PATTERNS OF FEMALE FERTILITY IN HETEROSTYLOUS PRIMULA-SIEBOLDII UNDERSEVERE POLLINATOR LIMITATION

1 In order to identify the mode and intensity of fertility selection i n a heterostylous population under severe pollinator limitation, morph composition and intra- and intermorph pollen loads, fruit and seed se t were investigated in an isolated population of the endangered, heter ostylous species Primula sieboldii. 2 Seventy-six genets were classifi ed as either of the pin or thrum morph on the basis of measurements of stigma-anther separation and relative anther height, except for one g enet that exhibited a short homostylous phenotype. There was a 1:1 rat io of pin and thrum plants in the population. 3 Measurements of stigma tic pollen load indicated that legitimate pollination (i.e. by the opp osite morph) was extremely limited. Female fertility was generally low but varied significantly among style morphs and among genets within m orphs. The pin morph showed significantly higher fertility than the th rum morph. High fruit and seed set was recorded for the short homostyl e. 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 fert ility 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.