Gene-expression novelty in allopolyploid cotton: a proteomic perspective

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.

An insight into the gene expression evolution in Gossypium species based on the leaf transcriptomes

BACKGROUND

Gene expression pattern is associated with biological phenotype and is widely used in exploring gene functions. Its evolution is also crucial in understanding species speciation and divergence. The genus Gossypium is a bona fide model for studying plant evolution and polyploidization. However, the evolution of gene expression during cotton species divergence has yet to be extensively discussed.

RESULTS

Based on the seedling leaf transcriptomes, this work analyzed the transcriptomic content and expression patterns across eight cotton species, including six diploids and two natural tetraploids. Our findings indicate that, while the biological function of these cotton transcriptomes remains largely conserved, there has been significant variation in transcriptomic content during species divergence. Furthermore, we conducted a comprehensive analysis of expression distances across cotton species. This analysis lends further support to the use of G. arboreum as a substitute for the A-genome donor of natural cotton polyploids. Moreover, our research highlights the evolution of stress-responsive pathways, including hormone signaling, fatty acid degradation, and flavonoid biosynthesis. These processes appear to have evolved under lower selection pressures, presumably reflecting their critical role in the adaptations of the studied cotton species to diverse environments.

CONCLUSIONS

In summary, this study provided insights into the gene expression variation within the genus Gossypium and identified essential genes/pathways whose expression evolution was closely associated with the evolution of cotton species. Furthermore, the method of characterizing genes and pathways under unexpected high or slow selection pressure can also serve as a new strategy for gene function exploration.

Transcriptome Shock in Developing Embryos of a Brassica napus and Brassica rapa Hybrid

Interspecific crosses that fuse the genomes of two different species may result in overall gene expression changes in the hybrid progeny, called 'transcriptome shock'. To better understand the expression pattern after genome merging during the early stages of allopolyploid formation, we performed RNA sequencing analysis on developing embryos of Brassica rapa, B. napus, and their synthesized allotriploid hybrids. Here, we show that the transcriptome shock occurs in the developing seeds of the hybrids. Of the homoeologous gene pairs, 17.1% exhibit expression bias, with an overall expression bias toward B. rapa. The expression level dominance also biases toward B. rapa, mainly induced by the expression change in homoeologous genes from B. napus. Functional enrichment analysis revealed significant differences in differentially expressed genes (DEGs) related to photosynthesis, hormone synthesis, and other pathways. Further study showed that significant changes in the expression levels of the key transcription factors (TFs) could regulate the overall interaction network in the developing embryo, which might be an essential cause of phenotype change. In conclusion, the present results have revealed the global changes in gene expression patterns in developing seeds of the hybrid between B. rapa and B. napus, and provided novel insights into the occurrence of transcriptome shock for harnessing heterosis.

Does one subgenome become dominant in the formation and evolution of a polyploid?

BACKGROUND

Polyploids are common in flowering plants and they tend to have more expanded ranges of distributions than their diploid progenitors. Possible mechanisms underlying polyploid success have been intensively investigated. Previous studies showed that polyploidy generates novel changes and that subgenomes in allopolyploid species often differ in gene number, gene expression levels and levels of epigenetic alteration. It is widely believed that such differences are the results of conflicts among the subgenomes. These differences have been treated by some as subgenome dominance, and it is claimed that the magnitude of subgenome dominance increases in polyploid evolution.

SCOPE

In addition to changes which occurred during evolution, differences between subgenomes of a polyploid species may also be affected by differences between the diploid donors and changes which occurred during polyploidization. The variable genome components in many plant species are extensive, which would result in exaggerated differences between a subgenome and its progenitor when a single genotype or a small number of genotypes are used to represent a polyploid or its donors. When artificially resynthesized polyploids are used as surrogates for newly formed genotypes which have not been exposed to evolutionary selection, differences between diploid genotypes available today and those involved in the formation of the natural polyploid genotypes must also be considered.

CONCLUSIONS

Contrary to the now widely held views that subgenome biases in polyploids are the results of conflicts among the subgenomes and that one of the parental subgenomes generally retains more genes which are more highly expressed, available results show that subgenome biases mainly reflect legacy from the progenitors and that they can be detected before the completion of polyploidization events. Further, there is no convincing evidence that the magnitudes of subgenome biases have significantly changed during evolution for any of the allopolyploid species assessed.

