Genetic variation and interspecific hybridisation in the genus Scaevola

Abstract

Australia is the centre of diversity for wild Scaevola species. Scaevola. aemula is the most common representative of the genus in international horticulture, but there is little variation for flower colour in current commercial varieties. Some species that are closely related to S. aemula represent potential sources of novel variation that may be used in breeding programmes to generate improved varieties. The major impediments to improve Scaevola for ornamental horticulture include a poor understanding of barriers to hybridisation, low hybrid seed germination, and the relationships between Scaevola species. Work described in this thesis aimed to: investigate the cytological and genetic variation within the genus characterise the genetic control of self incompatibility investigate the barriers to hybridisation in Scaevola and to explore approaches to overcome them investigate the merits of various approaches of hybrid confirmation that may be used in breeding. Cytological examination of root tips revealed that all Scaevola accessions studied (wild collections, commercial varieties and some interspecific hybrids) had 2n = 16 chromosomes which was consistent with reports that the genus Scaevola has a base chromosome number of n = 8 (Peacock, 1963). PMC (pollen mother cell) meiotic analysis showed that cultivated varieties and wild collections were all diploid and exhibited stable pairing of eight ring bivalents (8 II), whilst interspecific hybrids exhibited less stability in pairing patterns with different numbers of ring or open bivalents and the appearance of occasional univalents. RAPD analysis of DNA was applied to 16 Scaevola accessions representing 13 species. This technique differentiated between all of the accessions studied and revealed the genetic relationships between them. The results derived from this study were in general agreement with established taxonomy of the genus. However, the most important discrepancy between these results and current classification concerned the relationship between small and large flowered forms of S. aemula. The genetic distance between these two groups was much greater than that between most closely related species in the genus. More importantly, the large flowered forms of S. aemula were genetically more closely related to other species with large flowers, and the small flowered forms of S. aemula were more closely related to other species with small flowers. Further work is required to confirm this finding but from these results it appears that the large and small flowered form of S. aemula do not belong to the same species. Investigations of the nature of self incompatibility (SI) in large flower S. aemula (1) showed that SI is sporophytically determined and controlled by a single locus with multiple alleles. Dominance relationships between S-alleles in the pollen were observed while the S-alleles in the stigma were co-dominantly inherited in a normal Mendelian fashion. The investigation of Fi interspecific hybrids (S. albida x S. microphylla) proved that the inheritance of SI was the same pattern as that in S. aemula (1). This is the first finding that the same SI system is functioning in intraspecific and interspecific hybrid crosses in the genus Scaevola. Barriers limiting hybrid formation in Scaevola include SI, pre- and post-zygotic incompatibility and interspecific incongruity. Methods to overcome SI, such as killed compatible pollen (KCP) bridged hybridisation and stigma treatments with chemicals, proved to be effective in generating seed in both self pollination and intraspecific and interspecific crosses. Depending on their parental genetic compatibility or genetic distance, the hybrids generated between some closely related species ranged in fertility and vigour. Seeds derived from selfing were difficult to germinate in vitro and did not develop into healthy plantlets, suggesting that inbreeding depression was occurring which is consistent with the designation of most species in the genus as outcrossing. It can be concluded that SI in the genus Scaevola generally governs intraspecific hybrid formation and probably maintains the level of heterozygosity in populations. Putative intraspecific and interspecific hybrids that were produced in this research were characterised by a number of approaches in order to assess the efficiency of the approaches to confirm hybrid status, and to validate that the plants were true hybrids. Studies of the progeny of both intraspecific and interspecific plants revealed that simple dominant morphological characters that were unique to one parent may be used to assess their hybrid status. However, because of a limited understanding of the interactions regulating the expression of these morphological characters and the mode of inheritance of genes involved in their expression the results are not as certain as those derived from self incompatibility studies, meiotic analyses (interspecific crosses only) or RAPD PCR where a more definitive answer may be achieved. Information derived from these studies will vastly improve the efficiency of Scaevola breeding and allow for the generation of improved Scaevola varieties.