Reverse breeding: a novel breeding approach based on engineered meiosis

Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental lines for any heterozygous plant, one of the most sought after goals in plant breeding. RB generates perfectly complementing homozygous parental lines through engineered meiosis. The method is based on reducing genetic recombination in the selected heterozygote by eliminating meiotic crossing over. Male or female spores obtained from such plants contain combinations of non-recombinant parental chromosomes which can be cultured in vitro to generate homozygous doubled haploid plants (DHs). From these DHs, complementary parents can be selected and used to reconstitute the heterozygote in perpetuity . Since the fixation of unknown heterozygous genotypes is impossible in traditional plant breeding, RB could fundamentally change future plant breeding. In this review, we discuss various other applications of RB, including breeding per chromosome.     

抗生素菌种选育的研究进展

传统的诱变育种仍是目前抗生素菌种选育中最常用的育种技术,根据抗生素生物合成和代谢调控机制设计的育种方法——推理选育,可大大减少随机筛选的盲目性,提高筛选效率。原生质体融合技术和基因工程技术也被用于抗生素的菌种选育,以基因工程技术为主的多元化的育种方式是今后抗生素菌种选育的主导方向。     

Genetic differences based on a beef terminal index are reflected in future phenotypic performance differences in commercial beef cattle

The increased demand for animal-derived protein and energy for human consumption will have to be achieved through a combination of improved animal genetic merit and better management strategies. The objective of the present study was to quantify whether differences in genetic merit among animals materialised into phenotypic differences in commercial herds. Carcass phenotypes on 156 864 animals from 7301 finishing herds were used, which included carcass weight (kg), carcass conformation score (scale 1 to 15), carcass fat score (scale 1 to 15) at slaughter as well as carcass price. The price per kilogram and the total carcass value that the producer received for the animal at slaughter was also used. A terminal index, calculated in the national genetic evaluations, was obtained for each animal. The index was based on pedigree index for calving performance, feed intake and carcass traits from the national genetic evaluations. Animals were categorised into four terminal index groups on the basis of genetic merit estimates that were derived before the expression of the phenotypic information by the validation animals. The association between terminal index and phenotypic performance at slaughter was undertaken using mixed models; whether the association differed by gender (i.e. young bulls, steers and heifers) or by early life experiences (animals born in a dairy herd or beef herd) was also investigated. The regression coefficient of phenotypic carcass weight, carcass conformation and carcass fat on their respective estimated breeding values (EBVs) was 0.92 kg, 1.08 units and 0.79 units, respectively, which is close to the expectation of one. Relative to animals in the lowest genetic merit group, animals in the highest genetic merit group had, on average, a 38.7 kg heavier carcass, with 2.21 units greater carcass conformation, and 0.82 units less fat. The superior genetic merit animals were, on average, slaughtered 6 days younger than their inferior genetic merit contemporaries. The superior carcass characteristics of the genetically elite animals materialised in carcasses worth €187 more than those of the lowest genetic merit animals. Although the phenotypic difference in carcass traits of animals divergent in terminal index differed statistically by animal gender and early life experience, the detected interactions were generally biologically small. This study clearly indicates that selection on an appropriate terminal index will produce higher performing animals and this was consistent across all production systems investigated.     

Efficiency of direct selection on quantitative trait loci for a two-trait breeding objective

Selection on known loci affecting quantitative traits (DSQ) was compared to phenotypic selection index for a single and a two-trait selection objective. Two situations were simulated; a single known quantitative locus, and ten identified loci accounting for all the additive genetic variance. Selection efficiency of DSQ relative to traitbased selection was higher for two-trait selection, than was selection on a single trait with the same heritability. The advantage of DSQ was greater when the traits were negatively correlated. Relative selection efficiency (RSE) for a single locus responsible for 0.1 of the genetic variance was 1.11 with heritabilities of 0.45 and 0.2 and zero genetic and phenotypic correlations between the traits. RSE of DSQ for ten known loci was 1.5 to 1.8 in the first 3 generations of selection, but declined in each subsequent generation. With DSQ most loci reached fixation after 7 generations. Response to trait-based selection continued through generation 15 and approached the response obtained with DSQ after 10 generations. The cumulative genetic response after 10 generations of DSQ was only 93% to 97% of the economically optimum genotype because the less favorable allele reached fixation for some loci, generally those with effects in opposite directions on the two traits.     

