The cytonuclear dimension of allopolyploid evolution: an example from cotton using rubisco

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.

Genome-wide patterns of homoeologous gene flow in allotetraploid coffee

Premise

Allopolyploidy-a hybridization-induced whole-genome duplication event-has been a major driver of plant diversification. The extent to which chromosomes pair with their proper homolog vs. with their homoeolog in allopolyploids varies across taxa, and methods to detect homoeologous gene flow (HGF) are needed to understand how HGF has shaped polyploid lineages.

Methods

The ABBA-BABA test represents a classic method for detecting introgression between closely related species, but here we developed a modified use of the ABBA-BABA test to characterize the extent and direction of HGF in allotetraploid Coffea arabica.

Results

We found that HGF is abundant in the C. arabica genome, with both subgenomes serving as donors and recipients of variation. We also found that HGF is highly maternally biased in plastid-targeted-but not mitochondrial-targeted-genes, as would be expected if plastid-nuclear incompatibilities exist between the two parent species.

Discussion

Together, our analyses provide a simple framework for detecting HGF and new evidence consistent with selection favoring overwriting of paternally derived alleles by maternally derived alleles to ameliorate plastid-nuclear incompatibilities. Natural selection therefore appears to shape the direction and intensity of HGF in allopolyploid coffee, indicating that cytoplasmic inheritance has long-term consequences for polyploid lineages.

Cytonuclear interplay in auto- and allopolyploids: a multifaceted perspective from the Festuca-Lolium complex

Restoring cytonuclear stoichiometry is necessary after whole-genome duplication (WGD) and interspecific/intergeneric hybridization in plants. We investigated this phenomenon in auto- and allopolyploids of the Festuca-Lolium complex providing insights into the mechanisms governing cytonuclear interactions in early polyploid and hybrid generations. Our study examined the main processes potentially involved in restoring the cytonuclear balance after WGD comparing diploids and new and well-established autopolyploids. We uncovered that both the number of chloroplasts and the number of chloroplast genome copies were significantly higher in the newly established autopolyploids and grew further in more established autopolyploids. The increase in the copy number of the chloroplast genome exceeded the rise in the number of chloroplasts and fully compensated for the doubling of the nuclear genome. In addition, changes in nuclear and organelle gene expression were insignificant. Allopolyploid Festuca × Lolium hybrids displayed potential structural conflicts in parental protein variants within the cytonuclear complexes. While biased maternal allele expression has been observed in numerous hybrids, our results suggest that its role in cytonuclear stabilization in the Festuca × Lolium hybrids is limited. This study provides insights into the restoration of the cytonuclear stoichiometry, yet it emphasizes the need for future research to explore post-transcriptional regulation and its impact on cytonuclear gene expression stoichiometry. Our findings may enhance the understanding of polyploid plant evolution, with broader implications for the study of cytonuclear interactions in diverse biological contexts.

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.

Cytonuclear Interactions and Subgenome Dominance Shape the Evolution of Organelle-Targeted Genes in the Brassica Triangle of U

The interaction and coevolution between nuclear and cytoplasmic genomes are one of the fundamental hallmarks of eukaryotic genome evolution and, 2 billion yr later, are still major contributors to the formation of new species. Although many studies have investigated the role of cytonuclear interactions following allopolyploidization, the relative magnitude of the effect of subgenome dominance versus cytonuclear interaction on genome evolution remains unclear. The Brassica triangle of U features 3 diploid species that together have formed 3 separate allotetraploid species on similar evolutionary timescales, providing an ideal system for understanding the contribution of the cytoplasmic donor to hybrid polyploid. Here, we investigated the evolutionary pattern of organelle-targeted genes in Brassica carinata (BBCC) and 2 varieties of Brassica juncea (AABB) at the whole-genome level, with particular focus on cytonuclear enzyme complexes. We found partial evidence that plastid-targeted genes experience selection to match plastid genomes, but no obvious corresponding signal in mitochondria-targeted genes from these 2 separately formed allopolyploids. Interestingly, selection acting on plastid genomes always reduced the retention rate of plastid-targeted genes encoded by the B subgenome, regardless of whether the Brassica nigra (BB) subgenome was contributed by the paternal or maternal progenitor. More broadly, this study illustrates the distinct selective pressures experienced by plastid- and mitochondria-targeted genes, despite a shared pattern of inheritance and natural history. Our study also highlights an important role for subgenome dominance in allopolyploid genome evolution, even in genes whose function depends on separately inherited molecules.

