University of Wollongong Research Online University of Wollongong Thesis Collection 1954-2016 University of Wollongong Thesis Collections 1994 Genetic and ecological analysis of the putative natural hybrid zone formed Banksia robur Cav. and Banksia oblongifolia Cav. Suzanne Margaret Schibeci University of Wollongong Follow this and additional works at: https://ro.uow.edu.au/theses University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. 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Recommended Citation Schibeci, Suzanne Margaret, Genetic and ecological analysis of the putative natural hybrid zone formed Banksia robur Cav. and Banksia oblongifolia Cav., Doctor of Philosophy thesis, Department of Biological Sciences, University of Wollongong, 1994. https://ro.uow.edu.au/theses/1047 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Genetic and ecological analysis of the putative natural hybrid zone formed by Banksia robur Cav. and Banksia oblongifolia Cav. UNIVERSITY OF WOLLONGONG LIBRARY____ A thesis submitted in fulfilment of the requirements for the award of the degree of Doctor of Philosophy from The University of Wollongong by Suzanne Margaret Schibeci B.Sc (Hons) UNSW Department of Biological Sciences 1994 11 Declaration This thesis is submitted in accordance with the regulations of the University of Wollongong in fulfilment of the requirements of the degree of Doctor of Philosophy. The work described in this thesis was carried out by me and has not been submitted to any other university or institution. Suzanne Schibeci April 1994 Ill Abstract Natural hybridization provides the opportunity to study the processes involved in speciation, since hybridizing populations are thought to represent some intermediate stage between a common ancestral population and the complete divergence of two species. The presence of hybrids within a population indicates that reproductive isolation has at some stage been incomplete. Therefore, the determination of the current level of gene flow can indicate the degree to which speciation is complete. Further, the fitness of the hybrids in comparison to that of the parent species can indicate the mode of formation and maintenance of the hybrid zone: either (i) a genetic/morphological cline can form in association with an environmental cline if the species are suited to conditions on different extremes of the cline or if the hybrid are as fit as the parents in the ecotone (environmental cline) and (ii) a cline may be formed as a result of an equilibrium formed between the constant gene flow into, and the selection against hybrids within, the hybrid zone (tension zone). Banksia robur Cav. and Banksia oblongifolia Cav. form the most commonly reported hybrid zone within the genus. The two species are morphologically different and are also closely associated with very different soil water regimes. Morphologically intermediate individuals, present in regions where the two species coexist, are thought to be hybrids. This hybrid zone, therefore, provides the opportunity to compare the "environmental cline" versus "tension zone" modes of hybrid zone formation, because while the Banksia hybrid zone seems to be associated with an environmental cline, there is also evidence that hybrids are selectively disadvantaged. In order to make this comparison, there are three general aims of this thesis: 1. To describe the "parental species", hybrids, and the hybrid zones as they are at present; 2. To determine if hybrids are selected against at some stage IV of development; 3. To determine the present potential for hybrid production, through the determination of the extent of reproductive isolation within the populations. A genetic survey within allopatric populations of B. robur and B. oblongifolia showed that there were some useful genetic differences between the two species. Of an electrophoretic survey of 32 enzymes, none was variable in stands of B. robur. Four loci (Adh, NSdh, Sod and Gdh) were variable for B. oblongifolia. Three of these loci (Adh, Sod and Gdh) were useful in differentiating the two species: the alleles for which B. robur were fixed at these loci, were the least common B. oblongifolia alleles. A genetic hybrid index was constructed using the variation in the allopatric populations of the species. Each plant from the pure B. robur had genetic hybrid index scores (GHIS) of 0. "B. robur alleles" were also present in small frequencies even in the allopatric populations of B. oblongifolia, the pure B. oblongifolia plants therefore scored GHISs of either 5 or 6. Nine leaf characters were quantified for each plant in the allopatric population, and an additional five inflorescence characters were added when inflorescences were available. Again a hybrid index was constructed from these morphological characteristics. A comparison of a plant's morphological hybrid index score (MHIS) and its GHIS revealed a good association of GHIS with morphology. The hybrid indices developed using the allopatric populations of the species were then applied to the plants within the two hybrid zones. The plants within the hybrid zones encompassed the range of scores possible on both the genetic and morphological hybrid indices, indicating that not only are parental species present in the hybrid zones, but these are hybridizing and forming plants of intermediate morphology and genotype. Further, plants with genotypes between parental and hybrid genotypes indicated that there is backcrossing and introgression occurring extensively within the hybrid zone. Using multivariate analysis, the morphology could separate plants with the parental scores of 0 from 5 and 6. The plants of hybrid origin were intermediate to the parental plants, but V separation of the hybrid genotypes (i.e. GHISs of 1, 2, 3 and 4) by morphology was not possible, even when multivariate analysis was employed. The spatial arrangement of the individuals within the hybrid zones was genetically complex. In order to simplify this spatial arrangement, the frequency of the B. robur alleles were determined along a transect through each population. This approach provided the opportunity to observe the genetic cline through the hybrid zones. The cline obtained using the mean frequency of the three loci along the transect was used to determine several parameters describing the cline: cline width (w), the rate of decay of the tails (0) and the strength of the barrier to gene flow set up by the cline (B). The dines in both sites were narrow (~25 - 60 metres), and are amongst the narrowest reported. The value of 0 for the B. oblongifolia side of the cline was extremely small (0.01-0.04), indicating that there is substantial introgression of the B. robur alleles into the B. oblongifolia genome, but not vice versa. Similarly, this asymmetrical gene flow was supported by a high value of B0, indicating a relatively weak barrier to B. robur alleles going into the B. oblongifolia genome. There was, however, a strong barrier to B. oblongifolia alleles going into the B. robur genome. The coefficient of linkage disequilibrium, determined within the pure stand populations of B. oblongifolia, and for the plants within the hybrid zones showed that, while there were no significant linkage association between loci within the pure stand B. oblongifolia, significant linkage was detected between loci within the hybrid zone populations. Similar results have been obtained in many other hybrid zones, where significant linkage between loci increases towards the middle of the cline, because of the constant influx of parental genotypes. Cline width, the extent of introgression and the linkage between loci within the hybrid zone were used to determine the dispersal distance of the alleles within the hybrid zone VI and the selection coefficients within and outside the cline. Dispersal distance was fairly long (100 - 450 metres) compared to the cline width, indicating long distance gene flow. Selection against the B. robur alleles within the hybrid zone was extremely strong (-100%), whilst outside the cline, the same alleles were subject to fairly weak selection (-3%). Strong selection against hybrids within the centre of the cline was supported by a significant deficit of heterozygotes within the hybrid zone (while the pure populations of B. oblongifolia were in Hardy-Weinberg equilibrium).