Programmable Base Editing of the Sheep Genome Revealed No Genome-Wide Off-Target Mutations

Genome editing: An insight into disease resistance, production efficiency, and biomedical applications in livestock

One of the primary concerns for the survival of the human species is the growing demand for food brought on by an increasing global population. New developments in genome-editing technology present promising opportunities for the growth of wholesome and prolific farm animals. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. Genome editing entails modifying genetic material by removing, adding, or manipulating particular DNA sequences from a particular locus in a way that does not happen naturally. The three primary genome editors are CRISPR/Cas 9, TALENs, and ZFNs. Each of these enzymes is capable of precisely severing nuclear DNA at a predetermined location. One of the most effective inventions is base editing, which enables single base conversions without the requirement for a DNA double-strand break (DSB). As reliable methods for precise genome editing in studies involving animals, cytosine and adenine base editing are now well-established. Effective zygote editing with both cytosine and adenine base editors (ABE) has resulted in the production of animal models. Both base editors produced comparable outcomes for the precise editing of point mutations in somatic cells, advancing the field of gene therapy. This review focused on the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of ZFNs, TALENs, and CRISPR/Cas9 base editors, and prime editing in diverse lab and farm animals. Additionally, we address the methodologies that can be used for gene regulation, base editing, and epigenetic alterations, as well as the significance of genome editing in animal models to better reflect real disease. We also look at methods designed to increase the effectiveness and precision of gene editing tools. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. This review is an overview of the existing knowledge of the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of zinc finger nucleases (ZFNs), transcription-activator-like endonucleases (TALENs), and clustered regularly interspaced short palindromic repeats associated protein 9 (CRISPR/Cas 9), base editors and prime editing in diverse lab and farm animals, which will offer better and healthier products for the entire human race.

Perspectives in Genome-Editing Techniques for Livestock

Genome editing of farm animals has undeniable practical applications. It helps to improve production traits, enhances the economic value of livestock, and increases disease resistance. Gene-modified animals are also used for biomedical research and drug production and demonstrate the potential to be used as xenograft donors for humans. The recent discovery of site-specific nucleases that allow precision genome editing of a single-cell embryo (or embryonic stem cells) and the development of new embryological delivery manipulations have revolutionized the transgenesis field. These relatively new approaches have already proven to be efficient and reliable for genome engineering and have wide potential for use in agriculture. A number of advanced methodologies have been tested in laboratory models and might be considered for application in livestock animals. At the same time, these methods must meet the requirements of safety, efficiency and availability of their application for a wide range of farm animals. This review aims at covering a brief history of livestock animal genome engineering and outlines possible future directions to design optimal and cost-effective tools for transgenesis in farm species.

Japanese Regulatory Framework and Approach for Genome-edited Foods Based on Latest Scientific Findings

The food supply system is facing important challenges and its sustainability has to be considered. Genome-editing technology, which accelerates the development of new variety, could be used to achieve sustainable development goals, thereby protecting the environment and ensuring the stable production of food for an increasing global population. The most widely used genome-editing tool, CRISPR/Cas9, is easy to use, affordable, and versatile. Foods produced by genome-editing technologies have been developed worldwide to create novel traits. In the first half of the review, the latest scientific findings on genome-editing technologies are summarized, and the technical challenge in genome sequence analysis are clarified. CRISPR/Cas9 has versatile alternative techniques, such as base editor and prime editor. Genome sequencing technology has developed rapidly in recent years. However, it is still difficult to detect large deletions and structural variations. Long-read sequencing technology would solve this challenge. In the second part, regulatory framework and approach for genome-edited foods is introduced. The four government ministries, including the Ministry of Environment, the Ministry of Agriculture, Forestry and Fisheries, and the Ministry of Health, Labour and Welfare (MHLW), started to discuss how the regulation should be implemented in 2019. The SDN-1 technique is excluded from the current genetically modified organism (GMO) regulation. The Japanese regulatory framework includes pre-submission consultation and submission of notification form. In the last part of this review, transparency of regulatory framework and consumer confidence were described. Since maintaining consumer trust is vital, transparency of regulatory framework is a key to consumers. The information of notification process on approved genome-edited foods is made public immediately. This review will help regulators build regulatory frameworks, and lead to harmonization of the framework between the countries.