Genome-wide characterization of ovate family protein gene family associated with number of seeds per silique in Brassica napus

Ovate family proteins (OFPs) were firstly identified in tomato as proteins controlling the pear shape of the fruit. Subsequent studies have successively proved that OFPs are a class of negative regulators of plant development, and are involved in the regulation of complex traits in different plants. However, there has been no report about the functions of OFPs in rapeseed growth to date. Here, we identified the OFPs in rapeseed at the genomic level. As a result, a total of 67 members were obtained. We then analyzed the evolution from Arabidopsis thaliana to Brassica napus, illustrated their phylogenetic and syntenic relationships, and compared the gene structure and conserved domains between different copies. We also analyzed their expression patterns in rapeseed, and found significant differences in the expression of different members and in different tissues. Additionally, we performed a GWAS for the number of seeds per silique (NSPS) in a rapeseed population consisting of 204 natural accessions, and identified a new gene BnOFP13_2 significantly associated with NSPS, which was identified as a novel function of OFPs. Haplotype analysis revealed that the accessions with haplotype 3 had a higher NSPS than other accessions, suggesting that BnOFP13_2 is associated with NSPS. Transcript profiling during the five stages of silique development demonstrated that BnOFP13_2 negatively regulates NSPS. These findings provide evidence for functional diversity of OFP gene family and important implications for oilseed rape breeding.

Trajectories of Homoeolog-Specific Expression in Allotetraploid Tragopogon castellanus Populations of Independent Origins

Polyploidization can have a significant ecological and evolutionary impact by providing substantially more genetic material that may result in novel phenotypes upon which selection may act. While the effects of polyploidization are broadly reviewed across the plant tree of life, the reproducibility of these effects within naturally occurring, independently formed polyploids is poorly characterized. The flowering plant genus Tragopogon (Asteraceae) offers a rare glimpse into the intricacies of repeated allopolyploid formation with both nascent (< 90 years old) and more ancient (mesopolyploids) formations. Neo- and mesopolyploids in Tragopogon have formed repeatedly and have extant diploid progenitors that facilitate the comparison of genome evolution after polyploidization across a broad span of evolutionary time. Here, we examine four independently formed lineages of the mesopolyploid Tragopogon castellanus for homoeolog expression changes and fractionation after polyploidization. We show that expression changes are remarkably similar among these independently formed polyploid populations with large convergence among expressed loci, moderate convergence among loci lost, and stochastic silencing. We further compare and contrast these results for T. castellanus with two nascent Tragopogon allopolyploids. While homoeolog expression bias was balanced in both nascent polyploids and T. castellanus, the degree of additive expression was significantly different, with the mesopolyploid populations demonstrating more non-additive expression. We suggest that gene dosage and expression noise minimization may play a prominent role in regulating gene expression patterns immediately after allopolyploidization as well as deeper into time, and these patterns are conserved across independent polyploid lineages.

Evolutionary Contribution of Duplicated Genes to Genome Evolution in the Ginseng Species Complex

Genes duplicated by whole genome duplication (WGD) and small-scale duplication (SSD) have played important roles in adaptive evolution of all flowering plants. However, it still remains underinvestigated how the distinct models of duplication events and their contending evolutionary patterns have shaped the genome and epigenomes of extant plant species. In this study, we investigated the contribution of the WGD- and SSD-derived duplicate genes to the genome evolution of one diploid and three closely related allotetraploid Panax species based on genome, methylome, and proteome data sets. Our genome-wide comparative analyses revealed that although the ginseng species complex was recently diverged, they have evolved distinct overall patterns of nucleotide variation, cytosine methylation, and protein-level expression. In particular, genetic and epigenetic asymmetries observed in the recent WGD-derived genes are largely consistent across the ginseng species complex. In addition, our results revealed that gene duplicates generated by ancient WGD and SSD mechanisms exhibited distinct evolutionary patterns. We found the ancient WGD-derived genes (i.e., ancient collinear gene) are genetically more conserved and hypomethylated at the cytosine sites. In contrast, some of the SSD-derived genes (i.e., dispersal duplicated gene) showed hypermethylation and high variance in nucleotide variation pattern. Functional enrichment analyses of the duplicated genes indicated that adaptation-related traits (i.e., photosynthesis) created during the distant ancient WGDs are further strengthened by both the more recent WGD and SSD. Together, our findings suggest that different types of duplicated genes may have played distinct but relaying evolutionary roles in the polyploidization and speciation processes in the ginseng species complex.

