Cactus that made it to the Old World
Cactus-Cytogenetics, Cactus-Geographical distribution, Phylogeography, Biogeography, Plants, Genetics, Polyploidy, Cytology, Cactus, Speciation
Rhipsalis baccifera is the only species of cactus that grows endemically in both the Americas and the Old World. Furthermore, genome duplication events within the species resulted in diploid (2n), tetraploid (4n), and octoploid (8n) populations. Polyploidy contributes to species diversification resulting from instant speciation events. The subspecies of Rhipsalis baccifera are defined by their morphological characteristics. These subspecies delineations generally correlate with different polyploidy levels and geographic distributions. I conducted a population genetics study to assess the DNA sequence divergence of individual plants collected across a global distribution. I also estimated ploidy counts of individuals by measuring nuclear area to compare my data set with the putative ploidy levels associated with bio-geographic ranges. Results from chloroplast DNA suggest that the species dispersed rapidly and genetic divergence has yet to occur between the New World and the Old World populations. New World individuals morphologically identified as subspecies R. baccifera shaferi and R. baccifera hileiabaiana, which is found exclusively in the Bahia region of Brazil, have low sequence identity with other Rhipsalis baccifera subspecies. I used nuclear area as a proxy for ploidy counts and nuclear area to show that variation in polyploidy existed in my dataset. Pilot findings from the cytology suggest that polyploid populations do correspond with subspecies categorizations and biogeography. I found that forming connection between polyploidy and genetic divergence is necessary to determine speciation in plants.
Tan, Elizabeth Ann, "The phylogeography and cytogenetics of Rhipsalis baccifera : the cactus that made it to the Old World" (2016). Honors Project, Smith College, Northampton, MA.