Little evidence for homoeologous gene conversion and homoeologous exchange events in Gossypium allopolyploids.

PREMISE: A complicating factor in analyzing allopolyploid genomes is the possibility of physical interactions between homoeologous chromosomes during meiosis, resulting in either crossover (homoeologous exchanges) or non-crossover products (homoeologous gene conversion). Homoeologous gene conversion was first described in cotton by comparing SNP patterns in sequences from two diploid progenitors with those from the allopolyploid subgenomes. These analyses, however, did not explicitly consider other evolutionary scenarios that may give rise to similar SNP patterns as homoeologous gene conversion, creating uncertainties about the reality of the inferred gene conversion events. METHODS: Here, we use an expanded phylogenetic sampling of high-quality genome assemblies from seven allopolyploid Gossypium species (all derived from the same polyploidy event), four diploid species (two closely related to each subgenome), and a diploid outgroup to derive a robust method for identifying potential genomic regions of gene conversion and homoeologous exchange. RESULTS: We found little evidence for homoeologous gene conversion in allopolyploid cottons, and that only two of the 40 best-supported events were shared by more than one species. We did, however, reveal a single, shared homoeologous exchange event at one end of chromosome 1, which occurred shortly after allopolyploidization but prior to divergence of the descendant species. CONCLUSIONS: Overall, our analyses demonstrated that homoeologous gene conversion and homoeologous exchanges are uncommon in Gossypium, affecting between zero and 24 genes per subgenome (0.0-0.065%) across the seven species. More generally, we highlighted the potential problems of using simple four-taxon tests to investigate patterns of homoeologous gene conversion in established allopolyploids.

Genetic Regulatory Perturbation of Gene Expression Impacted by Genomic Introgression in Fiber Development of Allotetraploid Cotton.

Interspecific genomic introgression is an important evolutionary process with respect to the generation of novel phenotypic diversity and adaptation. A key question is how gene flow perturbs gene expression networks and regulatory interactions. Here, an introgression population of two species of allopolyploid cotton (Gossypium) to delineate the regulatory perturbations of gene expression regarding fiber development accompanying fiber quality change is utilized. De novo assembly of the recipient parent (G. hirsutum Emian22) genome allowed the identification of genomic variation and introgression segments (ISs) in 323 introgression lines (ILs) from the donor parent (G. barbadense 3-79). It documented gene expression dynamics by sequencing 1,284 transcriptomes of developing fibers and characterized genetic regulatory perturbations mediated by genomic introgression using a multi-locus model. Introgression of individual homoeologous genes exhibiting extreme low or high expression bias can lead to a parallel expression bias in their non-introgressed duplicates, implying a shared yet divergent regulatory fate of duplicated genes following allopolyploidy. Additionally, the IL N182 with improved fiber quality is characterized, and the candidate gene GhFLAP1 related to fiber length is validated. This study outlines a framework for understanding introgression-mediated regulatory perturbations in polyploids, and provides insights for targeted breeding of superior upland cotton fiber.

Origin and diversity of the wild cottons (Gossypium hirsutum) of Mound Key, Florida.

Elucidating genetic diversity within wild forms of modern crops is essential for understanding domestication and the possibilities of wild germplasm utilization. Gossypium hirsutum is a predominant source of natural plant fibers and the most widely cultivated cotton species. Wild forms of G. hirsutum are challenging to distinguish from feral derivatives, and truly wild populations are uncommon. Here we characterize a population from Mound Key Archaeological State Park, Florida using genome-wide SNPs extracted from 25 individuals over three sites. Our results reveal that this population is genetically dissimilar from other known wild, landrace, and domesticated cottons, and likely represents a pocket of previously unrecognized wild genetic diversity. The unexpected level of divergence between the Mound Key population and other wild cotton populations suggests that the species may harbor other remnant and genetically distinct populations that are geographically scattered in suitable habitats throughout the Caribbean. Our work thus has broader conservation genetic implications and suggests that further exploration of natural diversity in this species is warranted.

Evolutionary Dynamics of Chromatin Structure and Duplicate Gene Expression in Diploid and Allopolyploid Cotton.