Parental legacy and regulatory novelty in Brachypodium diurnal transcriptomes accompanying their polyploidy

Polyploidy is a widespread phenomenon in eukaryotes that can lead to phenotypic novelty and has important implications for evolution and diversification. The modification of phenotypes in polyploids relative to their diploid progenitors may be associated with altered gene expression. However, it is largely unknown how interactions between duplicated genes affect their diurnal expression in allopolyploid species. In this study, we explored parental legacy and hybrid novelty in the transcriptomes of an allopolyploid species and its diploid progenitors. We compared the diurnal transcriptomes of representative Brachypodium cytotypes, including the allotetraploid Brachypodium hybridum and its diploid progenitors Brachypodium distachyon and Brachypodium stacei. We also artificially induced an autotetraploid B. distachyon. We identified patterns of homoeolog expression bias (HEB) across Brachypodium cytotypes and time-dependent gain and loss of HEB in B. hybridum. Furthermore, we established that many genes with diurnal expression experienced HEB, while their expression patterns and peak times were correlated between homoeologs in B. hybridum relative to B. distachyon and B. stacei, suggesting diurnal synchronization of homoeolog expression in B. hybridum. Our findings provide insight into the parental legacy and hybrid novelty associated with polyploidy in Brachypodium, and highlight the evolutionary consequences of diurnal transcriptional regulation that accompanied allopolyploidy.

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.

Expression patterns and functional divergence of homologous genes accompanied by polyploidization in cotton (Gossypium hirsutum L.)

Naturally allotetraploid cotton has been widely used as an ideal model to investigate gene expression remodeling as a consequence of polyploidization. However, the global gene pattern variation during early fiber development was unknown. In this study, through RNA-seq technology, we comprehensively investigated the expression patterns of homologous genes between allotetraploid cotton (G. hirsutum) and its diploid progenitors (G. arboreum and G. raimondii) at the fiber early development stage. In tetraploid cotton, genes showed expression level dominance (ELD) bias toward the A genome. This phenomenon was explained by the up-/downregulation of the homologs from the nondominant progenitor (D genome). Gene ontology (GO) enrichment results indicated that the ELD-A genes might be a prominent cause responsible for fiber property change through regulating the fatty acid biosynthesis/metabolism and microtubule procession, and the ELD-D genes might be involved in transcription regulation and stress inducement. In addition, the number and proportion of completely A- and D-subfunctionalized gene were similar at different fiber development stages. However, for neofunctionalization, the number and proportion of reactivated D-derived genes were greater than those of A at 3 and 5 DPA. Eventually, we found that some homologous genes belonging to several specific pathways might create novel asymmetric transcripts between two subgenomes during polyploidization and domestication process, further making the fiber property meet the human demands. Our study identified determinate pathways and their involved genes between allotetraploid cotton and their progenitors at early fiber development stages, providing new insights into the mechanism of cotton fiber evolution.

Proteomic profiling of cotton fiber developmental transition from cell elongation to secondary wall deposition

Cotton fiber developmental transition from elongation to secondary cell wall biosynthesis is a critical growth shifting phase that affects fiber final length, strength, and other properties. Morphological dynamic analysis indicated an asynchronous fiber developmental pattern between two most important commercial cotton species, Gossypium hirsutum (Gh) and G. barbadense (Gb). Using isobaric tags for relative and absolute quantitation techniques, we examined the temporal changes of protein expression at three representative development periods (15-19, 19-23, and 23-27 dpa) in both species. Strikingly, a large proportion of differentially expressed proteins (DEPs) were identified at 19-23 dpa in Gh and at 23-27 dpa in Gb, corresponding to their fiber developmental transition timing. To better understand fiber transitional development, we comparatively analyzed those DEPs in 19-23 dpa of Gh vs. in 23-27 dpa of Gb, and noted that these cotton species indeed share fundamentally similar fiber developmental features under the biological processes. We also showed that there are limited overlaps in both specific upregulated and downregulated proteins between the two species, suggesting species-specific protein regulations in the development process. Proteomic profiling results revealed dynamic changes of several key proteins and biological processes that are potentially correlated with fiber developmental transition. During the transition, upregulated proteins are mainly involved in carbohydrate/energy metabolism, oxidation-reduction, cytoskeleton, protein turnover, Ca2+ signaling, etc., whereas important downregulated proteins are mostly involved in phenylpropanoid and flavonoid secondary metabolism pathways. The gene expressions of several changed proteins in this key stage were also examined by quantitative reverse transcription polymerase chain reaction. Overall, the present study provides accurate pictures of the regulatory networks of functional proteins during the fiber developmental transition, therefore highlighting candidate genes/proteins and related pathways for the cotton fiber improvement.

Genome-wide identification of the Dicer-like family in cotton and analysis of the DCL expression modulation in response to biotic stress in two contrasting commercial cultivars

BACKGROUND

Dicer-like proteins (DCLs) are essential players in RNA-silencing mechanisms, acting in gene regulation via miRNAs and in antiviral protection in plants and have also been associated to other biotic and abiotic stresses. To the best of our knowledge, despite being identified in some crops, cotton DCLs haven't been characterized until now. In this work, we characterized the DCLs of three cotton species and analyzed their expression profiles during biotic stress.

