Pollen source affects female reproductive success and early offspring traits in the rare endemic plant Polemonium vanbruntiae (Polemoniaceae)

Abstract

Understanding the relative magnitudes of inbreeding and outbreeding depression in rare plant populations is increasingly important for effective management strategies. There may be positive and negative effects of crossing individuals in fragmented populations. Conservation strategies may include introducing new genetic material into rare plant populations, which may be beneficial or detrimental based on whether hybrid offspring are of increased or decreased quality. Thus, it is important to determine the effects of pollen source on offspring fitness in rare plants. We established pollen crosses (i.e. geitonogamous-self, autonomous-self, intrasite-outcross, intersite-outcross and open-pollinated controls) to determine the effects of pollen source on fitness (seeds/fruit and seed mass) and early offspring traits (probability of germination, number of leaves, leaf area and seedling height) in the rare plant Polemonium vanbruntiae. Open-pollinated, intrasite-outcross and geitonogamous-self treatments did not differ in fitness. However, plants receiving autonomous-self pollen had the lowest fitness and the lowest probability of seed germination. Intersite-outcross plants contained fewer seeds/fruit, but seeds germinated at higher frequencies and seedlings were more vigorous. We also detected heterosis at the seed germination stage. These data may imply that natural populations of P. vanbruntiae exhibit low genetic variation and little gene flow. Evidence suggests that deleterious alleles were not responsible for reduced germination; rather environmental factors, dichogamy, herkogamy and/or lack of competition among pollen grains may have caused low germinability in selfed offspring. Although self-pollination may provide some reproductive assurance in P. vanbruntiae, the result is a reduction in germination and size-related early traits for selfed offspring.