Genetics of the phenotypic evolution in sheep: a molecular look at diversity-driving genes

BACKGROUND

After domestication, the evolution of phenotypically-varied sheep breeds has generated rich biodiversity. This wide phenotypic variation arises as a result of hidden genomic changes that range from a single nucleotide to several thousands of nucleotides. Thus, it is of interest and significance to reveal and understand the genomic changes underlying the phenotypic variation of sheep breeds in order to drive selection towards economically important traits.

REVIEW

Various traits contribute to the emergence of variation in sheep phenotypic characteristics, including coat color, horns, tail, wool, ears, udder, vertebrae, among others. The genes that determine most of these phenotypic traits have been investigated, which has generated knowledge regarding the genetic determinism of several agriculturally-relevant traits in sheep. In this review, we discuss the genomic knowledge that has emerged in the past few decades regarding the phenotypic traits in sheep, and our ultimate aim is to encourage its practical application in sheep breeding. In addition, in order to expand the current understanding of the sheep genome, we shed light on research gaps that require further investigation.

CONCLUSIONS

Although significant research efforts have been conducted in the past few decades, several aspects of the sheep genome remain unexplored. For the full utilization of the current knowledge of the sheep genome, a wide practical application is still required in order to boost sheep productive performance and contribute to the generation of improved sheep breeds. The accumulated knowledge on the sheep genome will help advance and strengthen sheep breeding programs to face future challenges in the sector, such as climate change, global human population growth, and the increasing demand for products of animal origin.

Application of Gene Editing Technology in Resistance Breeding of Livestock

As a new genetic engineering technology, gene editing can precisely modify the specific gene sequence of the organism’s genome. In the last 10 years, with the rapid development of gene editing technology, zinc-finger nucleases (ZFNs), transcription activator-like endonucleases (TALENs), and CRISPR/Cas9 systems have been applied to modify endogenous genes in organisms accurately. Now, gene editing technology has been used in mice, zebrafish, pigs, cattle, goats, sheep, rabbits, monkeys, and other species. Breeding for disease-resistance in agricultural animals tends to be a difficult task for traditional breeding, but gene editing technology has made this easier. In this work, we overview the development and application of gene editing technology in the resistance breeding of livestock. Also, we further discuss the prospects and outlooks of gene editing technology in disease-resistance breeding.

Enhancing Animal Disease Resistance, Production Efficiency, and Welfare through Precise Genome Editing

The major goal of animal breeding is the genetic enhancement of economic traits. The CRISPR/Cas system, which includes nuclease-mediated and base editor mediated genome editing tools, provides an unprecedented approach to modify the mammalian genome. Thus, farm animal genetic engineering and genetic manipulation have been fundamentally revolutionized. Agricultural animals with traits of interest can be obtained in just one generation (and without long time selection). Here, we reviewed the advancements of the CRISPR (Clustered regularly interspaced short palindromic repeats)/Cas (CRISPR associated proteins) genome editing tools and their applications in animal breeding, especially in improving disease resistance, production performance, and animal welfare. Additionally, we covered the regulations on genome-edited animals (GEAs) and ways to accelerate their use. Recommendations for how to produce GEAs were also discussed. Despite the current challenges, we believe that genome editing breeding and GEAs will be available in the near future.

Application of CRISPR/Cas9 System in Establishing Large Animal Models

The foundation for investigating the mechanisms of human diseases is the establishment of animal models, which are also widely used in agricultural industry, pharmaceutical applications, and clinical research. However, small animals such as rodents, which have been extensively used to create disease models, do not often fully mimic the key pathological changes and/or important symptoms of human disease. As a result, there is an emerging need to establish suitable large animal models that can recapitulate important phenotypes of human diseases for investigating pathogenesis and developing effective therapeutics. However, traditional genetic modification technologies used in establishing small animal models are difficultly applied for generating large animal models of human diseases. This difficulty has been overcome to a great extent by the recent development of gene editing technology, especially the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). In this review, we focus on the applications of CRISPR/Cas9 system to establishment of large animal models, including nonhuman primates, pigs, sheep, goats and dogs, for investigating disease pathogenesis and treatment. We also discuss the limitations of large animal models and possible solutions according to our current knowledge. Finally, we sum up the applications of the novel genome editing tool Base Editors (BEs) and its great potential for gene editing in large animals.

Efficient gene editing through an intronic selection marker in cells

BACKGROUND

Gene editing technology has provided researchers with the ability to modify genome sequences in almost all eukaryotes. Gene-edited cell lines are being used with increasing frequency in both bench research and targeted therapy. However, despite the great importance and universality of gene editing, the efficiency of homology-directed DNA repair (HDR) is too low, and base editors (BEs) cannot accomplish desired indel editing tasks.

RESULTS AND DISCUSSION

Our group has improved HDR gene editing technology to indicate DNA variation with an independent selection marker using an HDR strategy, which we named Gene Editing through an Intronic Selection marker (GEIS). GEIS uses a simple process to avoid nonhomologous end joining (NHEJ)-mediated false-positive effects and achieves a DsRed positive rate as high as 87.5% after two rounds of fluorescence-activated cell sorter (FACS) selection without disturbing endogenous gene splicing and expression. We re-examined the correlation of the conversion tract and efficiency, and our data suggest that GEIS has the potential to edit approximately 97% of gene editing targets in human and mouse cells. The results of further comprehensive analysis suggest that the strategy may be useful for introducing multiple DNA variations in cells.

A hypomorphic mutation in the mouse Csn1s1 gene generated by CRISPR/Cas9 pronuclear microinjection

Caseins are major milk proteins that have an evolutionarily conserved role in nutrition. Sequence variations in the casein genes affect milk composition in livestock species. Regulatory elements of the casein genes could be used to direct the expression of desired transgenes into the milk of transgenic animals. Dozens of casein alleles have been identified for goats, cows, sheep, camels and horses, and these sequence variants are associated with altered gene expression and milk protein content. Most of the known mutations affecting casein genes’ expression are located in the promoter and 3’-untranslated regions. We performed pronuclear microinjections with Cas9 mRNA and sgRNA against the first coding exon of the mouse Csn1s1 gene to introduce random mutations in the α-casein (Csn1s1) signal peptide sequence at the beginning of the mouse gene. Sanger sequencing of the founder mice identified 40 mutations. As expected, mutations clustered around the sgRNA cut site (3 bp from PAM). Most of the mutations represented small deletions (1–10 bp), but we detected several larger deletions as well (100–300 bp). Functionally most mutations led to gene knockout due to a frameshift or a start codon loss. Some of the mutations represented in-frame indels in the first coding exon. Of these, we describe a novel hypomorphic Csn1s1 (Csn1s1c.4-5insTCC) allele. We measured Csn1s1 protein levels and confirmed that the mutation has a negative effect on milk composition, which shows a 50 % reduction in gene expression and a 40–80 % decrease in Csn1s1 protein amount, compared to the wild-type allele. We assumed that mutation affected transcript stability or splicing by an unknown mechanism. This mutation can potentially serve as a genetic marker for low Csn1s1 expression.

Progression and application of CRISPR-Cas genomic editors

Base editing technology is an efficient tool for genome editing, particularly in the correction of base mutations. Diverse base editing systems were developed according to the dCas9 or nCas9 linked with different deaminase or reverse transcriptase in the editors, including ABEs, CBEs, PEs and dual-functional of base editor (such as CGBE1, A&C-BEmax, ACBE, etc.). Currently, Base editing technology has been widely applied to various fields such as microorganisms, plants, animals and medicine for basic research and therapeutics. Here, we reviewed the advancement of base editing technology. We also discussed the application of base editors in different areas in the future.

CRISPR base editing applications for identifying cancer-driving mutations

CRISPR base editing technology is a promising genome editing tool as (i) it does not require a DNA template to introduce mutations and (ii) it avoids creating DNA double-strand breaks, which can lead to unintended chromosomal alterations or elicit an unwanted DNA damage response. Given many cancers originate from point mutations in cancer-driving genes, the application of base editing for either modelling tumour development, therapeutic editing, or functional screening is of great promise. In this review, we summarise current DNA base editing technologies and will discuss recent advancements and existing hurdles for its usage in cancer research.

