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Hybridization is widely used. However, for a long time, systematic theories and technologies related to hybridization in fish have been lacking. In this study, through long-term systematic research, we investigated and obtained the main rules regarding inheritance and reproduction related to fish distant hybridization. Furthermore, we established one-step and multistep breeding technologies that were suitable for interspecific hybridization and intraspecific hybridization. Simultaneously, we used these two breeding technologies to produce a batch of diploid fish lineages and tetraploid fish lineages and improved fishes. In addition, we widely discuss the methods, technologies and results of hybridization breeding, referring to the domestic and foreign literature on fish hybridization. We hope that this paper will be beneficial for the research and application of fish hybrid breeding.

Food legumes are important for defeating malnutrition and sustaining agri-food systems globally. Breeding efforts in legume crops have been largely confined to the exploitation of genetic variation available within the primary genepool, resulting in narrow genetic base. Introgression as a breeding scheme has been remarkably successful for an array of inheritance and molecular studies in food legumes. Crop wild relatives (CWRs), landraces, and exotic germplasm offer great potential for introgression of novel variation not only to widen the genetic base of the elite genepool for continuous incremental gains over breeding cycles but also to discover the cryptic genetic variation hitherto unexpressed. CWRs also harbor positive quantitative trait loci (QTLs) for improving agronomic traits. However, for transferring polygenic traits, “specialized population concept” has been advocated for transferring QTLs from CWR into elite backgrounds. Recently, introgression breeding has been successful in developing improved cultivars in chickpea ( Cicer arietinum ), pigeonpea ( Cajanus cajan ), peanut ( Arachis hypogaea ), lentil ( Lens culinaris ), mungbean ( Vigna radiata ), urdbean ( Vigna mungo ), and common bean ( Phaseolus vulgaris ). Successful examples indicated that the usable genetic variation could be exploited by unleashing new gene recombination and hidden variability even in late filial generations. In mungbean alone, distant hybridization has been deployed to develop seven improved commercial cultivars, whereas in urdbean, three such cultivars have been reported. Similarly, in chickpea, three superior cultivars have been developed from crosses between C. arietinum and Cicer reticulatum . Pigeonpea has benefited the most where different cytoplasmic male sterility genes have been transferred from CWRs, whereas a number of disease-resistant germplasm have also been developed in Phaseolus . As vertical gene transfer has resulted in most of the useful gene introgressions of practical importance in food legumes, the horizontal gene transfer through transgenic technology, somatic hybridization, and, more recently, intragenesis also offer promise. The gains through introgression breeding are significant and underline the need of bringing it in the purview of mainstream breeding while deploying tools and techniques to increase the recombination rate in wide crosses and reduce the linkage drag. The resurgence of interest in introgression breeding needs to be capitalized for development of commercial food legume cultivars.

Distant hybridization between Brassica napus and related genera serves as an effective approach for rapeseed germplasm innovation. Isatis indigotica, a wild relative of Brassica, has emerged as a valuable genetic resource for rapeseed improvement due to its medicinal properties. This study employed anchor mapping of alien chromosomal fragment localization (AMAC) method to efficiently identify alien chromosomal fragments in the progeny derived from distant hybridization between I. indigotica and Brassica napus, ‘Songyou No. 1’. Based on the AMAC method, we developed 193,101 IP and SSR markers utilizing the I. indigotica reference genome (Woad-v1.0). Through Electronic-PCR analysis against the Brassica and I. indigotica pan-genome, 27,820 specific single-locus (SSL) IP and SSR markers were obtained. Subsequently, 205 pairs of IP primers and 50 pairs of SSR primers were synthesized randomly, among which 148 pairs of IP markers (72.20%) and 45 pairs of SSR markers (90%) were verified as SSL molecular markers for the I. indigotica genome with no amplification product in four Brassica crops. These 193 SSL markers enable precise identification of one complete I6 chromosome and three chromosomal fragments (I1:1.17 Mb, I5:2.61 Mb, I7:1.11 Mb) in ‘Songyou No. 1’. Furthermore, we traced 32 genes involved in bioactive compound biosynthesis within/near these alien segments in ‘Songyou No. 1’ and developed seven functional markers. This study not only validates the efficacy of SSL markers for detecting exogenous chromatin in intergeneric hybrids but also provides valuable insights for the precise identification and mapping of desired chromosomal fragments or genes embedded in the derivatives from distant hybridization and potential applications in marker-assisted breeding for medicinal plant via distant hybridization strategy between I. indigotica and Brassica crops.

Rapeseed, an allotetraploid oil crop, provides vegetable oil for human consumption. The growing demand for oilseeds has necessitated the development of rapeseed varieties with improved quality. Therefore, a clear understanding of the genetic basis underlying the seed oil content (SOC) is required. In this study, a natural population comprising 204 diverse accessions and recombinant inbred lines (RILs) derived from Brassica napus and Sinapis alba via distant hybridization were collected for genome-wide association analysis (GWAS) and quantitative trait loci (QTL) mapping of the SOC trait, respectively. The variable coefficient of the RIL and natural populations ranged from 7.43 to 10.43% and 8.40 to 10.91%. Then, a high-density linkage map was constructed based on whole genome re-sequencing (WGS); the map harbored 2,799 bin markers and covered a total distance of 1,835.21 cM, with an average marker interval of 0.66 cM. The QTLs for SOC on chromosome A07 were stably detected in both single and multiple environments. Finally, a novel locus qA07.SOC was identified as the major QTL for SOC based on the GWAS and RIL populations. In addition, the RNA-seq results showed that photosynthesis, lipid biosynthesis proteins, fatty acid metabolism, and unsaturated fatty acid biosynthesis were significantly different between the developed seeds of the two parents of the RIL population. By comparing the variation information and expression levels of the syntenic genes within qA07.SOC and its syntenic genomic regions, as well as through haplotype analysis via GWAS, BnaA07.STR18 , BnaA07.NRT1 , and BnaA07g12880D were predicted as candidate genes in the qA07.SOC interval. These stable QTLs containing candidate genes and haplotypes can potentially provide a reliable basis for marker-assisted selection in B. napus breeding for SOC.

SYCP1 and SYCP3 are essential testis‐specific genes for centromere pairing during meiosis, as well as for spermatogenesis and fertility in male germ cells. However, it is still unclear regarding the expression patterns in the fertile reciprocal hybrid offspring of Megalobrama amblycephala (blunt snout bream, BSB, 2n = 48) × Culter alburnus (topmouth culter, TC, 2n = 48). This research elucidated the genetic and expression characteristics of SYCP1 and SYCP3 through molecular cloning, sequence alignment, Western blotting, and immunohistochemistry to assess their roles in both hybrids and parents. The findings revealed that SYCP1 and SYCP3 exhibited high homology between M. amblycephala and C. alburnus, with varying degrees of chimerism in the BT and TB hybrids. The expression level of SYCP1 in these hybrids was intermediate between parents, while SYCP3 was more similar to M. amblycephala and significantly different from C. alburnus (p < 0.05). Western blotting confirmed the normal expression of both SYCP1 and SYCP3 proteins in the hybrid offspring. Immunohistochemistry verified the significant presence of these proteins in the testes of mature hybrids. These findings suggested that BT and TB hybrids retained the stability of the SYCP1 and SYCP3 genes inherited from their heterozygous parental origins, supporting independent protein expression despite slight variations in the CDS structure. Our results demonstrate that the normal expression of key meiotic genes plays an important role in overcoming reproductive barriers in distant hybridization, which is of great significance for genetic breeding in fish.

Background Developing novel germplasm by using wheat wild related species is an effective way to rebuild the wheat resource bank. The Psathyrostachys huashanica Keng ( P. huashanica , 2 n  = 2 x  = 14, NsNs) is regarded as a superior species to improve wheat breeding because of its multi-resistance, early maturation and numerous tiller traits. Introducing genetic components of P. huashanica into the common wheat background is the most important step in achieving the effective use. Therefore, the cytogenetic characterization and influence of the introgressed P. huashanica large segment chromosomes in the wheat background is necessary to be explored. Results In this study, we characterized a novel derived line, named D88-2a, a progeny of the former characterized wheat- P. huashanica partial amphiploid line H8911 (2 n  = 7 x  = 49, AABBDDNs). Cytological identification showed that the chromosomal composition of D88-2a was 2 n  = 44 = 22II, indicating the addition of exogenous chromosomes. Genomic in situ hybridization demonstrated that the supernumerary chromosomes were a pair of homologues from the P. huashanica and could be stably inherited in the common wheat background. Molecular markers and 15 K SNP array indicated that the additional chromosomes were derived from the sixth homoeologous group (i.e., 6Ns) of P. huashanica . Based on the distribution of the heterozygous single-nucleotide polymorphism sites and fluorescence in situ hybridization karyotype of each chromosome, this pair of additional chromosomes was confirmed as P. huashanica 6Ns large segment chromosomes, which contained the entire short arm and the proximal centromere portion of the long arm. In terms of the agronomic traits, the addition line D88-2a exhibited enhanced stripe rust resistance, improved spike characteristics and increased protein content than its wheat parent line 7182. Conclusions The new wheat germplasm D88-2a is a novel cytogenetically stable wheat- P. huashanica 6Ns large segment addition line, and the introgressed large segment alien chromosome has positive impact on plant spikelet number and stripe rust resistance. Thus, this germplasm can be used for genetic improvement of cultivated wheat and the study of functional alien chromosome segment.

Distant hybridization makes it possible to transfer the genome of one species to another, which results in changes in phenotypes and genotypes of the progenies. This study shows that distant hybridization or the combination of this method with gynogenesis or androgenesis lead to different ploidy fishes with genetic variation, including fertile tetraploid hybrids, sterile triploid hybrids, fertile diploid hybrids, fertile diploid gynogenetic fish, and their derived progenies. The formations of the different ploidy fishes depend on the genetic relationship between the parents. In this study, several types of distant hybridization, including red crucian carp ( Carassius auratus red var.) (2 n =100, abbreviated as RCC) (♀)×common carp ( Cyprinus carpio L.) (2 n =100, abbreviated as CC) (♂), and RCC (2 n =100) (♀)×blunt snout bream ( Megalobrama amblycephala ) (2 n =48, abbreviated as BSB) (♂) are described. In the distant hybridization of RCC (♀)×CC (♂), bisexual fertile F 3 –F 18 allotetraploid hybrids (4 n =200, abbreviated as 4 n AT) were formed. The diploid hybrid eggs and diploid sperm generated by the females and males of 4 n AT developed into diploid gynogenetic hybrids and diploid androgenetic hybrids, respectively, by gynogenesis and androgenesis, without treatment for doubling the chromosome. Improved tetraploid hybrids and improved diploid fishes with genetic variation were derived from the gynogenetic hybrid line. The improved diploid fishes included the high-body RCC and high-body goldfish. The formation of the tetraploid hybrids was related to the occurrence of unreduced gametes generated from the diploid hybrids, which involved in premeiotic endoreduplication, endomitosis, or fusion of germ cells. The sterile triploid hybrids (3 n =150) were produced on a large scale by crossing the males of tetraploid hybrids with females of diploid fish (2 n =100). In another distant hybridization of RCC (♀)×BSB (♂), different ploidy fishes were obtained, including diploid bisexual fertile natural gynogenetic fish (2 n =100), sterile triploid hybrids (3 n =124), and bisexual fertile tetraploid hybrids (4 n =148). Furthermore, two kinds of pentaploid hybrids (5 n =172 and 5 n =198) were formed. The biological characteristics and the mechanisms of formation of the different ploidy fish were compared and discussed at the cellular and molecular level. The results indicated distant hybridization or the combination of this method with gynogenesis or androgenesis affects the formation of different ploidy fish with genetic variation.

Distant hybridization refers to crosses between two different species, genera, or higher-ranking taxa, which can break species limits, increase genetic variation, and combine the biological characteristics of existing species. It is an important way of creating genetic variation, fertile strains, and excellent characteristics in new strains and populations. Combining analyses and summaries from many inter-related documents in plants and animals, both domestic and international, including examples and long-standing research on distant hybridization in fish from our laboratory, we summarize and compare the similarities and differences in plant and animal distant hybridization. In addition, we analyze and review the biological characteristics of their different ploidy progenies and the possible causes of disparity in survival rates. Mechanisms of sterility in animal and plant distant hybrids are also discussed, and research methods for the study of biological characteristics of hybrids, including morphology, cytology, and molecular cytogenetics are presented. This paper aims to provide comprehensive research materials and to systematically compare the general and specific characteristics of plant and animal hybrids with regards to reproduction, genetics, growth traits, and other biological characteristics. It is hoped that this paper will have great theoretical and practical significance for the study of genetic breeding and biological evolution of plant and animal distant hybridization.