Intraspecific and Interspecific Variation in Pollen Size in the Genus Rhexia (Melastomataceae)
Abstract
Rhexia (Melastomataceae) is a genus of thirteen species occurring primarily in wet areas along the Atlantic coastal plain of North America. These showy flowering plants have a complex evolutionary history, with evidence for ancient and recent hybridization (several species are 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 levels for individual specimens. Pollen size in some plant groups is positively correlated with genome size and/or ploidy; one of our goals is to determine whether pollen grain size can be used to assess the ploidy of individual Rhexia specimens. This initial study focuses on determining inter- and intra-specific variation in pollen grain size for six species of Rhexia. Eighty plants were collected from twenty-three 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 R. alifanus, R. petiolata, and R. nuttallii (which occur only as diploids) were sampled. Anthers collected in the morning on the first day of anthesis were macerated on a glass slide to release pollen, which was suspended in a drop of iodine potassium iodide (IKI) solution. Slides were digitized using a Canon EOS 5DS R camera equipped with a 200 mm lens coupled with a 10X objective lens, and the equatorial diameter of 100 viable pollen grains from each image was measured in Photoshop. Analysis of variance (ANOVA) and Tukey's multiple comparison tests supported significant differences in pollen grain size between certain species of Rhexia, while patterns of intraspecific variation between species of variable ploidy versus those of consistent ploidy remain inconclusive.
Intraspecific and Interspecific Variation in Pollen Size in the Genus Rhexia (Melastomataceae)
Rhexia (Melastomataceae) is a genus of thirteen species occurring primarily in wet areas along the Atlantic coastal plain of North America. These showy flowering plants have a complex evolutionary history, with evidence for ancient and recent hybridization (several species are 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 levels for individual specimens. Pollen size in some plant groups is positively correlated with genome size and/or ploidy; one of our goals is to determine whether pollen grain size can be used to assess the ploidy of individual Rhexia specimens. This initial study focuses on determining inter- and intra-specific variation in pollen grain size for six species of Rhexia. Eighty plants were collected from twenty-three 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 R. alifanus, R. petiolata, and R. nuttallii (which occur only as diploids) were sampled. Anthers collected in the morning on the first day of anthesis were macerated on a glass slide to release pollen, which was suspended in a drop of iodine potassium iodide (IKI) solution. Slides were digitized using a Canon EOS 5DS R camera equipped with a 200 mm lens coupled with a 10X objective lens, and the equatorial diameter of 100 viable pollen grains from each image was measured in Photoshop. Analysis of variance (ANOVA) and Tukey's multiple comparison tests supported significant differences in pollen grain size between certain species of Rhexia, while patterns of intraspecific variation between species of variable ploidy versus those of consistent ploidy remain inconclusive.