Doctoral Degrees (Genetics)

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Browsing Doctoral Degrees (Genetics) by Subject "Basic chromosome number"

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    The development of Lachenalia cultivars
    (University of the Free State, 2013-09) Kleynhans, Riana; Spies, J. J.
    The floriculture and ornamental industry is constantly looking for new products. South Africa is blessed with an exceptional rich bio-diversity and many South African plants have found their way onto international markets. The local development of products for the international market unfortunately is limited. The genus Lachenalia is one of the exceptions, with local development and production of cultivars for the international pot plant market. This thesis thus aimed to establish the different aspects and requirements needed for the development of new Lachenalia cultivars and to use the basic genetic information generated through research to develop specific breeding strategies for the development of new cultivars. The thesis established the wider requirements of the complete value chain for the development of new floriculture crops and identified the strong need to establish basic research information in order to successfully develop new cultivars in the genus Lachenalia. The diversity amongst the 133 described species of Lachenalia and the breeding and research on production that facilitated the release of cultivars to the international market indicated the suitability of the genus for development. The genetic variation present in the genus includes various different basic chromosome numbers, polyploidy, B-chromosomes, different karyotypes within the same basic chromosome number, different phylogenetic groups and the existence of possible hybrid species. Relationships between specific basic chromosome numbers were shown and possible evolutionary history was proposed, but conclusions in this regard needs further investigation. The development of new cultivars is possible from both conventional and mutation breeding processes, but the availability of basic genetic information is essential for future progress. Inter-specific as well as complex hybrid/hybrid crosses are used for the development of new cultivars. To facilitate future crosses the cross-ability among Lachenalia species was investigated. The cross-ability data supports the phylogenetic relationships identified by various authors and both are strongly linked to basic chromosome numbers. Phenotypic characters cannot be used to predict the success of inter-species crosses, except where clear mechanical isolation (female long style species crossed with male short style species) is present. Clear unilateral cross-ability is present among several species and this is not linked to self-incompatibility. Self-incompatibility seems to be present in specific species, but can be overcome by crossing different accessions of the same species. Clear differences in the level of success of crossing combinations were statistical shown through AHC cluster and principle component analysis. A limited number of crosses showed good cross-ability with the production of many normal seeds. Most of these crosses were between species with the same basic chromosome number with only four exceptions, which were between basic x = 7 and x = 8, confirming the close relationship between these two basic chromosome numbers. Some intermediate success rates between basic x = 11 with both x = 7 and x = 8 was also present possible supporting the basal nature of x = 11. Basic chromosome numbers are currently the best criterion for predicting the success rate of inter-species crossing combinations but it does not guarantee success. The data presented clearly indicated the importance of well characterized (phenotypic and genotypic) germplasm material, including the maintenance of various accessions of a species. Good breeding parents were identified to assist breeders to reach specific goals. The importance of an in-depth investigation on the nature and extent of the crossing barriers and continued research on the genetics and molecular systematic of the genus was determined. This study clearly shows that the availability of basic genetic information and data on the cross-ability among species is essential for the selection of breeding parents to ensure better success rates for inter-species crossing combinations and the future development of new Lachenalia cultivars. Afrikkans: