Analysis of genetic control of mating behavior in screwworm (Diptera: Calliphoridae) males through diallel crosses and artificial selection

Summary The mode of genetic control of male screw-worm (Diptera: Calliphoridae) mating behavior was examined using diallel cross and artificial selection. Diallel crosses showed strong dominance effects, with hybrids being uniformly more successful in copulation than their more inbred parental strains. Weaker additive and reciprocal effects were also noted. Environmental (replicate) effects were highly significant. Regression of array variances and covariances indicated that epistatic interactions or unequal allele distribution during gametogenesis may have occurred and that high courtship propensity polygenes show dominance over low propensity genes. Artificial selection on males from outbred strains from Guatemala and Belize resulted in a decreased number of mating attempts for lines selected for reduced activity, but mating attempts in lines selected for high mating activity did not increase. A combination of inbreeding during the selection cycles as well as selection for recessive traits would explain this response. The two types of experiments were in general agreement, indicating significant dominance and environmental influence on male mating behavior with weaker additive and possible maternal effects.     

Quantitative Genetics of Doubled Haploid Populations and Application to the Theory of Line Development

The line value of a genotype is defined as the expected value of all lines that can be derived from this genotype. Specific genetic effects are defined for this value: only additive and additive by additive epistatic effects are necessary. There is no dominance effect for such a value. A general expression for the covariances between related lines is given. From a design with several lines per haplodiploidized plant taken at random from a population it is possible to estimate the additive variance for line value and the variance of additive by additive epistasis for line value. Variances of higher order epistasis can be estimated with a two-factor mating design in which a cross is replaced by the population of lines that can be derived from it. With a diallel or a factorial design a direct test for the presence of homozygous by homozygous epistasis is possible. The application of the concept of line value to the theory of line development leads to simple expressions of genetic advance in one cycle of recurrent selection according to the testing system. A brief consideration of these expressions leads to the conclusion that single doubled haploid descent recurrent selection will be one of the most efficient methods for low heritabilities and with a rapid development of doubled haploid lines.     

Effect of gametic disequilibrium on selection in an autotetraploid population

Summary The effects of a gametic disequilibrium (DSE) in an autotetraploid population on response to selection as measured by the covariance of selection were investigated. The theoretical responses were calculated for mass selection [Mass (1)] and half-sib progeny test selection (HSPT) in a two-allele (B and b), single locus, autotetraploid population. The complexity of calculations precluded analytical expressions for the covariances so numerical analysis was used assuming the following genetic models: monoplex dominance, partial monoplex dominance, duplex dominance, partial duplex dominance, and additive gene action.The results indicated the DSE could greatly affect the covariance of selection. For a constant allele frequency the DSE might double the covariance expected with selection in a population at random mating equilibrium (RME) of gametes, but in other instances approach zero. For all genetic models and the two breeding methods investigated the covariance of selection was always increased when the frequency of BB gamete exceeded p² (where p is frequency of allele B) and decreased when the frequency of BB gamete was less than p². The possible incorporation of this information into a long term breeding program and some other ramifications were briefly discussed.With the DSE the covariances of selection with HSPT and Mass (1) had a proportionality of 1:2, respectively, with the additive genetic model, but this relationship rarely occurred for other genetic models. The deviations from this ratio were not large in comparison to differences between selection in populations in DSE and RME.Cooperative investigations of the Alfalfa Production Research Unit, United State Department of Agriculture, Agricultural Research Service, and the Nevada Agricultural Experiment Station, Reno, Nevada. Paper No. 512. Scientific Journal Series, Nevada Agricultural Experiment Station     

Estimates of genetic variance in an F2 maize population

Maize (Zea mays L.) breeders have used several genetic-statistical models to study the inheritance of quantitative traits. These models provide information on the importance of additive, dominance, and epistatic genetic variance for a quantitative trait. Estimates of genetic variances are useful in understanding heterosis and determining the response to selection. The objectives of this study were to estimate additive and dominance genetic variances and the average level of dominance for an F2 population derived from the B73 x Mo17 hybrid and use weighted least squares to determine the importance of digenic epistatic variances relative to additive and dominance variances. Genetic variances were estimated using Design III and weighted least squares analyses. Both analyses determined that dominance variance was more important than additive variance for grain yield. For other traits, additive genetic variance was more important than dominance variance. The average level of dominance suggests either overdominant gene effects were present for grain yield or pseudo-overdominance because of linkage disequilibrium in the F2 population. Epistatic variances generally were not significantly different from zero and therefore were relatively less important than additive and dominance variances. For several traits estimates of additive by additive epistatic variance decreased estimates of additive genetic variance, but generally the decrease in additive genetic variance was not significant.     

