Genome analysis in higher plants

Abstract

Gordon Kimber introduced the workshop topics that were designed to illustrate several facets of genome analysis in higher plants. Kimber's presentation covered genome analysis by chromosome pairing in hybrids. He emphasized that this is the only method in which at least some DNA is sampled along the entire length of the chromosomes. Moreover, originally based on total pairing, genome analysis has now been extended to include numerical assessments of the patterns of pairing. Genome analysis is most accurate in those hybrids where there is pairing competition; for example in diploid x allopolyploid hybrids. There are various models of pairing at increasing levels of ploidy that can be used to calculate relative measures of affinity among the genomes. In the ensuing discussion,, Kimber pointed out that genes affecting chromosome pairing do not alter the results of genome analysis as they influence overall pairing, but not the pattern of pairing. Limitations to genome analysis were also discussed. For example, chromosome pairing in diploid hybrids cannot be considered a reliable indication of relationship because of lack of competition for synaptic partners. Similarly, plants with small chromosomes are also unsuitable for genome analysis as differences between rod and ring bivalents cannot be distinguished. Second speaker, B. S. Gill, spoke on the use of heterochromatin banding and in situ hybridization in assaying subgenomic affinities. These techniques provide additional and complementary information to genome analysis by chromosome pairing. The subgenomic affinities that can be assayed include individual chromosome and chromosome arm homologies and specific DNA sequence homologies. Individual chromosome homologies have been assessed by banding analysis of metaphase I configurations in meiocytes. Somatic chromosome banding analysis provides additional information on the amount, location, and structure of heterochromatin. In situ hybridization has been used to study the amount and location of specific DNA sequences in different genomes. Such marked chromosomes can be traced in natural and experimental introgressive hybridization studies. Southern blotting analysis with defined DNA sequences (rDNA, as an example) at different 'stringencies of hybridization can be used to discern evolutionary relationship among taxa. The questions involved strategies for cloning of DNA sequences for molecular cytogenetics analysis. The third speaker, K. Tsunewaki described the role of plasmon analysis as a means of identifying cytoplasm donor to polyploid species. Cytoplasm of various Aegilops species have been substituted with the nucleus of Triticum aestivum. From the phenotypes of such alloplasmic lines, it has been possible to trace diploid donor of cytoplasm of polyploid Triticum and Aegilops species. More recently, restriction digestion analysis of chloroplast and mitochondrial DNA has been used for assaying cytoplasmic affinities. Some of the problems with the use of plasmon analysis in phylogenetic affinities were discussed. First of all, there is intraspecific variation in both mitochondrial and chloroplast DNAs. Another potential problem is the male transmission of cytoplasmic organelles, although in wheat no such male transmission has been documented. The final speaker, D. F. Weber, discussed the use of restriction fragment length polymorphisms (RFLPs) in corn improvement. He elaborated on the methodology and applications of RFLP analysis. In addition to their use as molecular markers, RFLPs are also useful in uncovering single gene or small chromosomal duplications or other aberrations that cannot be detected by pairing analysis. About 400 RFLP loci have been mapped in corn. About 20 % of the loci were found to be duplicated either on the same chromosome or were dispersed on many chromosomes. The A-B translocations were useful in chromosome arm assignment of RFLP loci, in mapping of centromeres, and in aligning RFLP linkage maps with chromosome maps and conventional linkage maps. There was considerable discussion on the advancement of RFLP maps in different taxa. It was suggested that there should be a coordination of the work on RFLP maps and on comparative gene mapping. Annual meetings of the Genetics Society of Canada or America could provide forums for joint meetings of RFLP workers in plants. The workshop ended on a note of thanks by Kimber to participating speakers and the audience. About 60 people participated and the workshop was considered a success.