Abstract
Here we test whether genome size is a predictor of pollen size. If it were, inferences of ancient genome size would be possible using the abundant paleo-palynolgical record. We performed regression analyses across 464 species of pollen width and genome size. We found a significant positive trend. However, regression analysis using phylogentically independent contrasts did not support the correlated evolution of these traits. Instead, a large split between angiosperms and gymnosperms for both pollen width and genome size was revealed. Sister taxa were not more likely to show a positive contrast when compared to deeper nodes. However, significantly more congeneric species had a positive trend than expected by chance. These results may reflect the strong selection pressure for pollen to be small. Also, because pollen grains are not metabolically active when measured, their biology is different than other cells which have been shown to be strongly related to genome size, such as guard cells. Our findings contrast with previously published research. It was our hope that pollen size could be used as a proxy for inferring the genome size of ancient species. However, our results suggest pollen is not a good candidate for such endeavors.
Highlights
IntroductionThe variation in pollen size may stem from strong selection pressures related to pollen dispersal strategies
Pollen range in size by over three orders of magnitude [1, 2] (Figure 1)
Of the 21 families that had more than five species represented in our sample, Onagraceae had the largest mean pollen width at 113.9 μm, while Plantagi naceae had the smallest mean pollen width at 23.6 μm
Summary
The variation in pollen size may stem from strong selection pressures related to pollen dispersal strategies. Some gymnosperms have large pollen but are wind pollinated (Pinaceae and Podocarpaceae) [4]. Two air-filled sacs (sacci) facilitate wind dispersal in these groups [3, 4]. Pollen of species that use insect facilitated dispersal can sometimes be quite large, but we are not aware of any study showing that pollen dispersed by insects is generally larger than pollen dispersed abiotically. There is greater interspecific variability for pollen grain size in species that use insect dispersal [5,6,7]. Understanding what controls pollen size from a developmental perspective will enhance our understanding of the ecological significance of variation in pollen size
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