Evidence for a conserved karyotype in Swartzia (Fabaceae, Papilionoideae): Implications for the taxonomy and evolutionary diversification of a species-rich neotropical tree genus

Abstract

To assess the taxonomic utility of cytogenetic variation in the species-rich neotropical papilionoid legume genus Swartzia and to ascertain the importance of cytogenetic evolution in the diversification history of the genus, a variety of cytogenetic data—chromosome number, chromosome lengths, relative chromosome length, total chromatin length (TCL), CMA/DAPI and FISH—were collected for 19 taxa of Swartzia and for a single species of the related genus Ateleia. In the sampled species of Swartzia, chromosome counts yielded a diploid number of 2n=2x=26. However, both diploid and tetraploid (2n=4x=52) counts were obtained for S. leptopetala. The species of Swartzia presented small chromosomes (0.25μm to 1.41μm), with gradual length variation, furthermore, each of them had two sites of CMA+/DAPI- and two sites of 45S and 5S rDNA. Cytogenetic data for the morphologically anomalous species S. euxylophora convey its close relationship to other species of Swartzia. Ateleia ovata was found to differ from all of the sampled taxa of Swartzia in diploid chromosome number (2n =28). Taken together, these results constitute preliminary evidence for a strongly conserved karyotype pattern in Swartzia and in combination with previously published data suggest that karyological characters, while useful for characterizing the genus, are of limited taxonomic utility within Swartzia. We conclude that cytogenetic evolution involving changes in chromosome number has not figured prominently in the explosive diversification history of Swartzia.