POLLEN-OVULE RATIO, POLLEN SIZE, AND THE RATIO OF STIGMATIC AREA TO THE POLLEN-BEARING AREA OF THE POLLINATOR: AN HYPOTHESIS

Abstract

The initial work on pollen-ovule ratios (P/O's) was predicated on the assumption that P/O's reflected the efficiency of pollination; the more efficient the transfer of pollen, the lower the P/O (Cruden, 1977). The observation that P/O's decrease in switches from xenogamy (cross-pollination) to autogamy (self-pollination) and autogamy to cleistogamy, without loss of fecundity is consistent with the initial assumption (Cruden, 1977). Additional support for the efficiency argument was provided by Cruden and Jensen (1979) who showed that species in Onagraceae have very low P/O's compared to species in other families with equivalent breeding systems and fecundities. Viscin threads, which hold pollen grains together in large clumps, and quite large stigmas contribute to the efficient transport and transfer of pollen in Onagraceae. The observation that xenogamous species with low P/O's (e.g., in Epilobium, Geranium and Mirabilis) have large pollen grains and those with high P/O's (various Boraginaceae) have quite small pollen grains suggested that P/O's might reflect pollen grain size, as well as number. Because pollen grain germination and penetration of the stigmatic surface by the pollen tube are a function of pollen number (Brewbaker and Majumder, 1961; Jennings and Topham, 1971) and products stored in the intine (Stanley and Linskens, 1974; Heslop-Harrison, 1975) we reasoned, with respect to germination and the contribution of gametes to successful fertilizations, that the individual pollen grains of species with large pollen grains must be more successful than the individual pollen grains of species with small pollen grains. The data presented below were gathered to test the hypothesis that P/O's are inversely related to 1) the likelihood of a pollen grain reaching a stigma and 2) pollen grain size. First, we present the underlying assumptions of our hypothesis and examine our data. Second, we discuss information in the literature on distylous species which provide an independent test of the hypothesis. We also comment briefly on resource allocation to male sexual function, a possible trade-off between pollen number and pollen grain size, and P/O's as indicators of breeding systems.