flowers plant-1 than homostyles. Damage to seed capsules occurred but did not differ between morphs. (6) Homostyles produced more seeds capsule-1 and plant-1 than heterostyles in 1983 but thrums out-produced pins and homostyles in 1984. (7) A significant negative relationship between seed number capsule-1 and mean seed weight occurred in all three morphs. This relationship could mean that in years of poor pollination when heterostyle seed set is low, heterostyle seeds will be significantly larger than homostyle seeds. If seed size is more important than seed number in determining the fitness of primrose morphs, the trade-off between seed size and seed number could handicap homostyles on the common occasions when heterostyle seed set is pollinator-limited.