Selection and stability of synthetic varieties of Lolium perenne

Summary The performance of three experimental cultivars of Lolium perenne selected for yield or water soluble carbohydrate content was monitored over four generations of seed multiplication under relaxed selection. In each variety the selected trait regressed towards that of the base population from which the selection line derived. This could be accounted for by residual genetic variation within the lines for the selected trait, and in some instances, by association of this variation with the fitness character, seed numbers produced. These results emphasize the need for practical breeding programmes to consider the nature of the gene action controlling the selected trait, if additive, directional selection should be effective in increasing the expression of the character. Where ambidirectional dominance and epistasis are important, consideration should be given to means of achieving reassortment of the controlling genes prior to selection.     

Inheritance of resistance in cucumber to race 2 of Colletotrichum lagenarium

Summary The resistant breeding line, AR79-95, and the susceptible cultivar, Model, were crossed to develop F₁, F₂, F₃, and backcross populations for genetic analysis of resistance in cucumbers to race 2 of Colletotrichum lagenarium (Pass.) Ellis & Halsted., the causal agent of cucurbit anthracnose. There was no maternal effect on resistance and a small amount of F₁ heterosis toward the susceptible parent. Generation means analysis showed that there was additive and dominance but no epistatic gene action detected on the scale used. Additive and dominance genetic variances were estimated, and narrow-sense heritability was low to moderate. Based on effective factor formulae, at least five effective factors contrtolled the resistance. Some of these factors were dominant and others recessive. Implications for breeding procedures are discussed.     

Identification of Indica rice chromosome segments for the improvement of Japonica inbreds and hybrids

Exploitation of heterosis has brought significant advance in plant breeding and agricultural production, although its genetic basis is still poorly understood. In this study, a total of 66 chromosome segment substitution (CSS) lines, derived from a cross between japonica rice inbred line Asominori (as the recurrent parent) and indica rice inbred line IR24 (as the donor parent), were used to investigate the genetic basis of heterosis in indica × japonica inter-subspecific rice hybrids. Each CSS line was crossed with the background parent Asominori, and the heterosis of F(1) hybrids was estimated by comparing the F(1) performance with its two parental lines. Field experiments were carried out across six different environments to evaluate yield and yield-related traits in the 66 CSS lines and their 66 corresponding F(1) hybrids. Quantitative trait loci (QTL) analyses were conducted using a likelihood ratio test based on the stepwise regression. Thirty-six QTL were identified with significant effects in CSSL, 21 with significant effects in hybrids and 13 with significant effects in both. On the basis of average dominance degree, of all the 70 QTL affecting yield-related agronomic traits, 28.6% (20) showed an overdominance, 35.7% (25) a partial dominance and 30% (21) an additive effect, indicating that all effects contribute to trait variation in japonica-indica rice hybrids. Effects of these QTL were examined to identify Indica rice chromosome segments of interest for the improvement of japonica inbred lines and hybrids.     

