Recent progression and future perspectives in cotton genomic breeding

Upland cotton is an important global cash crop for its long seed fibers and high edible oil and protein content. Progress in cotton genomics promotes the advancement of cotton genetics, evolutionary studies, functional genetics, and breeding, and has ushered cotton research and breeding into a new era. Here, we summarize high-impact genomics studies for cotton from the last 10 years. The diploid Gossypium arboreum and allotetraploid Gossypium hirsutum are the main focus of most genetic and genomic studies. We next review recent progress in cotton molecular biology and genetics, which builds on cotton genome sequencing efforts, population studies, and functional genomics, to provide insights into the mechanisms shaping abiotic and biotic stress tolerance, plant architecture, seed oil content, and fiber development. We also suggest the application of novel technologies and strategies to facilitate genome-based crop breeding. Explosive growth in the amount of novel genomic data, identified genes, gene modules, and pathways is now enabling researchers to utilize multidisciplinary genomics-enabled breeding strategies to cultivate "super cotton", synergistically improving multiple traits. These strategies must rise to meet urgent demands for a sustainable cotton industry.

The Gossypium herbaceum L. Wagad genome as a resource for understanding cotton domestication

Gossypium herbaceum is a species of cotton native to Africa and Asia that is one of the 2 domesticated diploids. Together with its sister-species G. arboreum, these A-genome taxa represent models of the extinct A-genome donor of modern polyploid cotton, which provide about 95% of cotton grown worldwide. As part of a larger effort to characterize variation and improve resources among diverse diploid and polyploid cotton genomes, we sequenced and assembled the genome of G. herbaceum cultivar (cv.) Wagad, representing the first domesticated accession for this species. This chromosome-level genome was generated using a combination of PacBio long-read technology, HiC, and Bionano optical mapping and compared to existing genome sequences in cotton. We compare the genome of this cultivar to the existing genome of wild G. herbaceum subspecies africanum to elucidate changes in the G. herbaceum genome concomitant with domestication and extend these analyses to gene expression using available RNA-seq. Our results demonstrate the utility of the G. herbaceum cv. Wagad genome in understanding domestication in the diploid species, which could inform modern breeding programs.

Features of Chromosome Introgression from Gossypium barbadense L. into G. hirsutum L. during the Development of Alien Substitution Lines

The creation of G. barbadense L./G. hirsutum L. chromosome-substitution lines is an important method to transfer agronomically valuable traits from G. barbadense into G. hirsutum. In this study, 30 monosomic lines of G. hirsutum from the Cytogenetic Collection of Uzbekistan, created in the genotypic background of line L-458, were used in crosses with the G. barbadense line Pima 3-79 to create substitution lines. In the course of this work, new monosomic lines were identified for chromosome 12 and monotelodisome 6 of the A_tsubgenome and for chromosomes 17, 21, and 22 of the D_tsubgenome using chromosome-specific SSR markers and a well-defined tester set of cotton translocation lines (USA). Compared to those in the F₁ hybrids, a strong decrease in the crossing and setting rates was found in the BC₁F₁ backcross lines, with the substitution of chromosomes 2, 4, 6, 7, and 12 of the A_tsubgenome and 17, 18, 21, and 22 of the D_tsubgenome. The F₁ and BC₁F₁ offspring from interspecific crosses differed in their transmission of univalents. Despite the regular pairing of chromosomes and the high meiotic index, interspecific aneuploid hybrids were characterized by a decrease in pollen fertility, which may indicate hidden structural variability in these genomes that did not affect meiotic division. The identification of chromosomes using chromosome-specific SSR markers in the early stages of plant development has greatly accelerated the detection of monosomic plants. The analysis of morphobiological traits revealed that monosomic F₁ hybrids were more similar to the donor line, while BC₁F₁ hybrids were more similar to the recurrent parent but also showed previously undetected traits.

