Patterns of female fertility in heterostylous Primula sieboldii under pollinater limitation

1 In order to identify the mode and intensity of fertility selection in a heterostylous population under severe pollinator limitation, morph composition and intraand intermorph pollen loads, fruit and seed set were investigated in an isolated population of the endangered, heterostylous species Primula sieboldii. 2 Seventy-six genets were classified as either of the pin or thrum morph on the basis of measurements of stigma-anther separation and relative anther height, except for one genet that exhibited a short homostylous phenotype. There was a 1:1 ratio of pin and thrum plants in the population. 3 Measurements of stigmatic pollen load indicated that legitimate pollination (i.e. by the opposite morph) was extremely limited. Female fertility was generally low but varied significantly among style morphs and among genets within morphs. The pin morph showed significantly higher fertility than the thrum morph. High fruit and seed set was recorded for the short homostyle. 4 Legitimate pollination by hand resulted in a significant increase in fruit and seed set except for the homostyle where high spontaneous seed set already occurred. Intragenet pollination or illegitimate (i.e. intramorph) pollination resulted in no seed set. 5 It was concluded that pollen limitation is largely responsible for the low female fertility of plants in the population. Under severe pollinator limitation, it is possible that the self-fertile homostyle will have a selective advantage and will spread in the population.

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