Efficiency of phenotypic and DNA markers for a genetic diversity study of alfalfa

Information on genetic diversity and germplasm characterization is essential for successful crop improvement. Diverse data sets (pedigree, morphological, biochemical, DNA based-markers) are employed in various aspects of plant analysis. The objective of this study was to determine the efficiency of phenotypic and RAPD markers in diversity assessment of ten alfalfa (Medicago spp.) accessions from Europe, North America and Australia. Field experiment was designed as a randomised complete block with three replications over two consecutive years (2004, 2005) at one location. Twelve morpho-agronomic traits were recorded on 50 plants per each accession. Genomic DNA’s from 16–20 randomly selected individual plants per accession were used for RAPD analysis. Six primers selected in this study generated a total of 93 polymorphic RAPD bands. The number of polymorphic bands detected per primer ranged from 11 to 20. Genetic distances (GD) among investigated accessions and two-dimensional principal coordinate analysis (2D PCoA) based on phenotypic and molecular data were obtained. The average GD between (0.283–0.416) and within (0.247–0.332) accessions based on RAPD data was higher than GD values obtained by morpho-agronomic traits (0.171–0.354 and 0.157–0.261, respectively). 2D PCoA based on GD from RAPD data grouped most of the studied individual plants to four clusters according to their geographical or taxonomy origin. 2D PCoA based only on morpho-agronomic data did not group plants congruently to their origin, probably due to a strong environmental influence on studied traits. Our results indicated that the RAPD markers were effective in assessing genetic diversity within and between studied alfalfa accessions. In addition, the obtained results suggested that the RAPD markers might be useful for grouping of germplasm with similar genetic background and for pre-screening of potential heterotic groups in our breeding programme.     

Estimating genetic correlations based on phenotypic data: a simulation-based method

Knowledge of genetic correlations is essential to understand the joint evolution of traits through correlated responses to selection, a difficult and seldom, very precise task even with easy-to-breed species. Here, a simulation-based method to estimate genetic correlations and genetic covariances that relies only on phenotypic measurements is proposed. The method does not require any degree of relatedness in the sampled individuals. Extensive numerical results suggest that the propose method may provide relatively efficient estimates regardless of sample sizes and contributions from common environmental effects.     

The Norwegian sheep breeding scheme: description, genetic and phenotypic change

The developments in Norwegian sheep breeding since the early 1990s are reviewed. For the largest breeding population, the Norwegian White Sheep, results are presented for both genetic and phenotypic changes. Of the nine traits that make up the aggregate genotype, the largest gain per year, in per cent of the corresponding phenotypic average, was found for carcass grade (1.66%) and carcass weight (0.99%), number of lambs born at 1, 2 and 3 years of age (0.32% to 0.60%) and the maternal effect on weaning weight (0.26%). For fat grade, a genetic deterioration was estimated. This may be due to the too small weighting of this trait in the aggregate genotype and the true genetic parameters being somewhat different from the estimates in the prediction of breeding values. For lamb as well as ewe fleece weight, genetic change was close to zero - interpreted as mainly a correlated response to other traits in the aggregate genotype. Data for the two traits of fleece weight were, respectively, selected and few. Thus, phenotypic change was calculated for all traits except for fleece weight, and in addition for number of lambs at weaning, being indirectly selected for through number of lambs born. For all traits, with the exception of fat grade, advantageous phenotypic change was estimated. For weaning and carcass weight, the phenotypic change was less than the genetic change, while the opposite was observed for carcass and fat grade and number of lambs born. The latter traits can be more easily controlled by environmental actions, and the results thus exemplify the interdependency between environmental and genetic change.     

Melanin coloration has temperature‐dependent effects on breeding performance that may maintain phenotypic variation in a passerine bird

Fluctuating selection pressure may maintain phenotypic variation because of different types of individuals being adapted to different environmental conditions. We show that the extensive variation in the coloration of male pied flycatchers (Ficedula hypoleuca) can be maintained through differences in the reproductive success of male phenotypes under different conditions. The effects of weather conditions on the relative success of different male phenotypes varied between different phases of breeding. The reproductive output of black males was the highest when it was cold during egg‐laying but warm during the nestling period, whereas the fledgling production of brown males was highest when it was continuously warm. In addition, male forehead and wing patch sizes had context‐dependent effects on timing of breeding and nestling mortality, respectively. These results indicate that environmental heterogeneity plays a role in maintaining phenotypic variation. As melanin‐based coloration is heritable, climate change may alter phenotype frequencies depending on the patterns of warming.     

