Search

Many wheat (Triticum aestivum L.) landrace cultivars (LCs) conserved in seed banks are not sufficiently characterized to inspire breeders' interest for their efficient exploitation. Patterns of genetic variation within and among wheat LCs are usually unknown. Two sets of wheat LCs stored in CIMMYT's plant genetic resources center were assessed for genetic diversity by means of 76 (Set 1) and 44 simple sequence repeat (SSR) markers (Set 2). Set 1 included 36 LC accessions originating from different countries, either collected as bulks, composed of a single LC subline, or an unknown collection method. Set 2 consisted of three to 25 sublines of five Mexican and four Turkish LCs already included in Set 1. In a principal coordinate analysis based on modified Rogers' distance (MRD), only three Turkish LC accessions formed a distinct cluster in Set 1. The Mexican accessions clustered together with a Spanish accession and a close relationship between a Chilean and Nigerian accession was observed. In Set 2, gene diversity ([H.sub.e]) among the Turkish LCs (0.43) was higher than among the Mexican LCs (0.35). Analyses of molecular variance (AMOVA) revealed considerable genetic diversity within Mexican (52.7%) and within Turkish (67.6%) LCs. Pairwise fixation indices ([F.sub.ST]) were significant, except between two Turkish LCs. Results were discussed in relation to the most suitable collection method of wheat LCs (bulk or individual sublines) as well as to the use of SSRs as a tool for seed bank management., WHEAT LANDRACES are genetically diverse and dyamic populations but are still morphologically recognizable because of a certain integrity (Harlan, 1975). Thousands of landrace cultivars (LCs) in wheat are stored in [...]

Diverse and varied crop genetic resources are necessary (though not sufficient) for adequate food production in a rapidly changing world. Since the scientific community first raised public concern several decades ago, modern cultivars have been viewed as the cause of declining diversity in the world's crop genetic resources. This paper tests the hypothesis of increasing genetic uniformity in modern spring bread wheat (Triticum aestivum L.) cultivars from 1965, a year which marks the release of some of the first modern semidwarf cultivars carrying Rht1 and Rht2 genes in the developing world. Results from previously published studies are summarized. Preliminary molecular analyses, and new analyses of cultivar numbers, areas, ages, and genealogies are presented. An estimated 77% of the spring bread wheat area in the developing world today is sown to CIMMYT-related wheats, but this does not imply that they are genetically uniform. The hypothesis of increasing genetic uniformity is tested by assessing changes in the diversity of leading progenitors over three decades, in terms of several dimensions of diversity. Latent dimensions include genetic distance and genealogies. Apparent dimensions include performance with respect to yield potential, maintenance and stability across management (input use), and growing environments. The data are not consistent with the view that the genetic diversity of modern semidwarf wheat grown in the developing world has decreased over time. Moreover, since national programs in developing countries cross CIMMYT lines with their own materials before releasing them, the genetic diversity in their cultivars is at least as great as that present among CIMMYT lines., NEARLY 30 YR HAVE PASSED since in a landmark study, the National Research Council (1972) alerted the scientific community and the public about the dangers of restricting crop improvement to [...]

Breeders can increase genetic diversity by selecting lines with novel parentage from international nurseries. A database program, WCOP, an application of the International Center for Maize and Wheat Improvement (CIMMYT) Wheat Pedigree Management System (WPMS), calculated the coefficient of parentage (COP) among cultivars and was used to study 169 wheats from the International Spring Wheat Yield Nurseries (ISWYN) No. 17 to 27, and a set of 27 reference cultivars. The average COP (excluding similarity values of cultivars with themselves) among the 196 lines was 0.14. The similarity between the ISWYN cultivar group and the reference cultivar group was 0.08. Average internal COP values for U.S.-Canadian, Australian, and ISWYN sets were 0.18,0.30, and 0.14, respectively. The ISWYN cultivars were clustered by means of COP values into genealogically similar groups. The largest ISWYN clusters were divergent from U.S., Canadian, and Australian standard spring cultivars. The clusters highlight divergent germplasm that may broaden the genetic base of breeding programs in temperate zones., Genetic variation is required in a breeding program to achieve genetic gains. How do plant breeders search for useful diversity for crop improvement? COP can estimate of genetic diversity among [...]

Crop genetic diversity may not be apparent from plant phenotypes. Coefficients of parentage were used to examine genetic diversity and compare trends in spring wheat cultivars grown in the Yaqui Valley of Mexico (1972-1991) and the Punjab of Pakistan (1978-1990). Coefficients of parentage estimate probabilities of retaining latent diversity rather than focusing on the apparent diversity of highly heritable traits (e.g., rust resistance alleles). A small trend towards increased genetic diversity was found in the Pakistani Punjab. In the Yaqui Valley, large oscillations in diversity occurred without significant trends. In comparing the two regions, the latent genetic diversity in the Yaqui Valley based on parentage and area of production in a given year (weighted diversity) was 20% less than in the Punjab. However, the lower diversity was partially offset by rapid turnover in germplasm in the Yaqui Valley. The rate of change in germplasm (temporal diversity) was 22% lower in the Pakistani Punjab than in the Yaqui Valley. The temporal diversity of cultivars used by growers in the Yaqui Valley was greater than among cultivars recommended for use by the research services, indicating that growers replace germplasm faster than the changes in recommended cultivars. The opposite was found in the Punjab. In the Yaqui Valley, individual cultivars dominated production for 1 to 3 yr, while in the Punjab, one cultivar, 'Yecora' dominated production for 7 yr. We conclude that (i) cultivar improvement programs did not erode genetic diversity in production areas currently growing improved cultivars and (ii) patterns of cultivar use by farmers were more important than the composition of recommended cultivars in determining genetic diversity.

Reports on a study which used the data program `WCOP' which is an application of the International Center for Maize and Wheat Improvement Wheat Pedigree Management System to calculate the coefficient parentage of cultivars. Indication that several wheats were used from the International Spring Wheat Yield Nurseries (ISWYN) No. 17 to 27; Similarities between the ISWYN and the reference cultivar group; Results of the study.