The hexokinase Gene Family in Cotton: Genome-Wide Characterization and Bioinformatics Analysis

Hexokinase (HXK) is involved in hexose phosphorylation, sugar sensing, and signal transduction, all of which regulate plant growth and adaptation to stresses. Gossypium hirsutum L. is one of the most important fiber crops in the world, however, little is known about the HXKs gene family in G. hirsutum L. We identified 17 GhHXKs from the allotetraploid G. hirsutum L. genome (AADD). G. raimondii (DD) and G. arboreum (AA) are the diploid progenitors of G. hirsutum L. and contributed equally to the At_genome and Dt_genome GhHXKs genes. The chromosomal locations and exon-intron structures of GhHXK genes among cotton species are conservative. Phylogenetic analysis grouped the HXK proteins into four and three groups based on whether they were monocotyledons and dicotyledons, respectively. Duplication event analysis demonstrated that HXKs in G. hirsutum L. primarily originated from segmental duplication, which prior to diploid hybridization. Experiments of qRT-PCR, transcriptome and promoter cis-elements demonstrated that GhHXKs' promoters have auxin and GA responsive elements that are highly expressed in the fiber initiation and elongation stages, while the promoters contain ABA-, MeJA-, and SA-responsive elements that are highly expressed during the synthesis of the secondary cell wall. We performed a comprehensive analysis of the GhHXK gene family is a vital fiber crop, which lays the foundation for future studies assessing its role in fiber development.

CottonGen: The Community Database for Cotton Genomics, Genetics, and Breeding Research

Over the last eight years, the volume of whole genome, gene expression, SNP genotyping, and phenotype data generated by the cotton research community has exponentially increased. The efficient utilization/re-utilization of these complex and large datasets for knowledge discovery, translation, and application in crop improvement requires them to be curated, integrated with other types of data, and made available for access and analysis through efficient online search tools. Initiated in 2012, CottonGen is an online community database providing access to integrated peer-reviewed cotton genomic, genetic, and breeding data, and analysis tools. Used by cotton researchers worldwide, and managed by experts with crop-specific knowledge, it continuous to be the logical choice to integrate new data and provide necessary interfaces for information retrieval. The repository in CottonGen contains colleague, gene, genome, genotype, germplasm, map, marker, metabolite, phenotype, publication, QTL, species, transcriptome, and trait data curated by the CottonGen team. The number of data entries housed in CottonGen has increased dramatically, for example, since 2014 there has been an 18-fold increase in genes/mRNAs, a 23-fold increase in whole genomes, and a 372-fold increase in genotype data. New tools include a genetic map viewer, a genome browser, a synteny viewer, a metabolite pathways browser, sequence retrieval, BLAST, and a breeding information management system (BIMS), as well as various search pages for new data types. CottonGen serves as the home to the International Cotton Genome Initiative, managing its elections and serving as a communication and coordination hub for the community. With its extensive curation and integration of data and online tools, CottonGen will continue to facilitate utilization of its critical resources to empower research for cotton crop improvement.

The Gossypium anomalum genome as a resource for cotton improvement and evolutionary analysis of hybrid incompatibility

Cotton is an important crop that has been the beneficiary of multiple genome sequencing efforts, including diverse representatives of wild species for germplasm development. Gossypium anomalum is a wild African diploid species that harbors stress-resistance and fiber-related traits with potential application to modern breeding efforts. In addition, this species is a natural source of cytoplasmic male sterility and a resource for understanding hybrid lethality in the genus. Here, we report a high-quality de novo genome assembly for G. anomalum and characterize this genome relative to existing genome sequences in cotton. In addition, we use the synthetic allopolyploids 2(A2D1) and 2(A2D3) to discover regions in the G. anomalum genome potentially involved in hybrid lethality, a possibility enabled by introgression of regions homologous to the D3 (Gossypium davidsonii) lethality loci into the synthetic 2(A2D3) allopolyploid.