RESULTS

As main results, 11 DCLs in the allotetraploid cotton Gossypium hirsutum, 7 and 6 in the diploid G. arboreum and G. raimondii, were identified, respectively. Among some DCLs duplications observed in these genomes, the presence of an extra DCL3 in the three cotton species were detected, which haven't been found in others eudicots. All the DCL types identified by in silico analysis in the allotetraploid cotton genome were able to generate transcripts, as observed by gene expression analysis in distinct tissues. Based on the importance of DCLs for plant defense against virus, responses of cotton DCLs to virus infection and/or herbivore attack using two commercial cotton cultivars (cv.), one susceptible (FM966) and another resistant (DO) to polerovirus CLRDV infection, were analyzed. Both cvs. Responded differently to virus infection. At the inoculation site, the resistant cv. showed strong induction of DCL2a and b, while the susceptible cv. showed a down-regulation of these genes, wherever DCL4 expression was highly induced. A time course of DCL expression in aerial parts far from inoculation site along infection showed that DCL2b and DCL4 were repressed 24 h after infection in the susceptible cotton. As CLRDV is aphid-transmitted, herbivore attack was also checked. Opposite expression pattern of DCL2a and b and DCL4 was observed for R and S cottons, showing that aphid feeding alone may induce DCL modulation.

CONCLUSIONS

Almost all the DCLs of the allotetraploide G. hirsutum cotton were found in their relative diploids. Duplications of DCL2 and DCL3 were found in the three species. All four classes of DCL responded to aphid attack and virus infection in G. hirsutum. DCLs initial responses against the virus itself and/or herbivore attack may be contributing towards virus resistance.

Next-generation transcriptome assembly and analysis: Impact of ploidy

Whole genome duplications (WGD) occur widely in plants, but the effects of these events impact all branches of life. WGD events have major evolutionary impacts, often leading to major structural changes within the chromosomes and massive changes in gene expression that facilitate rapid speciation and gene diversification. Even for species that currently have diploid genomes, the impact of ancestral duplication events is still present in the genomes, especially in the context of highly similar gene families that are retained from WGD. However, the impact of these ploidies on various bioinformatics workflows has not been studied well. In this review, we overview biological significance of polyploidy in different organisms. We describe the impact of having polyploid transcriptomes on bioinformatics analyses, especially focusing on transcriptome assembly and transcript quantification. We discuss the benefits of using simulated benchmarking data when we examine the performance of various methods. We also present an example strategy to generate simulated allopolyploid transcriptomes and RNAseq datasets and how these benchmark datasets can be used to assess the performance of transcript assembly and quantification methods. Our benchmarking study shows that all transcriptome assembly methods are affected by having polyploid genomes. Quantification accuracy is also impacted by polyploidy depending on the method. These simulated datasets can be adapted for testing, such as, read mapping, variant calling, and differential expression using biologically realistic conditions.

Fiber Quality Improvement in Upland Cotton (Gossypium hirsutum L.): Quantitative Trait Loci Mapping and Marker Assisted Selection Application

Genetic improvement in fiber quality is one of the main challenges for cotton breeders. Fiber quality traits are controlled by multiple genes and are classified as complex quantitative traits, with a negative relationship with yield potential, so the genetic gain is low in traditional genetic improvement by phenotypic selection. The availability of Gossypium genomic sequences facilitates the development of high-throughput molecular markers, quantitative trait loci (QTL) fine mapping and gene identification, which helps us to validate candidate genes and to use marker assisted selection (MAS) on fiber quality in breeding programs. Based on developments of high density linkage maps, QTLs fine mapping, marker selection and omics, we have performed trait dissection on fiber quality traits in diverse populations of upland cotton. QTL mapping combined with multi-omics approaches such as, RNA sequencing datasets to identify differentially expressed genes have benefited the improvement of fiber quality. In this review, we discuss the application of molecular markers, QTL mapping and MAS for fiber quality improvement in upland cotton.

