School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemDevelopment of molecular markers for the genetic improvement of lentil( 1999)Molecular markers were applied in this study to explore two essential aspects of cultivated lentil breeding programs: 1) the genetic resources available within the species and within the Lens genus, including the wild relative species and 2) the genetics of resistance to foliar infection by the fungal pathogen Ascochyta lentis, examining both the host resistance mechanism and the diversity of the pathogen. 1) The genetic diversity within the Australian lentil breeding program was shown to be limited, representing a potential major constraint to the genetic improvement of lentil in Australia (Ford et al., 1997). Wild relative germplasm may be a source of variability for the transfer of desirable traits into superior breeding lines. A phylogeny of the Lens genus, based on nucleotide data from the 5S rRNA mutligene family, showed that L. culinaris ssp. orientalis was the closest and L. odemensis was the next closest related taxa to the cultivated species. Due to their close genetic relationships and crossibility with the cultivated species, these (sub-) species may provide new sources of useful germplasm. 2) The current major limiting factor to lentil production in Australia is ascochyta blight disease caused by Ascochyta lentis. A major strategy for disease control is breeding for resistance. However, pathogen variability is a threat to resistance breeding in that the pathogen may quickly adapt to overcome host resistance. A. lentis isolates in Australia were found to be as genetically variable as A. lentis isolates originating outside Australia. The genetics of resistance to foliar infection of A. lentis within cultivar ILL5588 was determined to be controlled by a single major dominant gene (AbRI). Molecular markers linked to the AbRI gene locus were identified with bulked segregant analysis and random amplified polymorphic DNA analysis. The two closest flanking markers being 6 and 14 cM away from AbRI. These markers were investigated for the potential to produce stable sequence characterised amplified regions (SCARS) for the identification of the AbRI locus in other lentil germplasm and the consequent use of the AbRI locus in marker assisted selection.
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ItemThe development of molecular markers for shatter resistance in Brassica rapa L( 1999)Shattering is a major problem in canola production worldwide and especially in Australia because the crop matures in summer under hot and windy condition. Little variation for shatter resistance is available in Brassica napus, which is the major canola species. Considerable variation for shatter resistance is available in B. rapa, which is a relatively minor oilseed species. Shatter resistance is being transferred from B. rapa into B. napus by backcrossing. Screening for shatter resistance in canola breeding programs is currently based on mechanical tests of individual siliqua strength. The test, which is time consuming, can only take place with completely dry and mature siliquae. This study aimed to identify molecular markers linked to shatter resistance in a population segregating for this trait, and to develop specific and robust primers to be used in a routine test, for screening for shatter resistance in canola breeding programs. Crosses were made between the shatter-susceptible Canadian cv. Torch of B. rapa L. ssp. oleifera (Metzg.) Sinsk and a shatter-resistant Indian line, DS-17-D of B. rapa L. ssp. oleifera var. Brown Sarson (Singh) Prakash. Shatter resistance was found to be a recessive trait controlled by two major genes, designated sh1 and sh2. The phenotypic segregation in the F3 population of this cross fitted a Mendelian ratio of 12: 3: 1 for shatter-susceptible (S), intermediate-shattering (M) and shatter-resistant (R) respectively. Further evidence from the phenotypic segregation in 19 F3 families was in agreement with the FZ ratio of 12: 3: 1. Bulked segregant analysis coupled with random amplified polymorphic DNA (RAPD) analysis was applied to both F, and F3 populations of the cross. Three RAPD markers, designated RAC-3, RX-7 and SAC-20, were identified from the F3 population. RAC-3 and RX-7 appeared to be linked in coupling to sh1 and sh2 at approximate distances of 13 cM and 20 cM respectively, whereas SAC-20 appeared to be linked in repulsion to both of these alleles at approximately 20 cM. The linkage of SAC-20 to both loci supported a theory of chromosome duplication during evolution of the B. rapa genome. Three pairs of sequence characterized amplified region (SCAR) primers designated SCAC-3, SCX-7 and SCAC-20, 21 to 24 bp in length were designed based on the sequences of RAPD markers RAC-3, RX-7 and SAC-20 respectively. Only the SCX-7 primer produced polymorphic markers corresponding to RX-7 amplified from 'shatter-resistant' DNA individuals, while SCAC-3 and SCAC-20 produced amplified products corresponding to RAC-3 and SAC-20 to both shatter-resistant and shatter-susceptible DNA individuals. SCAR SCX-7 was subsequently tested with other Brassica populations including B. napus and BC,F, of B. napus introgressed with B. rapa to investigate the presence of the sh2 gene. All lines from the backcross populations possessed the expected SCX-7 marker with the exception of one line where the marker was absent. This indicated that shatter-resistant gene sh2 was present in all these populations except for the one line.
