Systematics and evolution of Tarasa Philippi (Malvaceae): An enigmatic Andean polyploid genus.

摘要

This work represents a progression from a large scale molecular phylogeny to a species level study of breeding systems. A phylogenetic approach utilizing molecular sequence data was employed to evaluate the current infratribal circumscription and generic relationships within the tribe Malveae (subfamily Malvoideae, Malvaceae). The resulting ITS phylogeny indicates that many of the alliances as well as several genera are not monophyletic and suggests that the current classification needs revision. An in depth phylogenetic examination of one Malveae genus, Tarasa Philippi, and suggested close relatives, Nototriche Turcz. and Sphaeralcea St.-Hil, was also conducted using nuclear and chloroplast sequence data. The monophyly of Tarasa was rejected in both the nuclear and chloroplast trees. Surprisingly, the morphologically similar tetraploid species also did not form a single clade, which indicates that the tetraploids have been formed multiple times. Geographic and molecular data suggest that the tetraploid species may have been derived from multiple encounters between different sympatric diploid annual species. Although multiple origins of polyploidy in Tarasa is not a novel finding, the apparent morphological convergence of the tetraploid species is most intriguing particularly because they violate two traditional dogmas of polyploids: they are annuals and have smaller pollen grains than their diploid relatives. A study of the breeding systems and pollen characteristics for Tarasa diploid and tetraploid species also revealed unexpected findings. Pollen/ovule (p/o) ratios were used as an indirect measure of the degree of outcrossing versus selfing and greenhouse studies provided direct evidence of self-compatibility as well as autogamy in the plants. Tarasa species follow the predicted trend of low p/o ratios in autogamous species and high p/o ratios in xenogamous species. However, the overall positive correlation observed in Tarasa between p/o ratio and pollen size contradicts the expected negative relationship between these two factors. Statistically significant differences were detected between the tetraploids and diploid annuals and perennials for anther number per flower, number of grains per anther, p/o ratio, and pollen size. The tetraploid species occupy the highest elevation habitats in the Andes and have repeatedly adopted a floral morphology which is typical of autogamous species.