Divergent Expression Patterns and Function of Two cxcr4 Paralogs in Hermaphroditic Epinephelus coioides

Chemokine receptor Cxcr4 evolved two paralogs in the teleost lineage. However, cxcr4a and cxcr4b have been characterized only in a few species. In this study, we identified two cxcr4 paralogs from the orange-spotted grouper, Epinephelus coioides. The phylogenetic relationship and gene structure and synteny suggest that the duplicated cxcr4a/b should result from the teleost-specific genome duplication (Ts3R). The teleost cxcr4 gene clusters in two paralogous chromosomes exhibit a complementary gene loss/retention pattern. Ec_cxcr4a and Ec_cxcr4b show differential and biased expression patterns in grouper adult tissue, gonads, and embryos at different stages. During embryogenesis, Ec_cxcr4a/b are abundantly transcribed from the neurula stage and mainly expressed in the neural plate and sensory organs, indicating their roles in neurogenesis. Ec_Cxcr4a and Ec_Cxcr4b possess different chemotactic migratory abilities from the human SDF-1α, Ec_Cxcl12a, and Ec_Cxcl12b. Moreover, we uncovered the N-terminus and TM5 domain as the key elements for specific ligand⁻receptor recognition of Ec_Cxcr4a-Ec_Cxcl12b and Ec_Cxcr4b-Ec_Cxcl12a. Based on the biased and divergent expression patterns of Eccxcr4a/b, and specific ligand⁻receptor recognition of Ec_Cxcl12a/b⁻Ec_Cxcr4b/a, the current study provides a paradigm of sub-functionalization of two teleost paralogs after Ts3R.

Comparative proteomic analysis of Methanothermobacter thermautotrophicus reveals methane formation from H2 and CO2 under different temperature conditions

The growth of all methanogens is limited to a specific temperature range. However, Methanothermobacter thermautotrophicus can be found in a variety of natural and artificial environments, the temperatures of which sometimes even exceed the temperature growth ranges of thermophiles. As a result, the extent to which methane production and survival are affected by temperature remains unclear. To investigate the mechanisms of methanogenesis that Archaea have evolved to cope with drastic temperature shifts, the responses of Methanothermobacter thermautotrophicus to temperature were investigated under a high temperature growth (71°C) and cold shock (4°C) using Isobaric tags for relative and absolute quantitation (iTRAQ). The results showed that methane formation is decreased and that protein folding and degradation are increased in both high‐ and low‐temperature treatments. In addition, proteins predicted to be involved in processing environmental information processing and in cell membrane/wall/envelope biogenesis may play key roles in affecting methane formation and enhancing the response of M. thermautotrophicus to temperature stress. Analysis of the genomic locations of the genes corresponding to these temperature‐dependent proteins predicted that 77 of the genes likely to form 32 gene clusters. Here, we assess the response of M. thermautotrophicus to different temperatures and provide a new level of understanding of methane formation and cellular putative adaptive responses.

Whole Genome Incorporation and Epigenetic Stability in a Newly Synthetic Allopolyploid of Gynogenetic Gibel Carp

Allopolyploidization plays an important role in speciation, and some natural or synthetic allopolyploid fishes have been extensively applied to aquaculture. Although genetic and epigenetic inheritance and variation associated with plant allopolyploids have been well documented, the relative research in allopolyploid animals is scarce. In this study, the genome constitution and DNA methylation inheritance in a newly synthetic allopolyploid of gynogenetic gibel carp were analyzed. The incorporation of a whole genome of paternal common carp sperm in the allopolyploid was confirmed by genomic in situ hybridization, chromosome localization of 45S rDNAs, and sequence comparison. Pooled sample-based methylation sensitive amplified polymorphism (MSAP) revealed that an overwhelming majority (98.82%) of cytosine methylation patterns in the allopolyploid were inherited from its parents of hexaploid gibel carp clone D and common carp. Compared to its parents, 11 DNA fragments in the allopolyploid were proved to be caused by interindividual variation, recombination, deletion, and mutation through individual sample-based MSAP and sequencing. Contrast to the rapid and remarkable epigenetic changes in most of analyzed neopolyploids, no cytosine methylation variation was detected in the gynogenetic allopolyploid. Therefore, the newly synthetic allopolyploid of gynogenetic gibel carp combined genomes from its parents and maintained genetic and epigenetic stability after its formation and subsequently seven successive gynogenetic generations. Our current results provide a paradigm for recurrent polyploidy consequences in the gynogenetic allopolyploid animals.

