School of Agriculture, Food and Ecosystem Sciences - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
Item
-
ItemEvolutionary studies of the rye genome and chromosome variation in wheat-rye hybrids( 1988)The present study was undertaken firstly to examine the evolutionary relationship between cultivated rye (Secale cereale L.) and the wild Secale species based on repetitive sequence DNA, and secondly the changes that occur during tissue culture in the rye and wheat genomes, using the above DNA sequences in addition to N-banding and chromosome pairing analyses. Two families of tandemly repeated DNA sequences from the Secale species were examined for their evolutionary changes, namely the 5S DNA and rDNA. Two size classes of 5S DNA repeat units with basic repeat lengths of 460 by and 480 bp, designated the short and the long class respectively, were studied in detail from S. cereale, S. vavilovii, S. montanunn and S. silvestre . Two to five repeat units of the short and the long class were sequenced from each species. Differences in sequence structure of repeating units of the two classes were limited to deletions, insertions and point mutations in the spacer region. The deletions and insertions contributed to the 20 by difference in length of the two classes. Repeat units from both size classes in S. cereale were localized by in situ hybridization to two distinct chromosomal loci, one in the satellite of chromosome 1R distal to the NOR locus and the second site in the middle of the short arm of chromosome 5R. A preferential hybridization of the short 5S DNA units to the chromosome 1R site and the long units to the chromosome 5R locus indicated that the two classes are organized as separate arrays. The presence of the long 5S DNA units on 5R was confirmed by Bam HI and Taq I restriction enzyme analysis of DNA from cytogenetic stocks containing chromosome 5R addition to wheat. rDNA spacer length variation in twelve species of Secale (total of 178 plants) was examined by Tag I restriction analysis. Extensive spacer length variation (ranging from 0.9-3.1 Kb) existed in most species. S. silvestre was the only species to exhibit a unique spacer length variant of 0.9 Kb, which was shown to result from the presence of an extra Taq I site in the spacer. Furthermore, S. silvestre was the only species observed to be divergent in sequence composition of the rDNA spacer region relative to S. cereale as detected by DNA-DNA hybridization studies. Computer comparisons of 5S DNA sequences and rDNA spacer lengths from the Secale species allowed phylogenetic trees and phenetic relationships to be derived respectively. Both phenetic and cladistic methods were employed. The relationship between species from the 5S DNA and rDNA analyses, were consistent, and agreed with relationships proposed from more traditional morphological and chromosome pairing studies. In addition to the above, somaclonal variation in wheat X rye hybrids regenerated from embryo tissue culture was examined with molecular and cytogenetic techniques. rDNA spacer length variation was compared with that observed in natural populations of S. cereale. No new length variants were observed in the regenerants. Chromosome pairing data of hybrids derived from crosses between rye and 'Chinese Spring' wheat as well as the ph1b and ph2a mutants of 'Chinese Spring' did not show any significant increase in the overall pairing levels between cultured versus non-cultured hybrids. However, gross structural variation was detected in mitotic chromosomes of the hybrid regenerants by N-banding analysis. The usefulness of repetitive sequence DNA as markers in plant breeding and in examining evolutionary relationships between groups of related species, such as the genus Secale, is highlighted in the present thesis
-
ItemHybridization between Lycopersicon esculentum mill. and L. peruvianum (L.) mill( 1979)The incompatibility barrier between L.esculentum and L.peruvianum was shown to be incomplete and to vary for different L.peruvianum strains. The frequency of hybrid production was not increased when both species were grafted on to each other before cross-pollination. Pollinations with L.esculentum pollen followed immediately by L.peruvianum pollen and pollinations with irradiated L.peruvianum pollen also did not increase the frequency of hybrid production. However, using tetraploid L.esculentum as the female parent gave slight increases in the frequency of hybrids produced compared with the cross, 2n L.esculentum x 2n L.peruvianum. Cytological studies of the parent species and their hybrid at various ploidy levels demonstrated close cytological similarity of L.esculentum and L.peruvianum chromosomes and indicated there may possibly be as high a level of chromosome pairing in the diploid hybrid as in both diploid gents. The diploid and tetraploid hybrids were shown to have very high levels of pollen fertility and the triploid to have a much lower level of pollen fertility due probably to chromosome imbalance after meiosis. The results indicate that it would be possible to select within L.peruvianum for strains having compatibility with L.esculentum. It would be most useful to be able to hybridize readily the species at the diploid level because of regular chromosome pairing in the diploid hybrid and a high level of pollen fertility. While the use of tetraploid L.esculentum would slightly increase the frequency of hybrid production, the pollen fertility of the hybrids is low and in a system of back- crossing to diploid L.esculentum, there would be incompatibility between plants having different chromosome numbers