Format: ORAL

Joseph H. Williams1

1 Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee, USA

Whole genome duplication (WGD) has well-known phenotypic effects at the cell and organismal level, with potential functional consequences for pollen - the haploid, single-celled male gametophyte phase of angiosperms. A review of species with diploid (2x) and autotetraploid (4x) cytotypes, showed 2x pollen from artificial and crop neo-autotetraploids was double the volume of 1x pollen from diploid progenitors (N = 65, 34 species, respectively). In contrast, 2x pollen of wild autotetraploid cytotypes was only 54% larger (significantly smaller-than-doubled volume; N = 54). These results suggest WGD doubles pollen volume, whereas post-WGD evolution is biased to pollen size reduction. To understand why, I asked if WGD disrupts sexual allocation patterns that reflect evolved size-number trade-offs in Galax urceolata, a species with natural diploid (2n = 2x = 12) and cryptic autotetraploid (2n = 4x = 24) populations. I measured floral organ sizes and numbers in four 2x and four 4x populations (20 plants/population), verified using flow cytometry. 2x and 4x plants had the same number of flowers and floral parts, including ovules, whereas 4x floral organs were 20-24% longer, reflecting conserved progenitor-like floral allometries. Autotetraploid anthers were 2.12 times the volume of diploid anthers, enough to accommodate doubled volume of 1x pollen numbers. Surprisingly, autotetraploid anthers produced 76% more viable pollen of 33% larger size than diploid anthers. Total per-plant sexual allocation to anther and ovule volume in 4x plants increased by 76% and 68%, respectively, but 4x plants produced 83% more male, but only 6% more female, gametes. In sum, WGD seems not to alter sexual allocation at the organ level, but instead WGD-enlarged microsporangia have enabled the rapid evolution of smaller and more neo-2x pollen, whereas WGD-enlargement of megasporangia are limited to one egg/ovule. WGD generates greater opportunity for sexual selection on male than on female gamete numbers.