First-pollination primacy and pollen selection in the morning glory, ipomoea purpurea

Abstract

The relationship between pollination order and the transmission of male genes to seed was investigated by performing a large number of genetically marked, double and mixed artificial pollinations in the common morning glory, Ipomoea purpurea. Second pollinations were substantially less effective than first pollinations, even when applied immediately afterwards. When delayed 30 and 60 minutes, effectiveness was reduced further to about 14 and 7 per cent of fertilisations, respectively. After 120 minutes, effectiveness decreased to less than 2 per cent. Among a set of mixed pollinations, pollen from some individuals suffered a strong disadvantage in competitive ability. This disadvantage is partially genetic. The primacy of first pollinations, and the presence of strong pollen competition, have implications for studies of pollination biology and genetic transmission in natural populations of Ipomoea purpurea and other plant species. Specifically, sequential pollination ineffectiveness, and strong competition within pollen loads, should reduce the numbers of paternal parents and thus increase the numbers of full sibs among outcross seed within capsules. This represents a substantial violation of one of the critical assumptions in the mixed mating model and could influence the genetic structure of Ipomoea populations. In addition, these data indicate that gametophytic selection may be an important component of natural selection in Ipomoea. The mechanisms underlying the sequential and competitive effects probably include pollen-stigma interactions, differential pollen production, viability differences, or pollen tube growth rate differences, and possibly embryo abortion. These experiments also establish that outcrossing rates vary among individuals in natural populations, due to genetic variation in the distance between anthers and the stigmatic surface. © 1987 The Genetical Society of Great Britain.