罗氏沼虾良种选育研究进展

罗氏沼虾（Macrobrachium rosenbergii）是世界重要的经济虾类,畅销国内外市场,备受消费者的喜爱。中国是罗氏沼虾养殖大国,年均养殖产量占世界一半以上。种质资源是我国罗氏沼虾产业发展的原动力,处于整个产业链的最上游,并支撑着整个产业的发展。世界多国为了壮大产业规模,也纷纷开展了罗氏沼虾种质资源库的建设与良种选育。本文回顾了我国自主选育的罗氏沼虾主要品种及品系,综述了大规模家系的构建以及不同来源群体的遗传多样性分析,并对国外的良种选育现状和趋势进行了概述,最后结合国内外的研究进展,提出了良种选育的研究要点与方向,以期为今后的种质创新研究提供思路和借鉴。     

Potential for the improvement of Salicornia bigelovii through selective breeding

Salicornia bigelovii Torr. is an annual salt marsh plant that produces seed oils, protein meal, fresh salad greens and forage on seawater irrigation. We compared S. bigelovii lines produced in two breeding programs with wild germplasm in greenhouse trials on brackish water (10 ppt NaCl) irrigation. S. bigelovii is an out-crossing species that is also capable of selfing, and the breeding programs showed it is possible to use both hybridization and pedigree breeding to improve the germplasm. Lines selected in a breeding program carried out in Eritrea, Africa, had smaller plant size and lower biomass yields than the starting germplasm, due to the need to compress the growth cycle within the cool months of the year in that hot climate, but seed yields and harvest index were improved. Lines produced from wild germplasm by mass selection and hybridization in Tucson, Arizona had higher biomass yield than starting germplasm. We conclude that S. bigelovii has sufficient genetic diversity among wild accessions and cultivars to support a crop improvement program.     

Mendelian sampling terms as a selective advantage in optimum breeding schemes with restrictions on the rate of inbreeding

Quadratic indices are a general approach for the joint management of genetic gain and inbreeding in artificial selection programmes. They provide the optimal contributions that selection candidates should have to obtain the maximum gain when the rate of inbreeding is constrained to a predefined value. This study shows that, when using quadratic indices, the selective advantage is a function of the Mendelian sampling terms. That is, at all times, contributions of selected candidates are allocated according to the best available information about their Mendelian sampling terms (i.e. about their superiority over their parental average) and not on their breeding values. By contrast, under standard truncation selection, both estimated breeding values and Mendelian sampling terms play a major role in determining contributions. A measure of the effectiveness of using genetic variation to achieve genetic gain is presented and benchmark values of 0.92 for quadratic optimisation and 0.5 for truncation selection are found for a rate of inbreeding of 0.01 and a heritability of 0.25.     

Genetics and selective breeding of variation in wing truncation in a flightless aphid control agent

Augmentative biological control by predaceous ladybird beetles can be improved by using flightless morphs, which have longer residence times on the host plants. The two‐spot ladybird beetle, Adalia bipunctata (L.) (Coleoptera: Coccinellidae), is used for the biological control of aphids in greenhouses and on urban trees. Flightlessness due to truncated wings occurs at very low frequency in some natural populations of A. bipunctata. Pure‐breeding strains of this 'wingless' genotype of A. bipunctata can easily be obtained in the laboratory. Such strains have not been commercialized yet due to concerns about their reduced fitness compared to wild‐type strains, which renders mass production more expensive. Wingless strains exhibit, however, wide intra‐population phenotypic variation in the extent of wing truncation which is related to fitness traits. We here use classical quantitative genetic techniques to study the heritability and genetic architecture of variation in wing truncation in a wingless strain of A. bipunctata. Split‐families reared at one of two temperatures revealed strong family‐by‐temperature interaction: heritability was estimated as 0.64 ± 0.09 at 19 °C and 0.29 ± 0.06 at 29 °C. Artificial selection in opposite directions at 21 °C demonstrated that the degree of wing truncation can be altered within a few generations resulting in wingless phenotypes without any wing tissue (realized h² = 0.72), as well as those with minimal truncations (realized h² = 0.61) in two replicates. The latter lines produced more than twice as many individuals. This indicates that selective breeding of wing truncation may be exploited to improve mass rearing of flightless strains of A. bipunctata for commercial biological control. Our work illustrates that cryptic variation can also be a source for the selective breeding of natural enemies.     