Polyploidy is a prominent mechanism of plant speciation and adaptation, yet the mechanistic understandings of duplicated gene regulation remain elusive. Chromatin structure dynamics are suggested to govern gene regulatory control. Here, we characterized genome-wide nucleosome organization and chromatin accessibility in allotetraploid cotton, Gossypium hirsutum (AADD, 2n = 4X = 52), relative to its two diploid parents (AA or DD genome) and their synthetic diploid hybrid (AD), using DNS-seq. The larger A-genome exhibited wider average nucleosome spacing in diploids, and this intergenomic difference diminished in the allopolyploid but not hybrid. Allopolyploidization also exhibited increased accessibility at promoters genome-wide and synchronized cis-regulatory motifs between subgenomes. A prominent cis-acting control was inferred for chromatin dynamics and demonstrated by transposable element removal from promoters. Linking accessibility to gene expression patterns, we found distinct regulatory effects for hybridization and later allopolyploid stages, including nuanced establishment of homoeolog expression bias and expression level dominance. Histone gene expression and nucleosome organization are coordinated through chromatin accessibility. Our study demonstrates the capability to track high-resolution chromatin structure dynamics and reveals their role in the evolution of cis-regulatory landscapes and duplicate gene expression in polyploids, illuminating regulatory ties to subgenomic asymmetry and dominance.

Polyploid plants take cytonuclear perturbations in stride.

Hybridization in plants is often accompanied by nuclear genome doubling (allopolyploidy), which has been hypothesized to perturb interactions between nuclear and organellar (mitochondrial and plastid) genomes by creating imbalances in the relative copy number of these genomes and producing genetic incompatibilities between maternally derived organellar genomes and the half of the allopolyploid nuclear genome from the paternal progenitor. Several evolutionary responses have been predicted to ameliorate these effects, including selection for changes in protein sequences that restore cytonuclear interactions; biased gene retention/expression/conversion favoring maternal nuclear gene copies; and fine-tuning of relative cytonuclear genome copy numbers and expression levels. Numerous recent studies, however, have found that evolutionary responses are inconsistent and rarely scale to genome-wide generalities. The apparent robustness of plant cytonuclear interactions to allopolyploidy may reflect features that are general to allopolyploids such as the lack of F2 hybrid breakdown under disomic inheritance, and others that are more plant-specific, including slow sequence divergence in organellar genomes and preexisting regulatory responses to changes in cell size and endopolyploidy during development. Thus, cytonuclear interactions may only rarely act as the main barrier to establishment of allopolyploid lineages, perhaps helping to explain why allopolyploidy is so pervasive in plant evolution.

Regulatory controls of duplicated gene expression during fiber development in allotetraploid cotton.

Polyploidy complicates transcriptional regulation and increases phenotypic diversity in organisms. The dynamics of genetic regulation of gene expression between coresident subgenomes in polyploids remains to be understood. Here we document the genetic regulation of fiber development in allotetraploid cotton Gossypium hirsutum by sequencing 376 genomes and 2,215 time-series transcriptomes. We characterize 1,258 genes comprising 36 genetic modules that control staged fiber development and uncover genetic components governing their partitioned expression relative to subgenomic duplicated genes (homoeologs). Only about 30% of fiber quality-related homoeologs show phenotypically favorable allele aggregation in cultivars, highlighting the potential for subgenome additivity in fiber improvement. We envision a genome-enabled breeding strategy, with particular attention to 48 favorable alleles related to fiber phenotypes that have been subjected to purifying selection during domestication. Our work delineates the dynamics of gene regulation during fiber development and highlights the potential of subgenomic coordination underpinning phenotypes in polyploid plants.

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.

Evolutionary divergence of duplicated genomes in newly described allotetraploid cottons.

Allotetraploid cotton (Gossypium) species represents a model system for the study of plant polyploidy, molecular evolution, and domestication. Here, chromosome-scale genome sequences were obtained and assembled for two recently described wild species of tetraploid cotton, Gossypium ekmanianum [(AD)6, Ge] and Gossypium stephensii [(AD)7, Gs], and one early form of domesticated Gossypium hirsutum, race punctatum [(AD)1, Ghp]. Based on phylogenomic analysis, we provide a dated whole-genome level perspective for the evolution of the tetraploid Gossypium clade and resolved the evolutionary relationships of Gs, Ge, and domesticated G. hirsutum. We describe genomic structural variation that arose during Gossypium evolution and describe its correlates-including phenotypic differentiation, genetic isolation, and genetic convergence-that contributed to cotton biodiversity and cotton domestication. Presence/absence variation is prominent in causing cotton genomic structural variations. A presence/absence variation-derived gene encoding a phosphopeptide-binding protein is implicated in increasing fiber length during cotton domestication. The relatively unimproved Ghp offers the potential for gene discovery related to adaptation to environmental challenges. Expanded gene families enoyl-CoA δ isomerase 3 and RAP2-7 may have contributed to abiotic stress tolerance, possibly by targeting plant hormone-associated biochemical pathways. Our results generate a genomic context for a better understanding of cotton evolution and for agriculture.