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ItemThe effects of post-anthesis heat stress on wheat yield and quality( 1996)Post-anthesis temperature is a major determinant of wheat yield and quality. Post-anthesis temperatures in the moderately high range (ca 25-32C) are known to reduce grain yield but increase bread wheat quality, whereas very high (>32C) temperatures are known to significantly reduce both yield and quality. In Mediterranean and continental climates, such as Australia and the US., wheat is exposed to moderately high temperatures throughout most of the grain filling period, and very high temperatures may occur for an average 8% of grain growth. This thesis is primarily concerned with examining the effects of very high temperature on wheat yield and quality. Specifically, the study was designed to: 1) quantify the effects of short (3-5 day) periods of very high temperature on wheat yield and quality; and 2) determine the extent of genotypic variation in response of wheat yield and quality to very high temperature. Two varieties of wheat differing widely in heat tolerance were selected from 75 cultivars of wheat that were screened for tolerance to very high temperature. These two varieties (Oxley and Egret, heat sensitive and heat tolerant, respectively) were exposed to a variety of heat treatments in order to determine whether varietal differences in heat tolerance were maintained for heat treatments occurring at 3) different stages of grain growth and for 4) varying durations of heat stress. The 5) interaction of moderately high and very high temperatures was examined in order to determine whether cool temperatures following severe heat stress could alleviate the deleterious effects of very high temperature on yield and quality. In order to 6) examine the importance of acclimation to heat stress and to 7) establish a repeatable selection methodology, the impact of sudden increases to a high maximum temperature was compared with more gradual (6C h-1) rises to the same high temperature (40C). For each of the experiments 3 to 7 (above) results are presented for the effects of heat stress on: a) the accumulation of grain dry matter and water during grain growth; b) the accumulation during grain growth of total protein and its functionally-important fractions (SDS-soluble and SDS-insoluble polymer [glutenin], monomer [gliadin] and albumin/globulin), as determined by size-exclusion high-performance liquid chromatography and c) dough mixing behaviour using the 2-g mixograph. It is concluded that: 1) wheat genotypes vary widely in their responses of yield and quality to short periods of very high temperature; 2) the response to heat stress varies with the timing of stress: yield was reduced more by early than late-applied stress, whereas dough strength tended to decline most markedly in response to heat stress applied towards the end of grain filling; 3) both grain yield and dough strength declined linearly with increased duration of heat stress; 4) in a heat sensitive variety, moderately high and very high temperatures during grain filling each reduced grain yield and dough strength: cool temperatures following exposure to very high temperature did not reduce the effects of very high temperature on either yield or quality; 5) some varieties of wheat appear to acclimate rapidly to heat stress: a gradual (6C h-1) increase from ca 20-40C lessened the impact of heat stress on yield and quality when compared with a sudden increase over the same temperature range. These results are discussed with special reference to their implications for: 1) selecting and breeding for heat tolerance in wheat; 2) predictive modelling of the effects of very high temperature on wheat yield and particularly quality.
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ItemGenotype and environmental influences on phasic development in wheat (Triticum aestivum L.) and the expression of yield components, especially spikelet number per head( 1995)The variation in, and the influence of, certain environmental factors on preanthesis phases of development in wheat was examined with particular reference to the number of spikelets produced per head. When the pre-anthesis phase was divided into three phases; the Ieaf initiation, spikelet initiation and culm elongation phases, considerable cultivar variation was found in the durations and rates of the three phases. A cultivar was found that departed from the general negative correlation between rate and duration of spikelet initiation giving possible scope for breeding for increased spikelet number without altering the duration of spikelet initiation. Variation in the rate and duration of the three development phases was also found for a selection of diploid and tetraploid wheat. For these species, spikelet number was found to be more closely associated with the duration of spikelet initiation. This character could be of use in long-season wheat cultivars. When the effects of photoperiod and light intensity on wheat phasic development and spikelet number were compared, photoperiod was found to have more influence. The transfer of wheat cultivars between long and short photoperiods at double ridge and terminal spikelet determined that the rate of development was influenced by a "memory" effect; both prior and current photoperiods influenced the rate of development. It was also noted that initial exposure to long photoperiod could have a sustaining effect on wheat development. Subjecting wheat lines to increased temperature increased the durations of development, in terms of thermal time, indicating that the relationship was not linear. The durations of pre- and post-terminal spikelet phases were found to respond differently to temperature. A selection of 6 wheat cultivars, varying in time to anthesis were grown in the field and it was found that photoperiod responses exerted the major influence on the durations of development. Basic development responses and vernalisation were found to exert comparatively less influence on development. The importance of basic development responses were not discounted as a means for breeding wheat cultivars for specific environments.
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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
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ItemCytology and fertility of wheat-rye (Secale) hybrids with induced pairing between homoeologous chromosomes( 1986)Studies were made of the influence of individual rye chromosomes and their arms on crossability, chromosome pairing and pollen and spikelet fertility in hybrids of hexaploid wheat cv. 'Chinese Spring' and its two homoeologous pairing mutants, ph2a and phIb , each with six wheat (cv. 'Holdfast') rye ('King II') chromosome addition lines and their telocentrics. Crosses were also made of the three 'Chinese Spring' parents each with seven rye (Secale) accessions, including different species. Studies were made of crossability, hybrid viability, pollen and spikelet fertility, and chromosome pairing in both the amphihaploids and amphidiploids from these crosses. Studies were also made of crossability, chromosome pairing and pollen and spikelet fertility in the F1 of crosses of the three 'Chinese Spring' wheats with both an octoploid and a hexaploid triticale. In these studies new information was being sought, both on genetic and evolutionary affinities between rye and hexaploid wheat and on approaches for the more efficient exploitation of genetic variation in the rye gene pool for wheat improvement.