Pollination mode and life form strongly affect the relation between mating system and pollen to ovule ratios

Pollen to ovule (P : O) ratios have been hypothesized to correlate with the degree of outcrossing and thus with the mating system of a plant. Also, P : O ratios are likely to vary with respect to pollination mode (i.e. wind pollination or animal pollination). Furthermore, constraints on the evolution of mating systems depending on life form may affect P : O ratios. We compiled P : O ratios and outcrossing rates for 107 angiosperm species and analyzed the relation between these traits considering pollination mode, life form and phylogenetic relatedness among species. In general, P : O ratios correlated significantly with outcrossing rates. However, when taking additional factors into account, the relation became ambiguous. The correlation was significantly positive in wind-pollinated species, but only marginally so in animal-pollinated species. Wind-pollinated species had higher P : O ratios than animal-pollinated taxa. In woody perennials, outcrossing was the predominant mating system and outcrossing rates did not correlate with P : O ratios. The results were not altered by accounting for phylogenetic relatedness among species. The results indicate that P : O ratios vary more strongly with pollination mode and life form than with the mating system.