分子标记辅助选择改良水稻不育系臻达A及其杂交种的稻瘟病抗性

水稻抗稻瘟病基因Pi25是一个遗传传递能力强的广谱抗性基因。本研究以携带抗稻瘟病基因Pi25的BL27为抗源供体,与优质、配合力强、感稻瘟病的水稻保持系臻达B为受体亲本进行杂交、回交创制水稻抗病保持系新种质,再与臻达A测交和回交进行不育系转育,结合分子标记辅助选择和农艺性状筛选,获得3个抗性基因纯合、农艺性状和开花习性均与臻达A相似的改良不育系株系。利用福建省近年来致病性代表的22个稻瘟病菌株对3个改良不育系及其15个杂交种进行抗性鉴定,3个改良不育系的抗性频率为95.45%~100%,15个杂交种的抗性频率均达75%以上,而原始对照臻达A及其杂交种的抗性频率仅为54.55%和40.91%~63.64%。自然病圃诱发鉴定表明,3个改良不育系的叶瘟和穗颈瘟均为0级,表现高抗,而对照臻达A的叶瘟为5级,穗颈瘟为7级,表现感病;15个杂交种均表现良好的稻瘟病抗性。进一步分析比较15个杂交种的产量、农艺性状和稻米品质表现,结果表明臻达A-Pi25-3改良不育系的综合性状表现最优,继续回交转育,于2015年育成了稻瘟病抗性强、配合力好、群体整齐和性状稳定的不育系,命名为157A。研究表明,抗稻瘟病基因Pi25不仅在水稻不育系臻达A的遗传背景下的抗性表达完全,且在不同水稻恢复系测交种的背景下同样表现出较高水平的抗性,说明抗性基因Pi25对不育系稻瘟病改良的效果明显。创制的新不育系157A的稻瘟病抗性显著提高,还基本保留了原来不育系高配合力等优良特性,为选育高产、优质、抗病杂交稻新品种提供了不育系新种质。     

烤烟主要农艺性状的遗传与相关分析

利用包括基因型与环境互作的加性-显性遗传模型,对14个烤烟品种(系)及其配制的41个杂交组合在4个环境下的7个农艺性状表现进行遗传分析。结果表明,株高、节距、腰叶宽主要受加性效应控制,叶数、腰叶长受显性×环境互作效应影响最大,茎围以加性×环境互作效应、显性×环境互作效应为主,产量以加性效应、显性×环境互作效应为主。适应当地生态条件的品种（系）具有较高的正向加性效应。许多组合的显性主效应及在各试验点的显性×环境互作效应在方向上不尽一致,杂交组合的选配宜针对特定的生态环境进行。性状相关分析表明,大多数成对性状的各项相关系数为正值,且多以加性遗传相关为主,可利用株高对产量进行间接选择。
     

Selection theory for selfed progenies

Summary The purpose of this article was to extend the model used to predict selection response with selfed progeny from 2 alleles per locus to a model which is general for number and frequency of alleles at loci. To accomplish this, 4 areas had to be dealt with: 1) simplification of the derivation and calculation of the condensed coefficients of identity; 2) presentation of the genetic variances expressed among and within selfed progenies as linear function of 5 population parameters; 3) presentation of selection response equations for selfed progenies as functions of these 5 population parameters; and 4) to identify a set of progeny to evaluate, such that one might be able to estimate these 5 population parameters.The five population parameters used in predicting gains were the additive genetic variance, the dominance variance, the covariance of additive and homozygous dominance deviations, the variance of the homozygous dominance deviations and a squared inbreeding depression term.Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 9971     

An additive-dominance model to determine chromosomal effects in chromosome substitution lines and other gemplasms

When using chromosome substitution (CS) lines in a crop breeding improvement program, one needs to separate the effects of the substituted chromosome from the remaining chromosomes. This cannot be done with the traditional additive-dominance (AD) model where CS lines, recurrent parent, and their hybrids are used. In this study, we develop a new genetic model and software, called a modified AD model with genotype × environment interactions, which can predict additive and dominance genetic effects attributed to a substituted alien chromosome in a CS line as well as the overall genetic effects of the non-substituted chromosomes. In addition, this model will predict the additive and dominance effects of the same chromosome of interest (i.e. chromosome 25 of cotton in this study) in an inbred line, as well as the effects of the remaining chromosomes in the inbred line. The model requires a CS line, its recurrent parent and their F₁ and/or F₂ hybrids between the substitution lines and several inbred lines. Monte Carlo simulation results showed that genetic variance components were estimated with no or slight bias when we considered this modified AD model as random. The correlation coefficient between predicted effects and true effects due to the chromosomes of interest varied from zero to greater than 0.90 and it was positively relative to the difference between the CS line and the recurrent line. To illustrate the use of this new genetic model, an upland cotton, Gossypium hirsusum L, CS line (CS-B25), TM-1 (the recurrent parent), five elite cultivars, and the F₂ hybrids from test-crossing these two lines with the five elite cultivars were grown in two environments in Mississippi. Agronomic and fiber data were collected and analyzed. The results showed that the CS line, CS-B25, which has chromosome 25 from line 3 to 79, Gossypium barbadense substituted into TM-1, had positive genetic associations with several fiber traits. We also determined that Chromosome 25 from FiberMax 966 had significantly positive associations with fiber length and strength; whereas, chromosome 25 from TM-1 and SureGrow 747 had detectable negative genetic effects on fiber strength. The new model will be useful to determine effects of the chromosomes of interest in various inbred lines in any diploid or amphidiploid crop for which CS lines are available.     

