POLLEN AGGREGATION IN RELATION TO POLLINATION VECTOR

Premise of research. Angiosperms possess pollen dispersal units (PDUs) of varying size, from monads (single grains) to aggregates containing thousands of grains. It has been suggested that the degree of aggregation is related to the dispersal agent (in particular, animals vs. wind), but aggregation has rarely been measured, and its correlation with pollination vectors has been examined for only a few species. Methodology. Assuming a lognormal distribution, the expected distribution for a random disaggregation process, we tested the hypothesis that the distribution of PDU sizes depends on the pollination vector, using 32 anemophilous and zoophilous species. We also examined intraspecific variation in the lognormal parameter values, using 30 different individuals of the common weed Plantago lanceolata. Pivotal results. The mean and standard deviation of the lognormal distribution of PDUs and a third parameter, the proportion of solitary monads, all separated the species by pollination vector, with the mean slightly outperforming the other two metrics. The majority of species (75%) had a PDU distribution that was not significantly different from a lognormal expectation, as did most P. lanceolata individuals (57%). The proximate reason for the lack of fit of species with the lognormal was the overrepresentation of monads for many of the anemophilous species; a monad (1.0) represents the lower bound for a disaggregation process, and the lower limit for the lognormal is 0. Interestingly, the two Plantago species examined, thought by some authors to be ambophilous (using both wind and animals for dispersal), had lognormal parameter values intermediate between those for the animal-and wind-pollinated species. Conclusions. Our results indicate that aggregate size is a promising quantitative measure for distinguishing pollination vectors. Future work should focus on other factors governing PDU size, such as relative humidity and time since anther dehiscence, use a common methodology for generating disaggregation, and focus on putative ambophiles to ascertain whether they do indeed have a degree of aggregation intermediate between those of species whose pollen is dispersed solely by wind and species whose pollen is dispersed by animals.