Masters Theses

Date of Award

12-2025

Degree Type

Thesis

Degree Name

Master of Science

Major

Ecology and Evolutionary Biology

Major Professor

Joseph Williams

Committee Members

Jessica Budke, Jacob Suissa

Abstract

Polyploidy has played an important role in the diversification of flowering plants. In general, the initial phenotypic effects of whole genome duplication (WGD) occur at the cell level - increased cell size and longer cell cycles - and result in larger and delayed development of organs. However, rapid evolution of the rate and duration of the cell cycle can occur during early stages of polyploid evolution. The male gametophyte of seed plants is highly reduced, developing as a single-celled organism within the pollen grain. In most flowering plants, pollen is dispersed as in a sexually immature bicellular state, but remarkably, 30% of angiosperms have evolved to disperse pollen that has reached sexual maturity, having already formed its two sperm cells, the tricellular state. Little is known about the mechanisms that underline unidirectional evolutionary transitions from bicellular to tricellular pollen, because species are rarely stably polymorphic for pollen at both stages.

Galax urceolata is a mixed ploidy species, containing single-cytotype populations of diploids (2n = 2x = 12) and autotetraploids (4x = 24). To investigate the role of polyploidy in pollen evolution I examined the development of pollen in twelve single cytotype populations. Flow cytometry was used to confirm ploidy levels. Conventional methods of staining and microscopy were used to calculate the proportion of bicellular and tricellular pollen present in flowers at four stages of maturation. Temperature accumulation is summed from 18 days of data prior to each collection using the nearest weather stations.

Diploid populations produced a mix of both bicellular and tricellular pollen in flowers of all ages (mean = 23%), whereas flowers of autotetraploid anthers contained a high proportion of tricellular pollen at all floral stages (mean = 93%). Pollen of both cytotypes is dehydrated at dispersal and maintained > 65% viability for three days after collection. Temperature accumulation was similar for both 2x and 4x plants, and bud growth rates were similar. All indicators of the acceleration, not prolongation, of pollen development in autotetraploids. By duplicating gene dosage, WGD provides a mechanism for evolving faster biosynthetic rates, despite initial disadvantages of larger cells and slower cell cycles.

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