Cells, Genes & Molecules

How do you get more DNA down a pollen tube?

A common event for flowering plants is whole genome duplication (WGD), where a plant gets extra chromosomes. It’s often studied in mature plants, but there can also be extra copies of the genome in the pollen, and that’s a problem. When it lands on a suitable plant stigma, the pollen develops a pollen tube to deliver sperm to the ovules. If the pollen is polyploid, then that sperm is carrying more than one copy of the genome down that tube, and more than one copy will cause differences at the single-cell level. So how does that affect the pollen tube?

Measuring dimensions and growth rates of pollen tubes. Source: Williams and Olivera 2020.

Joseph Williams and Paulo Olivera examined diploid and polyploid near-relatives of Betula (Betulaceae) and Handroanthus (Bignoniaceae). Both are woody perennials, but Handroanthus grows a pollen tube faster than Betulia. They examined the pollen tubes for variations in tube wall thicknesses, tube circumferences, and pollen tube growth rates (PTGRs). They then used these measurements to calculate volume of solutes imported per unit of time (VGR) and cell wall production (wall production rate, WPR).

“The pollen tube is an excellent model for studying cell-level consequences of whole genome duplication because it is a single cell that functions during a single phase of the cell cycle and its elongation rate, PTGR, is determined entirely by the amount and rate of new tube cell wall production,” write the authors. “In both Betula and Handroanthus, the polyploid species had larger mature pollen grains that in turn formed larger pollen tubes, and hence required synthesis of more wall material and importation of more solutes per unit of tube length. That added materials cost, calculated as the change in PTGR that would occur if a polyploid pollen tube produced its walls at the same rate as its haploid relative, imposed a 16–20 % penalty on PTGR. The tube-size effect was largely compensated for by faster wall construction rates in both hexaploid species, resulting in an apparent evolutionary stasis of PTGR.”

“Despite the difficulties in measuring pollen tubes using light microscopy, our approach allowed us to decouple cell-level dimensional and energetic effects of polyploidy on pollen tube growth, with consequences for reproduction and evolution of angiosperms as a whole. Polyploidy imposed a substantial materials cost in the form of larger tube circumference, but also generated compensating energetic effects, as indicated by faster wall production rates, resulting in evolutionary stasis of PTGR.”“Given the pervasive cycles of WGDs in angiosperms relative to gymnosperms, if dimensional effects of polyploidy generally act as a brake on PTGR, our results suggest that selection on biosynthetic rates during the male gametophytic phase of angiosperm polyploids has probably contributed to their orders-of-magnitude faster PTGRs over those of gymnosperms.”

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