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Georgia Journal of Science

Article Title

POLLEN SIZE AS AN INDICATOR OF PLOIDY LEVEL IN RHEXIA, A SMALL GENUS OF FLOWERING PLANTS ENDEMIC TO THE NORTH AMERICAN COASTAL PLAIN, IN NORTH FLORIDA AND COASTAL GEORGIA**

Abstract

Rhexia (Melastomataceae) comprises a genus of 13 species that occur primarily in wet areas along the Atlantic coastal plain of North America. These showy plants, known as the meadow beauties, have a complex evolutionary history, with evidence for both ancient and recent hybridization (with several species of putative hybrid origin), as well as polyploidization (four species occur as a variety of cytotypes, e.g. in R. mariana n=11 or 22). Investigations into speciation and evolution in this group require identification of ploidy level for individual specimens. Pollen size in some plant groups is positively correlated with genome size and/or ploidy; in this study we set out to quantify the relationship between pollen grain size and ploidy level in species of Rhexia, potentially providing a means to determine ploidy for future studies without the labor intensive processes and specialized equipment required for chromosome counting and genome estimation by flow cytometry. Seventy-four live plants representing five species of Rhexia were collected from 21 separate populations in Florida and Georgia, and cultivated in the greenhouse. Rhexia mariana. R. cubensis and R. nashii (all of which occur at multiple ploidy levels) were well represented, and a few specimens of the invariably diploid R. alifanus and R. petiolata were also sampled. Pollen from flowers collected in the morning of the first day of anthesis was suspended in a glycerol/water solution on a glass slide, stained with acetocarmine and digitized under 200X magnification. From each image the equatorial diameter of 25 pollen grains was estimated by comparison to standards viewed at similar resolution and measured using a calibrated stage micrometer at the same magnification. We expect to determine whether pollen grain size is correlated with ploidy in three genomically variable species of Rhexia, which will prove useful for future studies of this evolutionarily complex group.

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