Abstract Detail

Reproductive Processes

Williams, Joseph [1], Oliveira, Paulo [2].

How does polyploidy affect pollen tube growth rate? A comparison among two diploid-polyploid species pairs.

In angiosperms and conifers, male gametophytes produce a pollen tube that transports sperm to the egg. Angiosperms have rapidly evolved orders-of-magnitude faster pollen tube growth rates (PTGRs) than conifers, potentially as a consequence of much more intense pollen competition under haploid selection. Yet, polyploidy is common in angiosperms and a recent comparative study indicates that rapid PTGRs might often have evolved in a non-haploid state. To date, there have been no studies on the effect of polyploidy on pollen tube cell size, which might affect PTGR. We compared such "nucleotypic" effects in two diploid-polyploid species complexes, Betula (Betulaceae) and Handroanthus (Bignoniaceae). We performed intra-specific, outcrossed hand-pollinations on pairs of flowers. In one flower, longitudinal whole mounts of styles were stained with aniline blue to measure pollen tube lengths over time (PTGR). In the second, styles were embedded in transverse orientation in glycol methacrylate, serial-sectioned to 5 µm thickness, and stained with purified aniline blue to measure wall thickness (W) and tube circumference (C). We used the means of these measurements for each replicate maternal plant to calculate wall production rate: WPR = W x C x PTGR. In Betulaceae, B. papyrifera (2n = 6x = 84), C was 13% larger than in B. occidentalis (2n = 2x = 28), but wall thickness (W) and PTGR did not differ. Using PTGR as a covariate, WPR was 23% higher in B. papyrifera than in the diploid (P = 0.001). In Bignoniaceae, C of tubes in H. chrysotrichus (2n = 4x = 80) and H. serratifolius (2n = 6x = 120) was 22-25% larger, respectively, than H. ochraceus (2n = 2x = 40), whereas W was similar among species. PTGRs of the tetraploid and hexaploid were 36% slower or non-significantly different than the diploid, respectively; which corresponded to 30% slower and 17% faster wall production rates. H. chrysotrichus was apomictic and its slower growth rates may reflect relaxed directional selection on speed. All other species were outcrossing. Thus, in both genera the hexaploids had larger tubes and faster WPRs, an indication they were working harder to maintain similar PTGRs as their diploid counterparts. Hexaploids are removed by at least two polyploid events from their ancestrally diploid condition, and have had more time to evolve genetic responses to the initial nucleotyic effects of larger tube cell size (which were also apparent in larger pollen volume and style lengths), incurred by genome doubling.

Related Links:
Williams lab website UTK
Paulo E. Oliveira information

1 - University Of Tennessee, Ecology And Evolutionary Biology, 1406 Circle Drive, Hesler 341, Knoxville, TN, 37996, United States
2 - Universidade Federal De Uberlandia, Instituto De Biologia. Campus Umuarama Bloco 2D, Uberlandia, MG, 38405-320, Brazil

Keywords:
pollen
polyploidy
Genome size
Growth rate
Evolution of development
Gametophyte biology
reproductive biology.

Presentation Type: Oral Paper
Session: REP1, Reproductive Processes 1
Location: San Luis 2/Starr Pass
Date: Wednesday, July 31st, 2019
Time: 9:00 AM
Number: REP1005
Abstract ID:254
Candidate for Awards:None