Monday, August 4, 2008
Exhibit Hall CD, Midwest Airlines Center
Background/Question/Methods
Chamaecrista is a diverse legume genus that contains over 260 species, most of which are Neotropical. Section Xerocalyx is a well-defined group within this genus, distributed from south of Mexico and the West Indies to north of Argentina, and is easily distinguished through flower and seed characters and chromosome number. This section includes three species, subdivided into 22 varieties. All varieties are buzz-pollinated by bees in search of pollen, and have extra-floral nectaries that attract ants as a defense. Although two or more varieties may be found in sympatry, hand-pollination studies have shown that co-occurring varieties do not form viable hybrids. Our goals were to assess the level of morphological variation within and among varieties and determine the amount geographic overlap of all taxa in section Xerocalyx. We conducted a morphometric study of specimens deposited in herbaria. We georeferenced all collection points to map taxon richness in a 1º x 1º grid, and produced a map of overlapping distributions to identify regions of high taxon richness. In addition, we modeled the distribution of each taxon using a 10' x 10' Worldclim dataset and the BIOCLIM model.
Results/Conclusions
Within section Xerocalyx, leaf length varies 14.3-fold, pedicel length 50-fold, and sepal length by 3.8-fold. No clear morphological gaps among varieties can be identified along the first eight principal component axes. In addition, distributions for two or more taxa overlap throughout most of the range of section Xerocalyx. More varieties co-occur in central and south-east Brazil and in Venezuela, where 11 and 6 varieties can be found within a 1º x 1º square, respectively. Taking advantage of the morphological dataset combined with location data, we will next investigate the occurrence of character displacement within section Xerocalyx, by testing if morphological traits of a given variety are more extreme when in sympatry with other varieties than when found alone.
Chamaecrista is a diverse legume genus that contains over 260 species, most of which are Neotropical. Section Xerocalyx is a well-defined group within this genus, distributed from south of Mexico and the West Indies to north of Argentina, and is easily distinguished through flower and seed characters and chromosome number. This section includes three species, subdivided into 22 varieties. All varieties are buzz-pollinated by bees in search of pollen, and have extra-floral nectaries that attract ants as a defense. Although two or more varieties may be found in sympatry, hand-pollination studies have shown that co-occurring varieties do not form viable hybrids. Our goals were to assess the level of morphological variation within and among varieties and determine the amount geographic overlap of all taxa in section Xerocalyx. We conducted a morphometric study of specimens deposited in herbaria. We georeferenced all collection points to map taxon richness in a 1º x 1º grid, and produced a map of overlapping distributions to identify regions of high taxon richness. In addition, we modeled the distribution of each taxon using a 10' x 10' Worldclim dataset and the BIOCLIM model.
Results/Conclusions
Within section Xerocalyx, leaf length varies 14.3-fold, pedicel length 50-fold, and sepal length by 3.8-fold. No clear morphological gaps among varieties can be identified along the first eight principal component axes. In addition, distributions for two or more taxa overlap throughout most of the range of section Xerocalyx. More varieties co-occur in central and south-east Brazil and in Venezuela, where 11 and 6 varieties can be found within a 1º x 1º square, respectively. Taking advantage of the morphological dataset combined with location data, we will next investigate the occurrence of character displacement within section Xerocalyx, by testing if morphological traits of a given variety are more extreme when in sympatry with other varieties than when found alone.
