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Evolution of the Satyrium longicauda (Orchidaceae) species complex.

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The identification of morphological variation in taxa grouped within a species complex is fundamental not only in systematics, but plays an important role in evolutionary biology as well. Finding diagnostic characters among populations in plant species undergoing speciation may be challenging due to the continuous nature and intraspecific overlap of variation in reproductive and vegetative traits. The orchid Satyrium longicauda currently comprises two varieties (jacottetianum and longicauda) which can be identified mainly by differences in spur length. Additionaly, S. buchananii and S. rhodanthum were proposed as part of the group which represents a species complex. The large variation in phenotype in the species detected across its broad range makes this complex a promising candidate for studying its systematics and evolutionary origin, through the implementation of multi-disciplinary tools. In this thesis, I have characterised the morphological variation of sympatric and allopatric natural populations of S. longicauda by traditional morphometrics and uni- and multivariate analyses leading to the identification of eight morphotypes, including S. rhodanthum, that partially overlap in traits. These taxa, together with S. buchananii which was not studied in detail, served as units of comparison in a molecular phylogeny that supported the monophyly of some of the morphotypes, but at the same time revealed the non-monophyly of S. longicauda. Extensive pollinator data from direct observations and motion-trigger cameras, revealed that most morphotypes are pollinated by nocturnal moths but pollination by long-tongued flies, sunbirds, and oil-collecting bees was also recorded. Analyses of pollinator assemblages using network tools led to identification of five modules representing potential pollination niches. Several evaluated floral traits such as colour, scent chemistry, nectar volume and concentration, and functional spur length co-varied with some or all of the pollination niches, suggesting they could represent functional traits. Traits associated with the sunbird pollination niche were the most divergent compared to traits of the moth, oil-collecting bee, and a mixed pollination niche. Subtle differences in nectar properties and functional spur length could be linked to the oil-collecting bee pollination niche and two nocturnal moth pollination niches, comprising pollination by settling moths and hawkmoths, respectively. A pollinator shift from nocturnal moth to oil-collecting bee pollination was demonstrated using a combination of field-based experiments and a phylogenetic analysis. This shift appears to be triggered by changes in the type of floral reward from nectar to oil. Morphological variation associated with genetic differences in combination with the identification of an exclusive pollination niche support the recognition of a new variety that is distinct from the two accepted varieties within the complex. Results presented in this thesis contribute new evidence on pollinator-driven evolution and systematics of S. longicauda. At the same time, the thesis provides a starting point for further studies of the processes involved in the diversification of species complexes and for testing the role of pollinators in shaping and maintaining such diversity.


Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.