Reproductive biology, ecology and historic biogeography of selected rare and endagered Oxalis L. (Oxalidaceae) species

Thesis (MSc)--University of Stellenbosch, 2007.

ENGLISH ABSTRACT: Twenty five percent of all southern African Oxalis species are rare/endangered and highly localized, making them especially vulnerable to threats of extinction (inbreeding, low genetic variation, disrupted biological interactions) associated with small, isolated populations. In addition, Oxalis also displays tristyly, a restrictive breeding system that includes a strong self-incompatibility component that promotes out-crossing within populations. As it requires equal availability of three different floral morph types as well as effective pollinators for seed production, this breeding system can be a further threat to small, isolated populations when it is fully expressed. Breakdown has been recorded in terms of all the components of this complex breeding system, but usually comprises a relaxation in the expression of self-incompatibility. This may significantly alter reproductive assurance regardless of the availability of pollinators and morph types, with positive consequences for at least the short-term future survival of rare species. Potential factors that may contribute towards rarity in Oxalis were investigated by focusing on the reproductive biology and ecology of two widespread and eight rare/highly localized Oxalis species. The expression of tristyly, levels of natural seed production and vegetative reproduction were investigated to identify possible limitations to reproductive success. Several ecological attributes, including habitat type and specificity, population size and density, rainfall patterns, potential pollinators, potential threats, etc. were investigated for the studied populations and compared to the reproductive biological expression in each case. In addition, phylogeographic patterns were assessed by investigating the degree of genetic variation within and among populations of a wellsupported monophyletic clade including two rare and highly localized species and one widespread species. Comparisons of results of different rare species together with comparisons between rare and related common species are presented to reveal the most likely threats to specific populations. Tristyly expression was unstable and extremely variable among the populations studied. In addition to limited reproductive success, most rare species are restricted by their highly specific habitat requirements and are particularly vulnerable to variation in rainfall patterns. Low genetic variation within rare species may also prevent the colonisation of new habitats or the adaptation to a changing environment.

AFRIKAANSE OPSOMMING: Vyf en twintig persent van alle suider Afrikaanse Oxalis spesies is skaars of bedreig met 'n baie gelokaliseerde verspreiding wat hulle veral kwesbaar maak vir gevare geassosieer met klein, geisoleerde populasies (inteling, lae genetiese variasie, versteurde biologiese interaksies) wat tot uitsterwing kan lei. Verder vertoon Oxalis tristylie, 'n beperkende kruisingsisteem wat 'n sterk selfonverenigbaarheidskomponent insluit om uitteling binne populasies te bevorder. Omdat gelyke beskikbaarheid van drie verskillende morfologiese blomtipes en effektiewe bestuiwers vir saadproduksie benodig word, kan hierdie kruisingsisteem 'n verdere bedreiging wees vir klein, geisoleerde populasies indien dit ten voile uitgedruk word. Die afbreek van al die komponente van hierdie komplekse kruisingsisteem is al gedokumenteer, maar behels meestal 'n verflouing in die uitdrukking van self-onverenigbaarheid. Dit kan die versekering van reproduksie aansienlik verbeter ongeag die beskikbaarheid van bestuiwers en blommorfologiese tipes, met positiewe nagevolge vir ten minste die korttermyn oorlewing van skaars spesies. Potensiele faktore wat tot skaarsheid in Oxalis kan hydra is ondersoek deur te fokus op die voortplantingsbiologie en ekologie van twee wydverspreide en agt skaars/baie gelokaliseerde Oxa/is spesies. Die uitdrukking van tristylie, vlakke van natuurlike saadproduksie en vegetatiewe voortplartting is ondersoek om moontlike beperkings tot voortplantingsukses te identifiseer. Verskeie ekologiese eienskappe wat habitat-tipe en -spesifiekheid, populasi~grootte en -digtheid, reenvalspatrone, potensiele bestuiwers, moontlike bedreigings ensovoorts insluit, is vir die bestudeerde populasies ondersoek en vergelyk met die voortplantingsbiologiese uitdrukking in elke geval. Verder is filogeografiese patrone bestudeer deur die graad van genetiese differensiasie binne en tussen populasies van 'n goed ondersteunde monofiletiese klade, wat twee skaars en gelokaliseerde spesies en een wydverspreide spesie insluit, vas te stel. Die vergelyking van resultate van verskillende skaars spesies tesame met vergelykings tussen skaars en verwante vollop spesies word voorgedra om die mees waarskynlike bedreigings vir spesifieke populasies te identifiseer. Die uitdrukking van tristylie was onstabiel en baie varierend tussen die bestudeerde populasies. Buiten beperkte voortplantingsukses word die meeste skaars spesies beperk deur hul baie spesifieke habitat vereistes en is veral kwesbaar vir varierende reenvalspatrone. Lae genetiese variasie binne skaars spesies kan ook die kolonisering van nuwe habitatte of die aanpassing by 'n veranderende omgewing bemoeilik.