Frequency-dependent male reproductive success in a polycross of Douglas fir

Frequency dependence of male reproductive success (RS) was investigated in a polycross of Douglas fir. We expect frequency-dependent RS in conifer polycrosses for two reasons. The first is the presence of "pollen chambers" in conifers, which limits the number of competing pollen grains and enhances the reproduction of poor males when at high frequency in the polymix. The second reason is the possibility of competitive interactions among pollen types, which generally gives more complicated patterns of RS. To measure frequency dependence, we performed four separate polycrosses with respective proportions of 1∶1∶1, 2∶1∶1, 1∶2∶1, and 1∶1∶2 of the same three males. Male RS was measured with isozyme markers in seed offspring, and three different estimation models were applied to the data. A general log-linear model detected frequency-dependent RS, but did not allow any specific characterization of its nature. A comparison of two more specific models, the first incorporating pollen chamber size and the second incorporating competitive interactions, indicated that competitive interactions were predominantly responsible for the observed frequency-dependent RS. The implications of frequency-dependent RS for forest tree breeding are discussed.