Variation in cytonuclear expression accommodation among allopolyploid plants.

Cytonuclear coevolution is a common feature among plants, which coordinates gene expression and protein products between the nucleus and organelles. Consequently, lineage-specific differences may result in incompatibilities between the nucleus and cytoplasm in hybrid taxa. Allopolyploidy is also a common phenomenon in plant evolution. The hybrid nature of allopolyploids may result in cytonuclear incompatibilities, but the massive nuclear redundancy created during polyploidy affords additional avenues for resolving cytonuclear conflict (i.e. cytonuclear accommodation). Here we evaluate expression changes in organelle-targeted nuclear genes for 6 allopolyploid lineages that represent 4 genera (i.e. Arabidopsis, Arachis, Chenopodium, and Gossypium) and encompass a range in polyploid ages. Because incompatibilities between the nucleus and cytoplasm could potentially result in biases toward the maternal homoeolog and/or maternal expression level, we evaluate patterns of homoeolog usage, expression bias, and expression-level dominance in cytonuclear genes relative to the background of noncytonuclear expression changes and to the diploid parents. Although we find subsets of cytonuclear genes in most lineages that match our expectations of maternal preference, these observations are not consistent among either allopolyploids or categories of organelle-targeted genes. Our results indicate that cytonuclear expression evolution may be subtle and variable among genera and genes, likely reflecting a diversity of mechanisms to resolve nuclear-cytoplasmic incompatibilities in allopolyploid species.

Organellar transcripts dominate the cellular mRNA pool across plants of varying ploidy levels.

Mitochondrial and plastid functions depend on coordinated expression of proteins encoded by genomic compartments that have radical differences in copy number of organellar and nuclear genomes. In polyploids, doubling of the nuclear genome may add challenges to maintaining balanced expression of proteins involved in cytonuclear interactions. Here, we use ribo-depleted RNA sequencing (RNA-seq) to analyze transcript abundance for nuclear and organellar genomes in leaf tissue from four different polyploid angiosperms and their close diploid relatives. We find that even though plastid genomes contain <1% of the number of genes in the nuclear genome, they generate the majority (69.9 to 82.3%) of messenger RNA (mRNA) transcripts in the cell. Mitochondrial genes are responsible for a much smaller percentage (1.3 to 3.7%) of the leaf mRNA pool but still produce much higher transcript abundances per gene compared to nuclear genome. Nuclear genes encoding proteins that functionally interact with mitochondrial or plastid gene products exhibit mRNA expression levels that are consistently more than 10-fold lower than their organellar counterparts, indicating an extreme cytonuclear imbalance at the RNA level despite the predominance of equimolar interactions at the protein level. Nevertheless, interacting nuclear and organellar genes show strongly correlated transcript abundances across functional categories, suggesting that the observed mRNA stoichiometric imbalance does not preclude coordination of cytonuclear expression. Finally, we show that nuclear genome doubling does not alter the cytonuclear expression ratios observed in diploid relatives in consistent or systematic ways, indicating that successful polyploid plants are able to compensate for cytonuclear perturbations associated with nuclear genome doubling.

Parental legacy versus regulatory innovation in salt stress responsiveness of allopolyploid cotton (Gossypium) species.