Multitrait index based on factor analysis and ideotype‐design: proposal and application on elephant grass breeding for bioenergy

This study proposes a new multitrait index based on factor analysis and ideotype‐design (FAI‐BLUP index), and validates its potential on the selection of elephant grass genotypes for energy cogeneration. Factor analysis was carried out, and afterwards, factorial scores of each ideotype were designed according to the desirable and undesirable factors, and the spatial probability was estimated based on genotype‐ideotype distance, enabling genotype ranking. In order to quantify the potential of the FAI‐BLUP index, genetic gains were predicted and compared with the Smith‐Hazel classical index. The FAI‐BLUP index allows ranking the genotypes based on multitrait, free from multicollinearity, and it does not require assigning weights, as in the case of the Smith‐Hazel classical index and its derived indices. Furthermore, the genetic correlation ‐ positive or negative ‐ within each factor was taken into account, preserving their traits relationship, and giving biological meaning to the ideotypes. The FAI‐BLUP index indicated the 15 elephant grass with the highest performance for conversion to bioenergy via combustion, and predicted balanced and desirable genetic gains for all traits. In addition, the FAI‐BLUP index predicted gains of approximately 62% of direct selection, simultaneously for all traits that are desired to be increased, and approximately 33% for traits which are desired to be decreased. The genotypes selected by the FAI‐BLUP index have potential to improve all traits simultaneously, while the Smith‐Hazel classical index predicted gains of 66% for traits that are desired to be increased, and ?32% for traits that are desired to be decreased, and it does not have potential to improve all traits simultaneously. The FAI‐BLUP index provides an undoubtable selection process and can be used in any breeding programme aiming at selection based on multitrait.     

Harvest index: a review of its use in plant breeding and crop physiology

This review charts the use of the concept of harvest index in crop improvement and physiology, concentrating on the literature from the last 20 years. Evidence from abstract journals indicates that the term has been applied most to small grain cereal crops and pulses, in India, Western Europe and the USA, and that it has been less useful for maize and tuber crops. Standard methods of measuring harvest index, the associated problems of measurement and interpretation, and representative values for a range of world species are reviewed. The values for modern varieties of most intensively-cultivated grain crops fall within the range 0.4 to 0.6. Variation between varieties of the same species is illustrated by trends in the harvest indices of old, outclassed and recent varieties of temperate and mediterranean wheat and barley (compared under uniform conditions); this shows a progressive increase throughout the present century, although improvement has been much slower in Australia and Canada than in the UK. In most cases, the improvement in harvest index has been a consequence of increased grain population density coupled with stable individual grain weight. The high heritability of harvest index is explored by examining its (rather weak) response to variation in environmental factors (fertilisation, population density, application of growth regulators) in the absence of severe stress. A fuller perspective is gained by reviewing aspects of the harvest index of rice, maize and tropical pulses. With rice, attention must be paid to the fact that the adhering lemma and palea (not primarily part of economic yield) can make up 20% of grain weight; and there are important interactions among biomass, grain yield and season length. Maize differs from most small grain crops in that harvest index (in N. American varieties) was already high at the start of this century, and increases in yield potential have been largely the consequence of increased biomass production. The harvest index of many pulse species and varieties tends to be low because selection has been for some yield in all seasons. Extension of the harvest index concept to express the partitioning of mineral nutrients as well as dry matter (e.g. the nitrogen harvest index) has provided a range of responses whose implications for production and breeding remain to be explored. It is concluded that even though the principal cereal crops appear to be approaching the upper limit of harvest index, and future yield gains will have to be sought by increased biomass production, there will still be a need for the concept of harvest index as a tool in interpreting crop response to different environments and climatic change.     

叶用芥菜种质表型性状的遗传多样性分析

为有效利用叶用芥菜种质资源,对筛选出的24份叶用芥菜品系资源的11个表型性状,应用相关系数、聚类分析和主成分分析等多元统计方法进行遗传多样性分析。结果表明:叶用芥菜种质资源具有丰富的遗传多样性;11个表型性状平均变异系数为44.74%,叶片数的变异系数最大,为150.80%;净菜率变异系数最小,仅为9.54%。通过系统聚类,将参试的24份品系分为3个类群,第Ⅰ类群有14份材料,第Ⅱ类群有5份材料,第Ⅲ类群有5份材料,各类群性状之间的差异较明显,明确了品系类群间存在的亲缘关系。在主成分分析中,可选取方差累计贡献率为86.62%的前5个主成分来评价24份叶用芥菜品系资源。本研究揭示了叶用芥菜不同品系的表型特异性和遗传多样性,筛选出一些特异品系资源,为高产优质叶用芥菜新品种(系)的选育提供重要的科学依据。