Molecular systematic study of Southern African Oxalis (Oxalidaceae)

[摘要] The genus Oxalis forms a major part of the flora of southern Africa, in particular theCape Floristic Region (CFR) at the southwestern tip of the continent, but the currenttaxonomy is outdated and ecological knowledge of the lineage is sadly incomplete. Inthis thesis I set out to address several aspects of Oxalis systematics that urgentlyrequire attention.Firstly, the current macro-morphological taxonomy requires phylogenetic testing, as itis acknowledged to be incomplete and artificial. I address this need by providing aDNA sequence-based phylogeny of three markers, using three different inferencemethods, for nearly three quarters of the indigenous species. This phylogenyconfirmed both the monophyly of the southern African taxa, and the artificiality of thecurrent classification system. It is congruent with previous sequence-basedreconstructions of smaller groups of southern African Oxalis species, and with thepalynological classification proposed for the genus.Secondly, previous phylogenetic work on the southern African members could notresolve basal relationships within the southern African clade. I attempt to address thisproblem by sequencing three extra chloroplast markers for a select group of taxa,followed by separate and combined (total evidence) molecular phylogenetic analyses.This approach did increase resolution at the base of the southern African lineage, butmany clades still showed poor resolution and support despite the use of more than 7000 bases of sequence data. Resolving these clades within the southern African Oxalisphylogeny remains a challenge, and should prove a fertile field for future research.Thirdly, the ages (and thus duration of presence) of many Cape plant lineages withinthe CFR are of major interest, given that the CFR represents a global biodiversityhotspot. The age of the genus in the Cape is estimated by analyzing combinedsequence data for all sampled taxa under both a Bayesian Relaxed Clock and a semiparametricPenalised Likelihood method, using calibration points inferred fromRelaxed Clock analyses of the entire order Oxalidales, for which fossil data areavailable. In an attempt to account for known problems with divergence timeestimation, I explored the potential bias introduced by method used, marker genomesource and different calibrations on the root. The results indicate substantial variationin the age of crown southern African Oxalis over a nearly twenty million year period,varying according to source data, calibration estimate and methodology employed inthe reconstruction. Despite this major variability, all average estimates are older than iv18 million years, which agrees with a growing body of evidence that there has been agradual accumulation of floristic diversity in the CFR, rather than a rapid, recent burstof speciation.Fourthly, as the produced phylogenies conclusively show the artificial nature of thecurrent taxonomy, I propose a new, almost completely different classification forsouthern African Oxalis taxa. Although a significant improvement, this classificationis considered informal due to the complete disagreement between the old andproposed new taxonomies, poor resolution in some of the proposed lineages, and aneed to confirm proposed groups (clades) with the identification of morphologicalsynapomorphies. Potential synapomorphies for various clades are proposed anddiscussed, which should guide future research.Fifthly, the presence of bulbs in this genus is of great interest as a potential preadaptationfor seasonally arid climates. The evolution of the bulbous habit in Oxalis ishere explored for the first time. I address the sequence of major morphologicalcharacter state changes leading to the suite of characters corresponding to the bulboushabit. The homology of basal leaf petioles, fleshy leaf scales and tunics is discussed,and it is shown that many bulb characters present in the southern African lineage arealso found in the close relatives of this lineage, and are thus older than this lineage.The ecological and evolutionary implications of bulb geophytism in the CFR Oxalisare also discussed.Finally, I address certain taxonomic issues that arose during the course of this study.Co-authors and I describe the new species O. saltusbelli and O. ericifolia. We alsoclarify issues surrounding the tremendously variable group species O. flava andpropose some nomenclatural changes and synonyms for related taxa. We also addressthe taxonomic position of the rare species O. purpurata, which was located too late inthe course of this study to include in the main analyses.

[发布机构] Stellenbosch University