TY - THES T1 - Assessing the Genetic Diversity in Crops with Molecular Markers: Theory and Experimental Results with CIMMYT Wheat and Maize Elite Germplasm and Genetic Resources T3 - Crop Science, Theoretical and Applied Genetics A1 - Reif,Jochen Christoph Y1 - 2004/10/11 N2 - Genetic diversity is a valuable natural resource and plays a key role in future breeding progress. Germplasm collections as a source of genetic diversity must be well-characterized for an efficient management and effective exploitation. The advent of PCR-based molecular markers such as sim-ple sequence repeats (SSRs) has created an opportunity for fine-scale genetic characterization of germplasm collections. The objective of this research was to optimize the utilization of genetic re-sources conserved at the International Wheat and Maize Improvement Center (CIMMYT), with the aid of DNA markers. Choice of suitable dissimilarity measures is important to facilitate the interpretation of findings from DNA marker studies on a theoretically sound basis. The objective of a theoretical study was to examine 10 dissimilarity coefficients widely used in germplasm surveys, with special focus on applications in plant breeding and seed banks. The distance and Euclidean properties of the dissimi-larity coefficients were investigated as well as the underlying genetic models. Application areas for different coefficients were suggested on the basis of the theoretical findings. It has been claimed that plant breeding reduces genetic diversity in elite germplasm, which could seriously jeopardize the continued ability to improve crops. The objectives of the presented ex-perimental study with wheat were to examine the loss of genetic diversity during (i) domestication of the species, (ii) change from traditional landrace cultivars (LC) to modern breeding varieties, and (iii) intensive selection over 50 years of international breeding. A sample of 253 CIMMYT or CIMMYT-related modern wheat cultivars, LC, and Triticum tauschii accessions were characterized with up to 90 SSR markers covering the entire wheat genome. A loss of genetic diversity was ob-served from T. tauschii to LC and from LC to the elite breeding germplasm. Wheat genetic diver-sity was narrowed from 1950 to 1989, but was enhanced from 1990 to 1997. The results indicate that breeders averted the narrowing of the wheat germplasm base and subsequently increased the genetic diversity through the introgression of novel materials. The LC and T. tauschii contain nu-merous unique alleles that were absent in modern wheat cultivars. Consequently, both LC and T. tauschii represent useful sources for broadening the genetic base of elite wheat breeding germ-plasm. In the 1980's, CIMMYT generated more than 100 maize populations and pools but little is known about the genetic diversity of this germplasm. The objective of the study with 23 CIMMYT maize populations was to characterize their population genetic structure with SSRs. The populations adapted to tropical, subtropical intermediate-maturity, subtropical early-maturity, and temperate mega-environments (ME) were fingerprinted with 83 SSR markers. Estimates of genetic differen-tiation between populations revealed that most of the molecular variation was found within the populations. Principal coordinate analysis based on allele frequencies of the populations revealed that populations adapted to the same ME clustered together and, thus, supported clearly the ME structure. Novel strategies were suggested to optimize the conservation of the genetic diversity within and among the populations. Heterotic groups and patterns are of fundamental importance in hybrid breeding. The objective of the presented study with a subset of 20 out of the 23 maize populations was to investigate the rela-tionship between heterosis and genetic distance determined with SSR markers. The published data of three diallels and one factorial trial evaluated for grain yield were re-analyzed to calculate het-erosis in population hybrids. Correlations of squared modified Rogers distance and heterosis were mostly positive and significant, but adaption problems caused deviations in some cases. For popu-lations adapted to the target regions, genetic distance can be used as a further criterion in the search for promising heterotic patterns and groups. For intermediate- and early-maturity subtropical germ-plasm, two heterotic groups were suggested, consisting of a flint and dent composite. For the tropi-cal germplasm, it was possible to assign population (Pop29) to the established heterotic group A and propose new heterotic groups (Pop25, Pop43). Our experimental results corroborate that SSRs are a powerful tool to (i) detect relationships among different germplasm, (ii) assess the level of genetic diversity present in germplasm pools and its flux over time, and (iii) search for promising heterotic groups for hybrid breeding in complementa-tion to field trials. KW - Heterosis KW - Hybridzüchtung KW - Genetische Variabilität KW - Genetische Ressourcen CY - Hohenheim PB - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim AD - Garbenstr. 15, 70593 Stuttgart UR - http://opus.uni-hohenheim.de/volltexte/2004/63 ER -