Abstract Detail

Evolutionary Developmental Biology (Evo-Devo)

Evidence for lower and upper limits on pollen size evolution after autopolyploidy.

Intraspecific autopolyploids are often cryptic, recognizable only by gigas traits, attributed to the effects of whole genome duplication (WGD) on cell size. To understand the degree to which WGD biases phenotypic evolution, the magnitude of gigas effects must be quantified before evolutionary processes begin to distort their expression. In angiosperms, the vegetative (“tube”) cell of the pollen grain is both organ and organism. It comprises the full inner volume of mature pollen, and functions ultimately to shelter, disperse and transfer (via a growing pollen tube) two sperm cells to the ovule. Increased pollen size can alter performance traits such as pollen hydrobiology, aperture number, ornamentation, and pollen tube diameter. Here I quantify the earliest effects of autopolyploidy on pollen size. I compared 252 pairs of 1n and autopolyploid 2n pollen from 165 species taken from the literature. I placed 2n pollen within a series of advancing evolutionary stages from: 1) abnormal meiosis in diploid sporophytes, to (2-4) normal meiosis in tetraploid sporophytes: 2) induced/spontaneous autopolyploids (< 5 generations), 3) cultivated autopolyploids (> 5 generations), and 4) established natural intraspecific autopolyploid cytotypes. 1n pollen parents were progenitor individuals of 2n pollen (stages 1, 2) or were from the same or closely-related populations (stages 3, 4). For each pair, y = log(2n pollen) – log(1n pollen) diameter. An ANCOVA found 2n pollen diameter > 1n diameter by a factor of 1.34^A, 1.26^B, 1.24^B and 1.16^C for stages 1-4, respectively (N = 24, 58, 29, 54 species; P < 0.0001), after accounting for a positive effect of chromosome number (P = 0.0081). Doubling of genome size is expected to double nuclear and cell volumes, increasing spherical cell diameter by 26%, almost exactly the magnitude found for stage two and three autopolyploids. The smaller-than-expected size of 2n pollen at stage four suggests natural selection reduces 2n pollen size after WGD in the wild. If so, wind-pollinated polyploids might experience stronger pollen size constraints than animal-pollinated polyploids. A test of pollination type versus evolutionary stage found that the 2n pollen diameter effect decreased faster with advancing stage of polyploidy in wind-pollinated than in animal-pollinated species (P = 0.022 for Pollination type x Stage interaction). Taken together, these data suggest that pollen size evolution is bound by a lower limit determined by genome size and by an upper limit associated with pollen dispersibility.

Related Links:
Joe Williams info page
Joe Williams lab website

1 - University of Tennessee, Ecology and Evolutionary Biology, Knoxville, TN, 37919, USA

Keywords:
Evolution of development
Single-cell biology
pollen
Pollen performance
polyploidy
Whole-genome duplication
reproductive ecology
Reproductive processes.

Presentation Type: Oral Paper
Session: EVDV1, Evolutionary Developmental Biology (Evo-Devo)
Location: Virtual/Virtual
Date: Thursday, July 30th, 2020
Time: 11:30 AM
Number: EVDV1007
Abstract ID:390
Candidate for Awards:None