Pollen Aggregation in Relation to Pollination Vector

Abstract

Editor: Pamela K. DigglePremise 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.

Turbulence-induced resonance vibrations cause pollen release in wind-pollinated Plantago lanceolata L. (Plantaginaceae)

“…We propose that a biomechanical approach, focused on accelerative and resistive forces (e.g. [3]), would provide a more productive understanding of the evolutionary ecology of plant reproduction in which theory could drive hypotheses (e.g. the role of a more open corolla or a more slender stamen in the evolutionary transition to or from wind pollination).…”

“…Whereas the syndrome of floral traits commonly associated with windpollinated angiosperms includes reduced petals and sepals, feathery stigmas, smooth pollen grains, small pollen diameter, lessened pollen clumping and long thin stamens [1,2], there are many exceptions to the classic pollination syndrome (e.g. [3]) and there have been few attempts to evaluate experimentally the biomechanical significance of any of these traits. Understanding the interaction of wind and flowers in pollen release is particularly important as release dynamics determine the quantity and timing of pollen delivered into the air column [4].…”

Beetle Pollination of Cycads in the Mesozoic

“…Another fact supporting this beetle-mediated pollination is that these Cycadopites pollen grains are in multiple aggregations, which comprise 3-14 grains in the specimen (Figures 2A' and 2B-2D). Many modern entomophilous cycad pollens and pollens of insect-pollinated angiosperms adhere in large aggregations, whereas wind-pollinated, or anemophilous, pollens are dispersed as single grains, or monads [17,18]. The Burmese amber has yielded a diverse Cretaceous flora including moss, liverworts, ferns, conifers, and angiosperms, but cycads remain unknown.…”

Kin selection and the evolution of plant reproductive traits

“…Indeed, pollen aggregations are associated with pollination by specialist pollinators [3,29]. Furthermore, species pollinated by wind, independent of phylogenetic constraints, often have few or one ovule(s) and single-seeded fruits [1,30,31].…”