Inheritance of emergence time at low temperatures in segregating generations of maize

Summary North Carolina Design III and generation means analyses were used to study the inheritance of seedling emergence time and a related seedling growth parameter in crosses between 5–154, a line from CIMMYT Pool 5 with rapid seedling emergence under cool conditions, and two Corn Belt Dent lines of maize (Zea mays L.). The crosses were evaluated at low temperatures in controlled environment rooms. Additive genetic variances were larger than dominance variances in both crosses and estimates of the average levels of dominance were in the partial dominance range. Dominance was in the direction of rapid seedling emergence and rapid utilization of seed reserve. Estimates of minimum numbers of effective factors provided evidence for polygenic inheritance.     

The ecology and genetics of fitness in forest plants. IV. Quantitative genetics of fitness components in Impatiens pallida (Balsaminaceae)

Genetic and environmental variances were estimated for a number of characters in the annual plant Impatiens pallida by planting seed obtained through controlled crosses into their native field site or pots maintained in the greenhouse. Significant additive genetic variance was detected for three of 11 characters studied—germination date, cotyledon area, and date of first flower production. Significant dominance and/or maternal variance was found for seed weight, proportion of seeds germinating, cotyledon area, plant height, and number of leaves produced. Environmental variance was greater in the field compared with the greenhouse. Characters found to be under strong directional selection in a previous study showed no detectable additive genetic variance. While these populations exhibit conditions that in theory could contribute to the maintenance of genetic variation (limited pollen and seed dispersal distances and small-scale variation for edaphic characteristics influencing plant growth), levels of additive genetic variance for most characters were not significantly different from zero.     

Genetic effects of individual chromosomes in cotton cultivars detected by using chromosome substitution lines as genetic probes

Determination of chromosomes or chromosome arms with desirable genes in different inbred lines and/or crosses should provide useful genetic information for crop improvement. In this study, we applied a modified additive-dominance model to analyze a data set of 13 cotton chromosome substitution lines and their recurrent parent TM-1, five commercial cultivars, and their 70 F(2) hybrids. The chromosome additive and dominance variance components for eight agronomic and fiber traits were determined. On average, each chromosome or chromosome arm was associated with 6.5 traits in terms of additive and/or dominance effects. The chromosomes or chromosome arms, which contributed significant additive variances for the traits investigated, included 2, 16, 18, 25, 5sh (short arm), 14sh, 15sh, 22sh, and 22Lo (long arm). Chromosome additive effects were also predicted in this study. The results showed that CS-B 25 was favorably associated with several fiber traits, while FM966 was favorably associated with both yield and fiber traits with alleles on multiple chromosomes or chromosome arms. Thus, this study should provide valuable genetic information on pure line development for several improved traits such as yield and fiber quality.     