Differential interferon system gene expression profiles in susceptible and resistant gynogenetic clones of gibel carp challenged with herpesvirus CaHV

Interferon (IFN) system plays a vital role in the first line of defense against viruses. In this study, we first identified multiple transcripts of 15 IFN system genes, including PRRs (TLR2, TLR3, RIG-I, and LGP2), PRR-mediated IFN signal pathway (MyD88, MITA, and MAVS), IFN regulatory factors (IRF1, IRF3, IRF7, and IRF9), IFNs (IFNφ1 and IFNφ3), and ISGs (Mx and viperin), and one transcript of TLR9 in de novo transcriptome assembly data of gibel carp head-kidney. Multiple nucleotide alignments and phylogenetic analysis of common region showed that the transcripts of every of the 15 IFN system genes were classified into two homologs with distinctly divergent sequences, indicating that hexaploid gibel carp may be an allopolyploid. During Carassius auratus herpesvirus (CaHV) infection, gibel carp resistant clone H significantly suppressed CaHV replication with markedly less viral loads than those in highly susceptible clone A⁺ and moderately resistant clone F. Then, qPCR analyses were performed to reveal their differential and dynamic expression changes during CaHV infection in head kidney, spleen and liver among three gibel carp gynogenetic clones. Through qPCR and hierarchical clustering analysis, 8 genes, such as RIG-Is, LGP2s, IRF1-B, IRF3s, IRF7s, IRF9-B, Mxs, and viperins, were identified as candidate resistant-related genes. They remarkably increased their expression in immune tissues of three clones after CaHV infection. Significantly, the up-regulation folds of these genes in clone A⁺, F and H were related to their resistance ability to CaHV, progressively increasing from susceptible clone to resistant clone at 1 dpi. The positive correlation to the resistance ability suggested that resistant clone H immediately triggered stronger IFN response. IFNφ3 showed a different dynamic change and was sharply induced in moderately resistant clone F at 3 dpi. The other 5 IFN system genes (TLR2, TLR3, TLR9, MyD88, and MITA) maintained a low expression level after CaHV challenge. Interestingly, the A or B copies/homologs of almost these IFN system genes exhibited differential transcript abundance in immune tissue after CaHV challenge, suggesting A or B homologs might occur dominant or biased expression of homeologs during gibel carp evolution. These data provide candidate resistant-related genes for disease-resistance breeding of gibel carp.

Asymmetric Evolution and Expansion of the NAC Transcription Factor in Polyploidized Cotton

Polyploidy in Gossypium hirsutum conferred different properties from its diploid ancestors under the regulation of transcription factors. The NAC transcription factor is a plant-specific family that can be related to plant growth and development. So far, little is known about the NAC family in cotton. This study identified 495 NAC genes in three cotton species and investigated the evolution and expansion of different genome-derived NAC genes in cotton. We revealed 15 distinct NAC subfamilies in cotton. Different subfamilies had different gene proportions, expansion rate, gene loss rate, and orthologous exchange rate. Paleohexaploidization (35%) and cotton-specific decaploidy (32%) might have primarily led to the expansion of the NAC family in cotton. Half of duplication events in G. hirsutum were inherited from its diploid ancestor, and others might have occurred after interspecific hybridization. In addition, NAC genes in the At and Dt subgenomes displayed asymmetric molecular evolution, as evidenced by their different gene loss rates, orthologous exchange, evolutionary rates, and expression levels. The dominant duplication event was different during the cotton evolutionary history. Different genome-derived NACs might have interacted with each other, which ultimately resulted in morphogenetic evolution. This study delineated the expansion and evolutionary history of the NAC family in cotton and illustrated the different fates of NAC genes during polyploidization.

Cytonuclear Variation of Rubisco in Synthesized Rice Hybrids and Allotetraploids

The allopolyploid speciation process faces the genomic challenge of stoichiometric disruption caused by merging biparental nuclear genomes with only one (usually maternal) of the two sets of progenitor cytoplasmic genomes. The photosynthetic protein 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is composed of nuclear-encoded small subunits (SSUs) and plastome-encoded large subunits (LSUs), making it an ideal enzyme to explore the evolution process of cytonuclear accommodation. We investigated the variation of SSUs and their encoding genes in synthetic nascent rice ( L.) allotetraploid lineages, formed from the parental subspecies and of Asian rice. The evolution of genes in rice subspecies involves both mutation and concerted homogenization. Within reciprocal rice hybrids and allopolyploids, there was no consistent pattern of biased expression of alleles or homeologs, nor was there biased gene conversion favoring the maternal gene copies. Instead, we observed an apparently stochastic pattern of intergenomic gene conversions and biased expression of homeologs. We conclude that in young rice allopolyploids, cytonuclear coordination either is not selectively favored because of high parental sequence similarity or because there has been insufficient time for subtle selective effects to become observable.

