The wheat cultivar Kariega expresses complete adult plant resistance to stripe rust in South Africa. The aim of this investigation was to determine the extent and nature of variability in stripe rust resistance in a population of 150 doubled haploid lines generated from a cross between Kariega and the susceptible cultivar Avocet S. Analysis of field data for adult plant stripe rust resistance identified two major QTLs and two minor QTLs in the resistant cultivar Kariega. The two major QTLs were located on chromosomes 7D ( QYr.sgi-7D) and 2B ( QYr.sgi-2B.1), contributing 29% and 30% to the phenotypic variance, respectively. QYr.sgi-2B.1 is primarily associated with a chlorotic and/or necrotic response, unlike QYr.sgi-7D, which is believed to be the adult plant resistance gene Yr18. These two QTLs for adult plant resistance in Kariega appear to represent different forms of resistance, where QYr.sgi-7D may represent potentially more durable resistance than QYr.sgi-2B.1. Mixture model analysis of the field leaf infection scores suggested a genetic model involving two independent genes combining in a classical, epistatic manner. The results of the QTL analysis demonstrate its higher resolution power compared to the mixture model analysis by detecting the presence of minor QTLs.
Genetic variation in chilling requirement was investigated over three growth periods using clonal progenies of six apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] families derived from crosses of high and low chill requiring cultivars. Two quantitative measurements related to chilling requirement, viz., the time of initial budbreak (vegetative and reproductive) and the number of breaking buds over a specified time interval, were used as evaluation criteria. Genetic and environmental variances of the traits are presented as intra-class correlation coefficients for clones within and between families. For budbreak time, reproductive and vegetative, broad-sense heritability averaged around 75% and 69% respectively, indicating a high degree of genetic determination in this material. For budbreak number, moderate to low genetic determination was found with broad-sense heritabilities around 30%. Estimates of genetic components of variance between families were generally very low in comparison to the variance within families and predict potentially favorable responses to truncation selection on the traits within these progeny groups. Analysis of the data showed that distribution of budbreak time is typical of quantitative traits with means distributed closely around midparent values. Skewed distributions towards low budbreak number were obtained in varying degrees in all families.
A locus affecting abdomen pigmentation of Drosophila melanogaster females is shown to have a large number of alleles in wild populations. Some of these also affect sternopleural bristle count.
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