Population bottlenecks increase additive genetic variance but do not break a selection limit in rain forest Drosophila

According to neutral quantitative genetic theory, population bottlenecks are expected to decrease standing levels of additive genetic variance of quantitative traits. However, some empirical and theoretical results suggest that, if nonadditive genetic effects influence the trait, bottlenecks may actually increase additive genetic variance. This has been an important issue in conservation genetics where it has been suggested that small population size might actually experience an increase rather than a decrease in the rate of adaptation. Here we test if bottlenecks can break a selection limit for desiccation resistance in the rain forest-restricted fly Drosophila bunnanda. After one generation of single-pair mating, additive genetic variance for desiccation resistance increased to a significant level, on average higher than for the control lines. Line crosses revealed that both dominance and epistatic effects were responsible for the divergence in desiccation resistance between the original control and a bottlenecked line exhibiting increased additive genetic variance for desiccation resistance. However, when bottlenecked lines were selected for increased desiccation resistance, there was only a small shift in resistance, much less than predicted by the released additive genetic variance. The small selection response in the bottlenecked lines was no greater than that observed in the control lines. Thus bottlenecks might produce a statistically detectable change in additive genetic variance but this change has no impact on the response to selection.     

Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster

Summary Six replicate lines of Drosophila melanogaster, which had been selected for increased abdominal bristle number for more than 85 generations, were assayed by hierarchical analysis of variance and offspring on parent regression immediately after selection ceased, and by single-generation realised heritability after more than 25 generations of subsequent relaxed selection.Half-sib estimates of heritability in 5 lines were as high as in the base population and much higher than observed genetic gains would suggest, excluding lack of sufficient additive genetic variance as a cause of ineffective selection in these lines. Also, there was considerable diversity among the six lines in composition of phenotypic variability: in addition to differences in the additive genetic component, one or more of the components due to dominance, epistasis, sex-linkage or genotype-environment interaction appeared to be important in different lines.Even after relaxed selection, single-generation realised heritabilities in four lines were as high as in the base population. As a large proportion of total genetic gain must have been made by fixation of favourable alleles, the compensatory increase of genetic variability has been sought in a genetic model involving genes at low initial frequencies, enhancement of gene effects during selection and/or new mutations.     

The inheritance of partial self-compatibility inBrassica oleracea L.: results from a half diallel homozygous for a highly recessive S-allele

Summary In a study of partial self-compatbility inBrassica oleracea, flower number, seeded siliqua and seed production were recorded on self- and cross-pollinated inflorescences of the progenies of a half diallel between five unrelated inbred plants homozygous for the same recessiveS-allele.On cross-pollinated inflorescences significant amounts of additively controlled genetic variation were found for seed set per flowering site and its two components, seeded siliquae per flower and seeds per seeded siliquae. Considerable heterosis and gene interaction were also present and a simple additive dominance model did not fully explain the observed variation.On self-pollinated inflorescences, additive gene action was absent for the seed production variates although differences between progenies were highly significant and heterosis was present. Complex gene interactions were considered to be responsible. The characteristics of the method of assessment used and the relationship between self- and outcross seed production are discussed. It appears unlikely that the component lines could be selected for reduced self seed set, but selection for higher outcross seed set may be possible.     

Genetic parameters for growth, wood density and pulp yield in Eucalyptus globulus

Genetic variation and co-variation among the key pulpwood selection traits for Eucalyptus globulus were estimated for a range of sites in Portugal, with the aim of improving genetic parameters used to predict breeding values and correlated response to selection. The trials comprised clonally replicated full-sib families (eight trials) and unrelated clones (17 trials), and exhibited varying levels of pedigree connectivity. The traits studied were stem diameter at breast height, Pilodyn penetration (an indirect measure of wood basic density) and near infrared reflectance predicted pulp yield. Univariate and multivariate linear mixed models were fitted within and across sites, and estimates of additive genetic, total genetic, environmental and phenotypic variances and covariances were obtained. All traits studied exhibited significant levels of additive genetic variation. The average estimated within-site narrow-sense heritability was 0.19 ± 0.03 for diameter and 0.29 ± 0.03 for Pilodyn penetration, and the pooled estimate for predicted pulp yield was 0.42 ± 0.14. When they could be tested, dominance and epistatic effects were generally not statistically significant, although broad-sense heritability estimates were slightly higher than narrow-sense heritability estimates. Averaged across trials, positive additive (0.64 ± 0.08), total genetic (0.58 ± 0.04), environmental (0.38 ± 0.03) and phenotypic (0.43 ± 0.02) correlation estimates were consistently obtained between diameter and Pilodyn penetration. This data argues for at least some form of pleiotropic relationship between these two traits and that selection for fast growth will adversely affect wood density in this population. Estimates of the across-site genetic correlations for diameter and Pilodyn penetration were high, indicating that the genotype by environment interaction is low across the range of sites tested. This result supports the use of single aggregated selection criteria for growth and wood density across planting environments in Portugal, as opposed to having to select for performance in different environments.     

