The evolution of the plant genus Tarasa (Malvaceae) has been influenced by polyploidy (doubling of chromosomes) and by the geological history of the Andean and Central American mountains. The 30 known species are found at elevations between 2000-4200 meters from central Peru to southern Chile and Argentina, and with two species disjunct in the highlands of central Mexico. The species of Tarasa are either diploid (2n) with the normal complement of chromosomes or tetraploid (4n) with twice the normal complement. Most polyploid species have larger cells (and often larger morphological features overall) than their diploid counterparts and are usually perennials rather than annuals. The tetraploid species of Tarasa, however, are unusual in that all are annuals and have reduced floral features as compared to the diploid species. Another interesting feature of this group is that the tetraploid species are all found at higher elevations in the Andes than the diploids. Because of their unusual features, the species present a unique opportunity to study the interactions among polyploidy, life-cycle habit, and morphology that have led to the present disjunct geographic distributions in Mexico and South America. Graduate student Jennifer Tate, under the direction of Dr. Beryl Simpson at the University of Texas, is studying the taxonomic classification and phylogenetic relationships of the species of Tarasa and their close relatives in the genera Sphaeralcea, Nototriche, and Acaulimalva. Using morphological and cytological (chromosomal) characters, supplemented with new molecular data from DNA sequences of nuclear and chloroplast genes, they plan to reconstruct the phylogeny or genealogical history of the species of Tarasa. With a phylogenetic framework in place, the investigators can explore specific evolutionary and biogeographic questions, such as whether the tetraploid species are derived from a single chromosome-doubling event or whether they have arisen multiple times, perhaps from different diploid species. Likewise, they will be able to explore ideas about the direction of species migration in the Andes, for example, whether the species of Tarasa moved northward from southern Chile and Argentina or whether they migrated southward from the northernmost Mexican area of the range. The phylogeny will help to determine whether the two species in Mexico represent a single lineage (clade) or whether they arrived independently from separate speciation events. A central goal of biogeography is to find similar patterns in unrelated taxonomic groups that provide a guide to major historical events. However, phylogenies for Andean groups of plants, based on rigorous methods and data, are few in number and hence limit the models necessary for explaining plant distributions. This study will contribute to our developing knowledge of the evolution of high Andean plants.
