Polhillia on the brink : taxonomy, ecophysiology and conservation assessment of a highly threatened Cape legume genus

Du Preez, Brian (2019-02)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Polhillia C.H.Stirt. is a small, highly threatened genus in the mega-diverse Fabaceae. It is poorly known, and thought to include 8 species restricted to Renosterveld remnants in the Greater Cape Floristic Region (GCFR). Most of the species delimitation has been based strictly on morphology, and the monophyly of the genus has not been tested using molecular phylogenetic analysis. This was the first aim of this study. Results proved Polhillia to be monophyletic, with P. involucrata (Thunb.) B.-E.van Wyk & A.L.Schutte resolving as sister to the rest of the genus. Based on this, and unique pink or white flowers and dehiscent fruit (versus yellow flowers and indehiscent fruit), this species was placed into a new monotypic subgenus Roseopolhillia B.du Preez. Given the confusing species delimitation, limited fieldwork history and weak herbarium specimen records of Polhillia, our next aim was to conduct a multidisciplinary systematic revision of the genus. This led to the recognition of 11 species in the genus, four of which are species new to science, while P. canescens was synonymised under P. connata. Conservation assessments based on IUCN criteria were conducted for all species. Two Critically Endangered species could be downgraded to Endangered based on the extensive fieldwork. Polhillia species are distributed through Renosterveld fragments of the GCFR, with six of eleven species restricted to the Overberg region. Species, however, never occur in sympatry. The third aim of the study was thus to determine if restricted distributions of Polhillia species may be due to edaphic factors such as preference to certain topographic habitat requirements, soils or to rhizobial symbionts. Habitat observations proved to be crucial as species with overlapping distributions occupied substantially different topographic habitats. Closely related allopatric species in the Overberg also often occupied distinct habitats. Soil chemical analysis showed that Overberg species shared fairly similar soils, while non-Overberg species occur on substantially different soils. A shift in soil and habitat preference was found to be an important contributing factor in the evolution of P. obsoleta (Harv.) B.-E.van Wyk and P. groenewaldii B.du Preez. Rhizobial symbionts were successfully isolated from nodules of eight Polhillia species. Five species were nodulating with Mesorhizobium representing three distinct lineages, and three species with a single Rhizobium lineage. These results show an extent rhizobial symbiont preference, but closely related species often shared symbionts. This also represents the first finding of Rhizobium nodulating with a member of the Genisteae tribe.

AFRIKAANSE OPSOMMING: Polhillia C.H.Stirt. is ‘n klein, hoogs bedreigde genus in die mega-diverse Fabaceae. Dit is swak bekend, en daar word tans geglo dat dit 8 spesies insluit wat tot Renosterveld fragmente in die Groter Kaapse Floristiese Streek (GKFS). Meeste van die spesie afbakening is streng slegs op morfologie gebaseer, en die monofilie van die genus is nog nie deur molekulêrfilogenetiese analise getoets nie. Dit was die eerste doelwit van hierdie studie. Resultate het bewys dat Polhillia wel monofileties is, met P. involucrata (Thunb.) B.-E.van Wyk & A.L.Schutte wat as suster tot die res van die genus uitkom. Gebaseer hierop, en die unieke pienk of wit blomme en oopspringende vrugte (versus geel blomme en nie-oopspringende vrugte), is hierdie spesies in ‘n nuwe monotipiese subgenus Roseopolhillia B.du Preez geplaas. Gegewe die verwarrende spesie-afbakening, beperkte veldwerk geskiedenis en swak herbarium eksemplaar rekord van Polhillia, was ons volgende doelwit om ʼn multidissiplinêre sistematiese hersiening van die genus te onderneem. Dit het gelei tot die herkenning van 11 spesies in die genus, vier waarvan nuut is tot die wetenskap, terwyl P. canescens onder P. connata in sinonimie geplaas is. Bewaringsassessering gebaseer op IUCN kriteria is vir alle spesies onderneem. Twee Krities Bedreigde spesies kon afgeskaap word na Bedreig gebaseer op uitgebreide veldwerk. Polhillia is versprei oor Renosterveld fragmente van die GKFS, met ses van die elf spesies beperk tot die Overberg omgewing. Spesies kom egter nooit simpatries voor nie. Die derde doelwit van die was dus om te bepaal of die beperkte verspreidings van Polhillia spesies toegeskryf kan word aan edafiese faktore soos ʼn voorkeur vir spesifieke topografiese habitatvereistes, gronde of rhizobium simbionte. Habitat waarnemings was baie belangrik, aangesien spesies met oorvleuelende verspreidings totaal verskillende topografiese habitatte bewoon. Naverwante allopatriese spesies in die Overberg bewoon ook dikwels kenmerkende habitatte. Die chemiese analise van grond het aangedui dat Overberg spesies relatief eenderse gronde deel, terwyl nie-Overberg spesies op substansieel verskillende gronde voorkom. ʼn Skuif in grond en habitat voorkeur is as belangrike bydraende faktore in die evolusie van P. obsoleta (Harv.) B.-E.van Wyk en P. groenewaldii B.du Preez geïdentifiseer. Rhizobium simbionte is suksesvol geïsoleer uit die nodule van agt Polhillia spesies geïsoleer. Vyf spesies noduleer met drie aparte ontwikkelingslyne van Mesorhizobium, en drie spesies met ʼn enkele Rhizobium ontwikkelingslyn. Hierdie resultate dui op ʼn mate van risobium simbiont voorkeur, maar naverwante spesies deel dikwels dieselfde simbionte. Dit verteenwoordig ook die eerste keer dat Rhizobium uit die nodule van ʼn lid van die tribus Genisteae geïsoleer is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105687
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