Segmental allotetraploidy generates extensive homoeologous expression rewiring and phenotypic diversity at the population level in rice

Allopolyploidization, that is, concomitant merging and doubling of two or more divergent genomes in a common nucleus/cytoplasm, is known to instantly alter genomewide transcriptome dynamics, a phenomenon referred to as "transcriptomic shock." However, the immediate effects of transcriptomic alteration in generating phenotypic diversity at the population level remain underinvestigated. Here, we employed the MassARRAY-based Sequenom platform to assess and compare orthologous, allelic and homoeologous gene expression status in two tissues (leaf and root) of a set of randomly chosen individuals from populations of parental rice subspecies (indica and japonica), in vitro "hybrids" (parental mixes), reciprocal F1 hybrids and reciprocal tetraploids at the 5th-selfed generation (S5). We show that hybridization and whole genome duplication (WGD) have opposing effects on allelic and homoeologous expression in the F1 hybrids and tetraploids, respectively. Whereas hybridization exerts strong attenuating effects on allelic expression differences in diploid hybrids, WGD augments the intrinsic parental differences and generates extensive and variable homoeolog content which triggers diversification in expression patterning among the tetraploid plants. Coupled with the vast phenotypic diversity observed among the tetraploid individuals, our results provide experimental evidence in support of the notion that allopolyploidy catalyses rapid phenotypic diversification in higher plants. Our data further suggest that largely stochastic homoeolog content reshuffling rather than alteration in total expression level may be an important feature of evolution in young segmental allopolyploids, which underlies rapid expression diversity at the population level.

Proteomic Changes Between Populus Allotriploids and Diploids Revealed Using an iTRAQ-based Quantitative Approach

Background

Polyploid breeding is a powerful approach for Populus genetic improve-ment because polyploid trees have valuable characteristics, including better timber quality and a higher degree of stress resistance compared with their full-sib diploids. However, the genetic mech-anism underlying this phenomenon remains unknown.

Objective

To better understand the proteomic changes between Populus allotriploids and diploids, we examined the proteomic profiles of allotriploid and diploid Populus by iTRAQ labeling coupled with two-dimensional liquid chromatography and MALDI-TOF/TOF mass spectrometry (MS).

Method

iTRAQ labeling coupled with two-dimensional liquid chromatography and MALDI-TOF/TOF mass spectrometry (MS).

Results

Between the Populus allotriploid and the full-sib diploid, 932 differentially expressed proteins (DEPs) were identified. These DEPs were primarily involved in stress, defense, transportation, transcriptional and/or translational modification, and energy production. The pathway analysis indi-cated that most of the DEPs were implicated in carbohydrate transport and metabolism, nitrogen me-tabolism and glycolysis, and the ribosome assembly pathway. These data suggest high protein di-vergence between Populus allotriploids and diploids, and rapid changes during hybridization.

Conclusion

The results provide new data for further understanding of the mechanisms of polyploid trees that generally display increased height growth compared with their full-sib diploids.

Genome-wide identification and characterization of phospholipase C gene family in cotton (Gossypium spp.)

Phospholipase C (PLC) are important regulatory enzymes involved in several lipid and Ca²⁺-dependent signaling pathways. Previous studies have elucidated the versatile roles of PLC genes in growth, development and stress responses of many plants, however, the systematic analyses of PLC genes in the important fiber-producing plant, cotton, are still deficient. In this study, through genome-wide survey, we identified twelve phosphatidylinositol-specific PLC (PI-PLC) and nine non-specific PLC (NPC) genes in the allotetraploid upland cotton Gossypium hirsutum and nine PI-PLC and six NPC genes in two diploid cotton G. arboretum and G.raimondii, respectively. The PI-PLC and NPC genes of G. hirsutum showed close phylogenetic relationship with their homologous genes in the diploid cottons and Arabidopsis. Segmental and tandem duplication contributed greatly to the formation of the gene family. Expression profiling indicated that few of the PLC genes are constitutely expressed, whereas most of the PLC genes are preferentially expressed in specific tissues and abiotic stress conditions. Promoter analyses further implied that the expression of these PLC genes might be regulated by MYB transcription factors and different phytohormones. These results not only suggest an important role of phospholipase C members in cotton plant development and abiotic stress response but also provide good candidate targets for future molecular breeding of superior cotton cultivars.

The evolutionary significance of polyploidy

Polyploidy, or the duplication of entire genomes, has been observed in prokaryotic and eukaryotic organisms, and in somatic and germ cells. The consequences of polyploidization are complex and variable, and they differ greatly between systems (clonal or non-clonal) and species, but the process has often been considered to be an evolutionary 'dead end'. Here, we review the accumulating evidence that correlates polyploidization with environmental change or stress, and that has led to an increased recognition of its short-term adaptive potential. In addition, we discuss how, once polyploidy has been established, the unique retention profile of duplicated genes following whole-genome duplication might explain key longer-term evolutionary transitions and a general increase in biological complexity.