Improvements in Gene Editing Technology Boost Its Applications in Livestock

Accelerated development of novel CRISPR/Cas9-based genome editing techniques provides a feasible approach to introduce a variety of precise modifications in the mammalian genome, including introduction of multiple edits simultaneously, efficient insertion of long DNA sequences into specific targeted loci as well as performing nucleotide transitions and transversions. Thus, the CRISPR/Cas9 tool has become the method of choice for introducing genome alterations in livestock species. The list of new CRISPR/Cas9-based genome editing tools is constantly expanding. Here, we discuss the methods developed to improve efficiency and specificity of gene editing tools as well as approaches that can be employed for gene regulation, base editing, and epigenetic modifications. Additionally, advantages and disadvantages of two primary methods used for the production of gene-edited farm animals: somatic cell nuclear transfer (SCNT or cloning) and zygote manipulations will be discussed. Furthermore, we will review agricultural and biomedical applications of gene editing technology.

Advances in engineering CRISPR-Cas9 as a molecular Swiss Army knife

The RNA-guided endonuclease system CRISPR-Cas9 has been extensively modified since its discovery, allowing its capabilities to extend far beyond double-stranded cleavage to high fidelity insertions, deletions and single base edits. Such innovations have been possible due to the modular architecture of CRISPR-Cas9 and the robustness of its component parts to modifications and the fusion of new functional elements. Here, we review the broad toolkit of CRISPR-Cas9-based systems now available for diverse genome-editing tasks. We provide an overview of their core molecular structure and mechanism and distil the design principles used to engineer their diverse functionalities. We end by looking beyond the biochemistry and toward the societal and ethical challenges that these CRISPR-Cas9 systems face if their transformative capabilities are to be deployed in a safe and acceptable manner.

New Directions in Pulmonary Gene Therapy

The lung has long been a target for gene therapy, yet efficient delivery and phenotypic disease correction has remained challenging. Although there have been significant advancements in gene therapies of other organs, including the development of several ex vivo therapies, in vivo therapeutics of the lung have been slower to transition to the clinic. Within the past few years, the field has witnessed an explosion in the development of new gene addition and gene editing strategies for the treatment of monogenic disorders. In this review, we will summarize current developments in gene therapy for cystic fibrosis, alpha-1 antitrypsin deficiency, and surfactant protein deficiencies. We will explore the different gene addition and gene editing strategies under investigation and review the challenges of delivery to the lung.

Highly efficient generation of sheep with a defined FecBB mutation via adenine base editing

Base editing has the potential to improve important economic traits in agriculture and can precisely convert single nucleotides in DNA or RNA sequences into minimal double-strand DNA breaks (DSB). Adenine base editors (ABE) have recently emerged as a base editing tool for the conversion of targeted A:T to G:C, but have not yet been used in sheep. ABEmax is one of the latest versions of ABE, which consists of a catalytically-impaired nuclease and a laboratory-evolved DNA-adenosine deaminase. The Booroola fecundity (FecB^B) mutation (g.A746G, p.Q249R) in the bone morphogenetic protein receptor 1B (BMPR1B) gene influences fecundity in many sheep breeds. In this study, by using ABEmax we successfully obtained lambs with defined point mutations that result in an amino acid substitution (p.Gln249Arg). The efficiency of the defined point mutations was 75% in newborn lambs, since six lambs were heterozygous at the FecB^B mutation site (g.A746G, p.Q249R), and two lambs were wild-type. We did not detect off-target mutations in the eight edited lambs. Here, we report the validation of the first gene-edited sheep generated by ABE and highlight its potential to improve economically important traits in livestock.

Insight into the Possible Formation Mechanism of the Intersex Phenotype of Lanzhou Fat-Tailed Sheep Using Whole-Genome Resequencing

Simple Summary

Individuals with hermaphroditism are a serious hazard to animal husbandry and production due to their abnormal fertility. The molecular mechanism of sheep intersex formation was unclear. This study was the first to locate the homologous sequence of the goat polled intersex syndrome (PIS) region in sheep and found that the intersex traits of Lanzhou fat-tailed sheep were not caused by the lack of this region. By detecting the selective sweep regions, vital genes associated with androgen biosynthesis and the follicle stimulating hormone response entry were found, including steroid 5 alpha-reductase 2 (SRD5A2), and pro-apoptotic WT1 regulator (PAWR). Additionally, the copy number variations of the four regions on chr9, chr1, chr4, and chr16 may affect the expression of the gonadal development genes, zinc finger protein, FOG family member 2 (ZFPM2), LIM homeobox 8 (LHX8), inner mitochondrial membrane peptidase subunit 2 (IMMP2L) and slit guidance ligand 3 (SLIT3), respectively, and further affect the formation of intersex traits.