Polyploidy provides an opportunity for evolutionary innovation and species diversification, especially under stressful conditions. In allopolyploids, the conditional dynamics of homoeologous gene expression can be either inherited from ancestral states pre-existing in the parental diploids or novel upon polyploidization, the latter potentially permitting a wider range of phenotypic responses to stresses. To gain insight into regulatory mechanisms underlying the diversity of salt resistance in Gossypium species, we compared global transcriptomic responses to modest salinity stress in two allotetraploid (AD-genome) cotton species, Gossypium hirsutum and G. mustelinum, relative to their model diploid progenitors (A-genome and D-genome). Multivariate and pairwise analyses of salt-responsive changes revealed a profound alteration of gene expression for about one third of the transcriptome. Transcriptional responses and associated functional implications of salt acclimation varied across species, as did species-specific coexpression modules among species and ploidy levels. Salt responsiveness in both allopolyploids was strongly biased toward the D-genome progenitor. A much lower level of transgressive downregulation was observed in the more salt-tolerant G. mustelinum than in the less tolerant G. hirsutum. By disentangling inherited effects from evolved responses, we show that expression biases that are not conditional upon salt stress approximately equally reflect parental legacy and regulatory novelty upon allopolyploidization, whereas stress-responsive biases are predominantly novel, or evolved, in allopolyploids. Overall, our work suggests that allopolyploid cottons acquired a wide range of stress response flexibility relative to their diploid ancestors, most likely mediated by complex suites of duplicated genes and regulatory factors.

Global Patterns of Subgenome Evolution in Organelle-Targeted Genes of Six Allotetraploid Angiosperms.

Whole-genome duplications (WGDs) are a prominent process of diversification in eukaryotes. The genetic and evolutionary forces that WGD imposes on cytoplasmic genomes are not well understood, despite the central role that cytonuclear interactions play in eukaryotic function and fitness. Cellular respiration and photosynthesis depend on successful interaction between the 3,000+ nuclear-encoded proteins destined for the mitochondria or plastids and the gene products of cytoplasmic genomes in multi-subunit complexes such as OXPHOS, organellar ribosomes, Photosystems I and II, and Rubisco. Allopolyploids are thus faced with the critical task of coordinating interactions between the nuclear and cytoplasmic genes that were inherited from different species. Because the cytoplasmic genomes share a more recent history of common descent with the maternal nuclear subgenome than the paternal subgenome, evolutionary "mismatches" between the paternal subgenome and the cytoplasmic genomes in allopolyploids might lead to the accelerated rates of evolution in the paternal homoeologs of allopolyploids, either through relaxed purifying selection or strong directional selection to rectify these mismatches. We report evidence from six independently formed allotetraploids that the subgenomes exhibit unequal rates of protein-sequence evolution, but we found no evidence that cytonuclear incompatibilities result in altered evolutionary trajectories of the paternal homoeologs of organelle-targeted genes. The analyses of gene content revealed mixed evidence for whether the organelle-targeted genes are lost more rapidly than the non-organelle-targeted genes. Together, these global analyses provide insights into the complex evolutionary dynamics of allopolyploids, showing that the allopolyploid subgenomes have separate evolutionary trajectories despite sharing the same nucleus, generation time, and ecological context.

Nuclear-cytoplasmic balance: whole genome duplications induce elevated organellar genome copy number.

The plant genome is partitioned across three distinct subcellular compartments: the nucleus, mitochondria, and plastids. Successful coordination of gene expression among these organellar genomes and the nuclear genome is critical for plant function and fitness. Whole genome duplication (WGD) events in the nucleus have played a major role in the diversification of land plants and are expected to perturb the relative copy number (stoichiometry) of nuclear, mitochondrial, and plastid genomes. Thus, elucidating the mechanisms whereby plant cells respond to the cytonuclear stoichiometric imbalance that follows WGDs represents an important yet underexplored question in understanding the evolutionary consequences of genome doubling. We used droplet digital PCR to investigate the relationship between nuclear and organellar genome copy numbers in allopolyploids and their diploid progenitors in both wheat and Arabidopsis. Polyploids exhibit elevated organellar genome copy numbers per cell, largely preserving the cytonuclear stoichiometry observed in diploids despite the change in nuclear genome copy number. To investigate the timescale over which cytonuclear stoichiometry may respond to WGD, we also estimated the organellar genome copy number in Arabidopsis synthetic autopolyploids and in a haploid-induced diploid line. We observed corresponding changes in organellar genome copy number in these laboratory-generated lines, indicating that at least some of the cellular response to cytonuclear stoichiometric imbalance is immediate following WGD. We conclude that increases in organellar genome copy numbers represent a common response to polyploidization, suggesting that maintenance of cytonuclear stoichiometry is an important component in establishing polyploid lineages.

