Browsing by Subject "Recurrent selection"

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Recurrent selection for increased outcrossing rates of barley from semi-arid regions of Syria and Jordan
(2010) Nandety, Aruna; Geiger, Hartwig H.
Improving the grain yield in drought stress environments such as the semi-arid areas of the West Asia North Africa (WANA) region has been a persistent problem since many years. Although barley (Hordeum vulgare L.) is widely grown in this region, the possibility of a crop failure is high. Being an autogamous crop, barley cultivars display almost complete homozygosity. Population genetic studies have shown that heterozygous barley genotypes possess a significantly increased stress tolerance, thus, being superior in both the level and stability of yielding performance. Therefore, increasing the level of heterozygosity in barley was the general aim of this study. For this purpose, a new marker-assisted recurrent selection (RS) approach was developed and applied to a genetically broad based world collection of barley germplasm. The specific objectives of this study were: (1) to investigate the efficacy of the above approach, (2) to determine the gain in heterozygosity over four RS cycles and to evaluate the selection results in a final experiment under common environmental conditions, (3) to estimate the selection differential, response to selection and realized heritability and (4) to provide barley materials with increased heterozygosity to plant breeding programs in the WANA region. Applying the RS approach, only plants showing superior levels of heterozygosity at co-dominant molecular marker (SSR) loci were advanced to successive selection cycles. These heterozygous plants were expected to carry a combination of advantageous alleles a) for open flowering from the female parent, and b) for pollen shedding from the male parent. For marker assessment, bulking of the plants and multiplexing of the SSR markers was practised in each selection cycle to save time and labour. The most polymorphic bulks were genotyped plant-wise and seed of the most heterozygous plants was advanced to the subsequent RS cycles. In the course of the RS experiment, a base population was compiled from 201 gene bank accessions held by the ?International Center for Agricultural Research in Dry Areas? (ICARDA) and the ?Institute of Plant Genetics and Crop Plant Research? (IPK) in Germany. Selection led to a stepwise increase in the heterozygosity from 0.60% in the base population to 3.24% after four cycles of selection. In the base population, the six-rowed landraces showed higher heterozygosity than the two-rowed. Selection response was highest in the first RS cycle which may be attributable to a major decline of the genetic variance from cycle to cycle and to a severe reduction of the population size due to strong dormancy among the entries selected in the first RS cycle. Very low realized heritabilities for observed heterozygosity were obtained in each RS cycle. Nevertheless, significant selection response was obtained. In order to compare the results of the individual RS cycles under common environmental conditions, preserved seed from each of the selected parent plants was grown in a final greenhouse experiment. Beside heterozygosity, various development, flowering and performance traits (days to ear emergence, anther extrusion, open flowering, number of ear bearing tillers, 100-grain weight and seed number) were additionally assessed in this experiment. The observed heterozygosity increased from 0.23% in population C1 via 0.69% in C2 and C3 to 1.29% in C4. The marker genotypes assessed in the final experiment were used to estimate multi-locus outcrossing rates. Values increased from 1.4% in C1 via 2.1% in C2 to 2.8% in C3 and C4. Generally, the increase from cycle to cycle was significant. Only the progress from C1 to C2 and from C3 to C4 did not reach the 5% significance level. All estimates were probably downward biased due to extremely high temperatures in the greenhouse during flowering. Great differences existed between the outcrossing rates of individual families within populations. Only non-significant weak to negligible correlations were obtained between floral traits and the outcrossing rate. The observed positive response to recurrent selection substantiates the efficacy of the present approach for enhancing the level of heterozygosity in barley, offering good perspectives for improving the productivity of the crop in the stress prone WANA region. The new selection approach, in principle, is applicable to other autogamous or partially autogamous crop plants as well.
Response to modified recurrent full-sib selection in two European F2 maize populations analyzed with quantitative genetic methods
(2006) Flachenecker, Christian; Melchinger, Albrecht E.
Many plant breeding strategies lead to a reduction in the genetic variance of the source population. However, a sufficient genetic variance is essential for the long-term selection response. Hence, the aim of recurrent selection (RS) is a continuous increase in the frequencies of favorable alleles while maintaining genetic variability in the population. Several intrapopulation RS methods have been proposed in maize: e.g., mass selection, half-sib selection, full-sib (FS) selection, S1 selection. Among them, recurrent FS selection is characterized by a short cycle length, complete parental control, and a high selection response. The goal of this thesis was to investigate the changes in the population structure over several cycles of a modified recurrent FS selection program in two European F2 maize populations. In detail, the objectives were to (i) monitor trends across selection cycles in the estimates of population mean, inbreeding coefficients, and variance components, (ii) determine selection response for per se and testcross performance, (iii) compare predicted with realized selection response, (iv) extend the population diallel analysis under full consideration of inbreeding depression due to random genetic drift, (v) separate genetic effects due to selection from those due to random genetic drift, and (vi) investigate the usefulness of best linear unbiased prediction (BLUP) estimates of parents for predicting progeny performance under the recurrent FS selection scheme applied. Four early maturing European flint inbreds were used as parents to produce two F2 populations (A×B and C×D). Both populations were three times intermated by chain crossing to reduce the gametic phase disequilibrium. Starting from the F2Syn3 population obtained in this manner, a modified recurrent FS selection program was conducted over four cycles in population A×B and over seven cycles in population C×D. In each cycle, 144 FS families were tested in field trials and, in parallel, six plants from each FS family were selfed. The selfed ears of the 36 families with the highest selection index (SI = 2 × dry matter content + grain yield) were selected and intermated according to a pseudo-factorial mating scheme. In this mating scheme, the gametic contribution of the best selected FS families is doubled compared with the gametic contribution of the remaining selected FS families. Afterwards, all cycles of both populations were tested in two population diallel analyses in six environments. Based on the known pedigree records, the inbreeding coefficient of each FS family and the coancestry coefficients among them were calculated. Variance components and BLUP values were obtained using phenotypic means and coancestry coefficients. For grain yield, the selection response per cycle, which could be expected after correcting for the effects of random genetic drift, was higher than reported in the literature (14.1% and 8.3% in populations A×B and C×D, respectively). We ascribe the comparatively high selection response mainly to the pseudo-factorial mating scheme. This mating scheme is expected to increase the selection response compared with commonly applied random mating schemes, without a major reduction in the effective population size (Ne). In this study, the expected Ne was 32, suggesting a minor influence of random genetic drift compared with that of selection. This assumption was verified by an extended population diallel analysis, showing that random genetic drift reduced the selection response only by about 1-2% in both populations. In contrast to an estimation of variance components with moment estimators, the REML procedure has no special requirements on the mating scheme and accounts for any relationship among families in a breeding population. As expected from the high Ne applied in our study, we observed only a moderate decrease in additive variance for grain yield and grain moisture in both populations. Nevertheless, the variance components were still associated with high standard errors, which prevented the revealing of trends across cycles. A larger number of test locations and larger population size would reduce the standard errors of variance components at the cost of oversized and expensive field trials. Methods for predicting the performance of progenies are important to optimize RS programs. Due to simplifying assumptions, a prediction with phenotypic means is often inaccurate. An alternative method is BLUP, which was suggested for predicting the performance of untested single-cross hybrids but has not been applied in RS programs. In our study, the prediction of progeny performance based on BLUP was only marginally better than prediction based on the phenotypic mean. However, higher degree of relationship between the entries and lower heritabilities would increase the advantage of BLUP compared with phenotypic means.