Abstract

Intersex, also known as hermaphroditism, is a serious hazard to animal husbandry and production. The mechanism of ovine intersex formation is not clear. Therefore, genome-wide resequencing on the only two intersex and two normal Lanzhou fat-tailed (LFT) sheep, an excellent but endangered Chinese indigenous sheep breed, was performed. Herein, the deletion of homologous sequences of the goat polled intersex syndrome (PIS) region (8787 bp, 247747059–247755846) on chromosome 1 of the LFT sheep was not the cause of the ovine intersex trait. By detecting the selective sweep regions, we found that the genes related to androgen biosynthesis and follicle stimulating hormone response items, such as steroid 5 alpha-reductase 2 (SRD5A2), steroid 5 alpha-reductase 3 (SRD5A3), and pro-apoptotic WT1 regulator (PAWR), may be involved in the formation of intersex traits. Furthermore, the copy number variations of the four regions, chr9: 71660801–71662800, chr1: 50776001–50778000, chr4: 58119201–58121600, and chr16: 778801–780800, may affect the expression of the zinc finger protein, FOG family member 2 (ZFPM2), LIM homeobox 8 (LHX8), inner mitochondrial membrane peptidase subunit 2 (IMMP2L) and slit guidance ligand 3 (SLIT3) genes, respectively, which contribute to the appearance of intersex traits. These results may supply a theoretical basis for the timely detection and elimination of intersex individuals in sheep, which could accelerate the healthy development of animal husbandry.

Redesigning small ruminant genomes with CRISPR toolkit: Overview and perspectives

Genetic modification is a rapidly developing field in which numerous significant breakthroughs have been achieved. Over the last few decades, genetic modification has evolved from insertional transgenesis to gene targeting and editing and, more recently, to base and prime editing using CRISPR-derived systems. Currently, CRISPR-based genome editing systems are showing great potential for generating gene-edited offspring with defined genetic characteristics. Domestic small ruminants (sheep and goats) have shown great potential as large animal models for genome engineering. Ovine and caprine genomes have been engineered using CRISPR-based systems for numerous purposes. These include generating superior agricultural breeds, expression of therapeutic agents in mammary glands, and developing animal models to be used in the study of human genetic disorders and regenerative medicine. The creation of these models has been facilitated by the continuous emergence and development of genetic modification tools. In this review, we provide an overview on how CRISPR-based systems have been used in the generation of gene-edited small ruminants through the two main pathways (embryonic microinjection and somatic cell nuclear transfer) and highlight the ovine and caprine genes that have been targeted via knockout, knockin, HDR-mediated point mutation, and base editing approaches, as well as the aims of these specific manipulations.

Cytosine base editor 4 but not adenine base editor generates off-target mutations in mouse embryos

Deaminase base editing has emerged as a tool to install or correct point mutations in the genomes of living cells in a wide range of organisms. However, the genome-wide off-target effects introduced by base editors in the mammalian genome have been examined in only one study. Here, we have investigated the fidelity of cytosine base editor 4 (BE4) and adenine base editors (ABE) in mouse embryos using unbiased whole-genome sequencing of a family-based trio cohort. The same sgRNA was used for BE4 and ABE. We demonstrate that BE4-edited mice carry an excess of single-nucleotide variants and deletions compared to ABE-edited mice and controls. Therefore, an optimization of cytosine base editors is required to improve its fidelity. While the remarkable fidelity of ABE has implications for a wide range of applications, the occurrence of rare aberrant C-to-T conversions at specific target sites needs to be addressed.