Parallel and Intertwining Threads of Domestication in Allopolyploid Cotton.

The two cultivated allopolyploid cottons, Gossypium hirsutum and Gossypium barbadense, represent a remarkable example of parallel independent domestication, both involving dramatic morphological transformations under selection from wild perennial plants to annualized row crops. Deep resequencing of 643 newly sampled accessions spanning the wild-to-domesticated continuum of both species, and their allopolyploid relatives, are combined with existing data to resolve species relationships and elucidate multiple aspects of their parallel domestication. It is confirmed that wild G. hirsutum and G. barbadense were initially domesticated in the Yucatan Peninsula and NW South America, respectively, and subsequently spread under domestication over 4000-8000 years to encompass most of the American tropics. A robust phylogenomic analysis of infraspecific relationships in each species is presented, quantify genetic diversity in both, and describe genetic bottlenecks associated with domestication and subsequent diffusion. As these species became sympatric over the last several millennia, pervasive genome-wide bidirectional introgression occurred, often with striking asymmetries involving the two co-resident genomes of these allopolyploids. Diversity scans revealed genomic regions and genes unknowingly targeted during domestication and additional subgenomic asymmetries. These analyses provide a comprehensive depiction of the origin, divergence, and adaptation of cotton, and serve as a rich resource for cotton improvement.

Homoeologous gene expression and co-expression network analyses and evolutionary inference in allopolyploids.

Polyploidy is a widespread phenomenon throughout eukaryotes. Due to the coexistence of duplicated genomes, polyploids offer unique challenges for estimating gene expression levels, which is essential for understanding the massive and various forms of transcriptomic responses accompanying polyploidy. Although previous studies have explored the bioinformatics of polyploid transcriptomic profiling, the causes and consequences of inaccurate quantification of transcripts from duplicated gene copies have not been addressed. Using transcriptomic data from the cotton genus (Gossypium) as an example, we present an analytical workflow to evaluate a variety of bioinformatic method choices at different stages of RNA-seq analysis, from homoeolog expression quantification to downstream analysis used to infer key phenomena of polyploid expression evolution. In general, EAGLE-RC and GSNAP-PolyCat outperform other quantification pipelines tested, and their derived expression dataset best represents the expected homoeolog expression and co-expression divergence. The performance of co-expression network analysis was less affected by homoeolog quantification than by network construction methods, where weighted networks outperformed binary networks. By examining the extent and consequences of homoeolog read ambiguity, we illuminate the potential artifacts that may affect our understanding of duplicate gene expression, including an overestimation of homoeolog co-regulation and the incorrect inference of subgenome asymmetry in network topology. Taken together, our work points to a set of reasonable practices that we hope are broadly applicable to the evolutionary exploration of polyploids.

Conservation and Divergence in Duplicated Fiber Coexpression Networks Accompanying Domestication of the Polyploid Gossypium hirsutum L.

Gossypium hirsutum L. (Upland cotton) has an evolutionary history involving inter-genomic hybridization, polyploidization, and subsequent domestication. We analyzed the developmental dynamics of the cotton fiber transcriptome accompanying domestication using gene coexpression networks for both joint and homoeologous networks. Remarkably, most genes exhibited expression for at least one homoeolog, confirming previous reports of widespread gene usage in cotton fibers. Most coexpression modules comprising the joint network are preserved in each subgenomic network and are enriched for similar biological processes, showing a general preservation of network modular structure for the two co-resident genomes in the polyploid. Interestingly, only one fifth of homoeologs co-occur in the same module when separated, despite similar modular structures between the joint and homoeologous networks. These results suggest that the genome-wide divergence between homoeologous genes is sufficient to separate their co-expression profiles at the intermodular level, despite conservation of intramodular relationships within each subgenome. Most modules exhibit D-homoeolog expression bias, although specific modules do exhibit A-homoeolog bias. Comparisons between wild and domesticated coexpression networks revealed a much tighter and denser network structure in domesticated fiber, as evidenced by its fewer modules, 13-fold increase in the number of development-related module member genes, and the poor preservation of the wild network topology. These results demonstrate the amazing complexity that underlies the domestication of cotton fiber.

Genomic diversifications of five Gossypium allopolyploid species and their impact on cotton improvement.