Using half-normal probability plot and regression analysis to differentiate complex traits: differentiating disease response of multigenic resistance and susceptibility in tomatoes to multiple pathogen isolates

The need for a new analytical approach was encountered in the course of characterizing newly developed tomato lines resistant to late blight. Late blight resistant tomato lines were created in independent breeding programs using the accession Solanum pimpinellifolium L. (formerly Lycopersicon pimpinellifolium (L.) Miller) L3708 as the source of the resistance. However, initial field observation suggested that the late blight resistance in the lines produced by two independent breeding programs differed. Possible causes included a partial transfer of the late blight resistance derived from S. pimpinellifolium L3708 or the possibility of race specificity of this resistance. A crucial issue was determining the most appropriate and robust analytical method to use with data from laboratory analyses of the responses of nine tomato lines against five P. infestans isolates. Prior analysis by standard ANOVA revealed significant differences across tomato lines but could not determine whether the disease responses in the CLN-R lines were different from those of the heterozygous F₁ hybrids, created by crossing susceptible tomatoes with the fixed CU-R lines. A different analytical method was needed. Therefore, sporangia numbers/leaflet and diseased area data were analyzed using a half-normal probability plot and regression analysis. The results of this analysis show its utility for genetic or pathology studies. Considering only populations of the uniform tomato lines, this method confirms the results obtained by using a standard ANOVA, but provides a clearer demonstration of the distributions of the individuals within the populations and how this distribution impacts variance and the difference among the populations. This method also allows a joint analysis of the uniform lines with an additional population that is less uniform, because it is segregating. Such an analysis would be invalid using a standard ANOVA. The results of this joint analysis determined that the additional population was divergent from the fixed CU-R lines, and, against some isolates, against the CLN-R lines as well. Half-normal probability plot analysis method would be applicable more broadly beyond analysis of disease resistance data. It could be useful for data from populations that are not normally distributed, for traits which are affected by epistatic gene action, and could be useful for selection of extremes.     

The successful application of a marker-assisted wheat breeding strategy

A number of useful marker-trait associations have been reported for wheat. However the number of publications detailing the integrated and pragmatic use of molecular markers in wheat breeding is limited. A previous report by some of these authors showed how marker-assisted selection could increase the genetic gain and economic efficiency of a specific breeding strategy. Here, we present a practical validation of that study. The target of this breeding strategy was to produce wheat lines derived from an elite Australian cultivar ‘Stylet’, with superior dough properties and durable rust resistance donated from ‘Annuello’. Molecular markers were used to screen a BC₁F₁ population produced from a cross between the recurrent parent ‘Stylet’ and the donor parent ‘Annuello’ for the presence of rust resistance genes Lr34/Yr18 and Lr46/Yr29. Following this, marker-assisted selection was applied to haploid plants, prior to chromosome doubling with cochicine, for the rust resistance genes Lr24/Sr24, Lr34/Yr18, height reducing genes, and for the grain protein genes Glu-D1 and Glu-A3. In general, results from this study agreed with those of the simulation study. Genetic improvement for rust resistance was greatest when marker selection was applied on BC₁F₁ individuals. Introgression of both the Lr34/Yr18 and Lr46/Yr29 loci into the susceptible recurrent parent background resulted in substantial improvement in leaf rust and stripe rust resistance levels. Selection for favourable glutenin alleles significantly improved dough resistance and dough extensibility. Marker-assisted selection for improved grain yield, through the selection of recurrent parent genome using anonymous markers, only marginally improved grain yield at one of the five sites used for grain yield assessment. In summary, the integration of marker-assisted selection for specific target genes, particularly at the early stages of a breeding programme, is likely to substantially increase genetic improvement in wheat.