Molecular mechanisms, off-target activities, and clinical potentials of genome editing systems

Methodologies of genome editing are rapidly developing with the improvement of gene science and technology, mechanism-based understanding, and urgent needs. In addition to the specificity and efficiency of on-target sites, one of the most important issues is to find and avoid off-targets before clinical application of gene editing as a therapy. Various algorithms, modified nucleases, and delivery vectors are developed to localize and minimize off-target sites. The present review aimed to clarify off-targets of various genome editing and explore potentials of clinical application by understanding structures, mechanisms, clinical applications, and off-target activities of genome editing systems, including CRISPR/Cas9, CRISPR/Cas12a, zinc finger nucleases, transcription activator-like effector nucleases, meganucleases, and recent developments. Current genome editing in cancer therapy mainly targeted immune systems in tumor microenvironment by ex vivo modification of the immune cells in phases I/II of clinical trials. We believe that genome editing will be the critical part of clinical precision medicine strategy and multidisciplinary therapy strategy by integrating gene sequencing, clinical transomics, and single cell biomedicine. There is an urgent need to develop on/off-target-specific biomarkers to monitor the efficacy and side-effects of gene therapy. Thus, the genome editing will be an alternative of clinical therapies for cancer with the rapid development of methodology and an important part of clinical precision medicine strategy.

Sheep and Goat Genome Engineering: From Random Transgenesis to the CRISPR Era

Sheep and goats are valuable livestock species that have been raised for their production of meat, milk, fiber, and other by-products. Due to their suitable size, short gestation period, and abundant secretion of milk, sheep and goats have become important model animals in agricultural, pharmaceutical, and biomedical research. Genome engineering has been widely applied to sheep and goat research. Pronuclear injection and somatic cell nuclear transfer represent the two primary procedures for the generation of genetically modified sheep and goats. Further assisted tools have emerged to enhance the efficiency of genetic modification and to simplify the generation of genetically modified founders. These tools include sperm-mediated gene transfer, viral vectors, RNA interference, recombinases, transposons, and endonucleases. Of these tools, the four classes of site-specific endonucleases (meganucleases, ZFNs, TALENs, and CRISPRs) have attracted wide attention due to their DNA double-strand break-inducing role, which enable desired DNA modifications based on the stimulation of native cellular DNA repair mechanisms. Currently, CRISPR systems dominate the field of genome editing. Gene-edited sheep and goats, generated using these tools, provide valuable models for investigations on gene functions, improving animal breeding, producing pharmaceuticals in milk, improving animal disease resistance, recapitulating human diseases, and providing hosts for the growth of human organs. In addition, more promising derivative tools of CRISPR systems have emerged such as base editors which enable the induction of single-base alterations without any requirements for homology-directed repair or DNA donor. These precise editors are helpful for revealing desirable phenotypes and correcting genetic diseases controlled by single bases. This review highlights the advances of genome engineering in sheep and goats over the past four decades with particular emphasis on the application of CRISPR/Cas9 systems.

Base pair editing in goat: nonsense codon introgression into FGF5 results in longer hair

The ability to alter single bases without homology directed repair (HDR) of double-strand breaks provides a potential solution for editing livestock genomes for economic traits, which are often multigenic. Progress toward multiplex editing in large animals has been hampered by the costly inefficiencies of HDR via microinjection of in vitro manipulated embryos. Here, we designed sgRNAs to induce nonsense codons (C-to-T transitions) at four target sites in caprine FGF5, which is a crucial regulator of hair length in mammals. Initial transfections of the third generation Base Editor (BE3) plasmid and four different sgRNAs into caprine fibroblasts were ineffective in altering FGF5. In contrast, all five progenies produced from microinjected single-cell embryos had alleles with a targeted nonsense mutation. The effectiveness of BE3 to make single base changes varied considerably based on sgRNA design. In addition, the rate of mosaicism differed between animals, target sites, and tissue type. The phenotypic effects on hair fiber were characterized by hematoxylin and eosin, immunofluorescence staining, and western blotting. Differences in morphology were detectable, even though mosaicism was probably affecting the levels of FGF5 expression. PCR amplicon and whole-genome resequencing analyses for off-target changes caused by BE3 were low at a genome-wide scale. This study provided the first evidence of base editing in large mammals produced from microinjected single-cell embryos. Our results support further optimization of BEs for introgressing complex human disease alleles into large animal models, to evaluate potential genetic improvement of complex health and production traits in a single generation.