Polyploidy is an evolutionary innovation for many animals and all flowering plants, but its impact on selection and domestication remains elusive. Here we analyze genome evolution and diversification for all five allopolyploid cotton species, including economically important Upland and Pima cottons. Although these polyploid genomes are conserved in gene content and synteny, they have diversified by subgenomic transposon exchanges that equilibrate genome size, evolutionary rate heterogeneities and positive selection between homoeologs within and among lineages. These differential evolutionary trajectories are accompanied by gene-family diversification and homoeolog expression divergence among polyploid lineages. Selection and domestication drive parallel gene expression similarities in fibers of two cultivated cottons, involving coexpression networks and N6-methyladenosine RNA modifications. Furthermore, polyploidy induces recombination suppression, which correlates with altered epigenetic landscapes and can be overcome by wild introgression. These genomic insights will empower efforts to manipulate genetic recombination and modify epigenetic landscapes and target genes for crop improvement.

The Gossypium longicalyx Genome as a Resource for Cotton Breeding and Evolution.

Cotton is an important crop that has made significant gains in production over the last century. Emerging pests such as the reniform nematode have threatened cotton production. The rare African diploid species Gossypium longicalyx is a wild species that has been used as an important source of reniform nematode immunity. While mapping and breeding efforts have made some strides in transferring this immunity to the cultivated polyploid species, the complexities of interploidal transfer combined with substantial linkage drag have inhibited progress in this area. Moreover, this species shares its most recent common ancestor with the cultivated A-genome diploid cottons, thereby providing insight into the evolution of long, spinnable fiber. Here we report a newly generated de novo genome assembly of G. longicalyx This high-quality genome leveraged a combination of PacBio long-read technology, Hi-C chromatin conformation capture, and BioNano optical mapping to achieve a chromosome level assembly. The utility of the G. longicalyx genome for understanding reniform immunity and fiber evolution is discussed.

Salt-tolerance diversity in diploid and polyploid cotton (Gossypium) species.

The development of salt-tolerant genotypes is pivotal for the effective utilization of salinized land and to increase global crop productivity. Several cotton species comprise the most important source of textile fibers globally, and these are increasingly grown on marginal or increasingly saline agroecosystems. The allopolyploid cotton species also provide a model system for polyploid research, of relevance here because polyploidy was suggested to be associated with increased adaptation to stress. To evaluate the genetic variation of salt tolerance among cotton species, 17 diverse accessions of allopolyploid (AD-genome) and diploid (A- and D-genome) Gossypium were evaluated for a total of 29 morphological and physiological traits associated with salt tolerance. For most morphological and physiological traits, cotton accessions showed highly variable responses to 2 weeks of exposure to moderate (50 mm NaCl) and high (100 mm NaCl) hydroponic salinity treatments. Our results showed that the most salt-tolerant species were the allopolyploid Gossypium mustelinum from north-east Brazil, the D-genome diploid Gossypium klotzschianum from the Galapagos Islands, followed by the A-genome diploids of Africa and Asia. Generally, A-genome accessions outperformed D-genome cottons under salinity conditions. Allopolyploid accessions from either diploid genomic group did not show significant differences in salt tolerance, but they were more similar to one of the two progenitor lineages. Our findings demonstrate that allopolyploidy in itself need not be associated with increased salinity stress tolerance and provide information for using the secondary Gossypium gene pool to breed for improved salt tolerance.

Unraveling cis and trans regulatory evolution during cotton domestication.

Cis and trans regulatory divergence underlies phenotypic and evolutionary diversification. Relatively little is understood about the complexity of regulatory evolution accompanying crop domestication, particularly for polyploid plants. Here, we compare the fiber transcriptomes between wild and domesticated cotton (Gossypium hirsutum) and their reciprocal F1 hybrids, revealing genome-wide (~15%) and often compensatory cis and trans regulatory changes under divergence and domestication. The high level of trans evolution (54%-64%) observed is likely enabled by genomic redundancy following polyploidy. Our results reveal that regulatory variation is significantly associated with sequence evolution, inheritance of parental expression patterns, co-expression gene network properties, and genomic loci responsible for domestication traits. With respect to regulatory evolution, the two subgenomes of allotetraploid cotton are often uncoupled. Overall, our work underscores the complexity of regulatory evolution during fiber domestication and may facilitate new approaches for improving cotton and other polyploid plants.