Comparative proteomic analyses of Asian cotton ovules with attached fibers in the early stages of fiber elongation process

BACKGROUND

Plenty of proteomic studies were performed to characterize the allotetraploid upland cotton fiber elongation process, whereas little is known about the elongating diploid cotton fiber proteome.

METHODS

In this study, we used a two-dimensional electrophoresis-based comparative proteomic approach to profile dynamic proteomes of diploid Asian cotton ovules with attached fibers in the early stages of fiber elongation process. One-way ANOVA and Student-Newman-Keuls test were used to find the differentially displayed protein (DDP) spots.

RESULTS

A total of 55 protein spots were found having different abundance ranging from 1 to 9 days post-anthesis (DPA) in a two-day interval. These 55 DDP spots were all successfully identified using high-resolution mass spectrometric analyses. Gene ontology analyses revealed that proteoforms involved in energy/carbohydrate metabolism, redox homeostasis, and protein metabolism are the most abundant. In addition, orthologues of the 13 DDP spots were also found in differential proteome of allotetraploid elongating cotton fibers, suggesting their possible essential roles in fiber elongation process.

CONCLUSIONS

Our results not only revealed the dynamic proteome change of diploid Asian cotton fiber and ovule during early stages of fiber elongation process but also provided valuable resource for future studies on the molecular mechanism how the polyploidization improves the trait of fiber length.

Evolution of Gene Duplication in Plants

Ancient duplication events and a high rate of retention of extant pairs of duplicate genes have contributed to an abundance of duplicate genes in plant genomes. These duplicates have contributed to the evolution of novel functions, such as the production of floral structures, induction of disease resistance, and adaptation to stress. Additionally, recent whole-genome duplications that have occurred in the lineages of several domesticated crop species, including wheat (Triticum aestivum), cotton (Gossypium hirsutum), and soybean (Glycine max), have contributed to important agronomic traits, such as grain quality, fruit shape, and flowering time. Therefore, understanding the mechanisms and impacts of gene duplication will be important to future studies of plants in general and of agronomically important crops in particular. In this review, we survey the current knowledge about gene duplication, including gene duplication mechanisms, the potential fates of duplicate genes, models explaining duplicate gene retention, the properties that distinguish duplicate from singleton genes, and the evolutionary impact of gene duplication.

Insights into the Ecology and Evolution of Polyploid Plants through Network Analysis

Polyploidy is a widespread phenomenon throughout eukaryotes, with important ecological and evolutionary consequences. Although genes operate as components of complex pathways and networks, polyploid changes in genes and gene expression have typically been evaluated as either individual genes or as a part of broad-scale analyses. Network analysis has been fruitful in associating genomic and other 'omic'-based changes with phenotype for many systems. In polyploid species, network analysis has the potential not only to facilitate a better understanding of the complex 'omic' underpinnings of phenotypic and ecological traits common to polyploidy, but also to provide novel insight into the interaction among duplicated genes and genomes. This adds perspective to the global patterns of expression (and other 'omic') change that accompany polyploidy and to the patterns of recruitment and/or loss of genes following polyploidization. While network analysis in polyploid species faces challenges common to other analyses of duplicated genomes, present technologies combined with thoughtful experimental design provide a powerful system to explore polyploid evolution. Here, we demonstrate the utility and potential of network analysis to questions pertaining to polyploidy with an example involving evolution of the transgressively superior cotton fibres found in polyploid Gossypium hirsutum. By combining network analysis with prior knowledge, we provide further insights into the role of profilins in fibre domestication and exemplify the potential for network analysis in polyploid species.

Polyploidy and the proteome

Although major advances have been made during the past 20 years in our understanding of the genetic and genomic consequences of polyploidy, our knowledge of polyploidy and the proteome is in its infancy. One of our goals is to stimulate additional study, particularly broad-scale proteomic analyses of polyploids and their progenitors. Although it may be too early to generalize regarding the extent to which transcriptomic data are predictive of the proteome of polyploids, it is clear that the proteome does not always reflect the transcriptome. Despite limited data, important observations on the proteomes of polyploids are emerging. In some cases, proteomic profiles show qualitatively and/or quantitatively non-additive patterns, and proteomic novelty has been observed. Allopolyploids generally combine the parental contributions, but there is evidence of parental dominance of one contributing genome in some allopolyploids. Autopolyploids are typically qualitatively identical to but quantitatively different from their parents. There is also evidence of parental legacy at the proteomic level. Proteomes clearly provide insights into the consequences of genomic merger and doubling beyond what is obtained from genomic and/or transcriptomic data. Translating proteomic changes in polyploids to differences in morphology and physiology remains the holy grail of polyploidy--this daunting task of linking genotype to proteome to phenotype should emerge as a focus of polyploidy research in the next decade. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.