Investigation of regulatory divergence between homoeologs in the recently formed allopolyploids, Tragopogon mirus and T. miscellus (Asteraceae)

Polyploidy is an important evolutionary process throughout eukaryotes, particularly in flowering plants. Duplicated gene pairs (homoeologs) in allopolyploids provide additional genetic resources for changes in molecular, biochemical, and physiological mechanisms that result in evolutionary novelty. Therefore, understanding how divergent genomes and their regulatory networks reconcile is vital for unraveling the role of polyploidy in plant evolution. Here, we compared the leaf transcriptomes of recently formed natural allotetraploids (Tragopogon mirus and T. miscellus) and their diploid parents (T. porrifolius X T. dubius and T. pratensis X T. dubius, respectively). Analysis of 35 400 expressed loci showed a significantly higher level of transcriptomic additivity compared to old polyploids; only 22% were non-additively expressed in the polyploids, with 5.9% exhibiting transgressive expression (lower or higher expression in the polyploids than in the diploid parents). Among approximately 7400 common orthologous regions (COREs), most loci in both allopolyploids exhibited expression patterns that were vertically inherited from their diploid parents. However, 18% and 20.3% of the loci showed novel expression bias patterns in T. mirus and T. miscellus, respectively. The expression changes of 1500 COREs were explained by cis-regulatory divergence (the condition in which the two parental subgenomes do not interact) between the diploid parents, whereas only about 423 and 461 of the gene expression changes represent trans-effects (the two parental subgenomes interact) in T. mirus and T. miscellus, respectively. The low degree of both non-additivity and trans-effects on gene expression may present the ongoing evolutionary processes of the newly formed Tragopogon polyploids (~80-90 years).

Subgenome evolutionary dynamics in allotetraploid ferns: insights from the gene expression patterns in the allotetraploid species Phegopteris decursivepinnata (Thelypteridacea, Polypodiales)

Allopolyploidization often leads to disruptive conflicts among more than two sets of subgenomes, leading to genomic modifications and changes in gene expression. Although the evolutionary trajectories of subgenomes in allopolyploids have been studied intensely in angiosperms, the dynamics of subgenome evolution remain poorly understood in ferns, despite the prevalence of allopolyploidization. In this study, we have focused on an allotetraploid fern-Phegopteris decursivepinnata-and its diploid parental species, P. koreana (K) and P. taiwaniana (T). Using RNA-seq analyses, we have compared the gene expression profiles for 9,540 genes among parental species, synthetic F1 hybrids, and natural allotetraploids. The changes in gene expression patterns were traced from the F1 hybrids to the natural allopolyploids. This study has revealed that the expression patterns observed in most genes in the F1 hybrids are largely conserved in the allopolyploids; however, there were substantial differences in certain genes between these groups. In the allopolyploids compared with the F1 hybrids, the number of genes showing a transgressive pattern in total expression levels was increased. There was a slight reduction in T-dominance and a slight increase in K-dominance, in terms of expression level dominance. Interestingly, there is no obvious bias toward the T- or K-subgenomes in the number and expression levels overall, showing the absence of subgenome dominance. These findings demonstrated the impacts of the substantial transcriptome change after hybridization and the moderate modification during allopolyploid establishment on gene expression in ferns and provided important insights into subgenome evolution in polyploid ferns.

Compensatory Genetic and Transcriptional Cytonuclear Coordination in Allopolyploid Lager Yeast (Saccharomyces pastorianus)

Cytonuclear coordination between biparental-nuclear genomes and uniparental-cytoplasmic organellar genomes in plants is often resolved by genetic and transcriptional cytonuclear responses. Whether this mechanism also acts in allopolyploid members of other kingdoms is not clear. Additionally, cytonuclear coordination of interleaved allopolyploid cells/individuals within the same population is underexplored. The yeast Saccharomyces pastorianus provides the opportunity to explore cytonuclear coevolution during different growth stages and from novel dimensions. Using S. pastorianus cells from multiple growth stages in the same environment, we show that nuclear mitochondria-targeted genes have undergone both asymmetric gene conversion and growth stage-specific biased expression favoring genes from the mitochondrial genome donor (Saccharomyces eubayanus). Our results suggest that cytonuclear coordination in allopolyploid lager yeast species entails an orchestrated and compensatory genetic and transcriptional evolutionary regulatory shift. The common as well as unique properties of cytonuclear coordination underlying allopolyploidy between unicellular yeasts and higher plants offers novel insights into mechanisms of cytonuclear evolution associated with allopolyploid speciation.

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.

Genome diploidization associates with cladogenesis, trait disparity, and plastid gene evolution

Angiosperm genome evolution was marked by many clade-specific whole-genome duplication events. The Microlepidieae is one of the monophyletic clades in the mustard family (Brassicaceae) formed after an ancient allotetraploidization. Postpolyploid cladogenesis has resulted in the extant c. 17 genera and 60 species endemic to Australia and New Zealand (10 species). As postpolyploid genome diploidization is a trial-and-error process under natural selection, it may proceed with different intensity and be associated with speciation events. In Microlepidieae, different extents of homoeologous recombination between the two parental subgenomes generated clades marked by slow ("cold") versus fast ("hot") genome diploidization. To gain a deeper understanding of postpolyploid genome evolution in Microlepidieae, we analyzed phylogenetic relationships in this tribe using complete chloroplast sequences, entire 35S rDNA units, and abundant repetitive sequences. The four recovered intra-tribal clades mirror the varied diploidization of Microlepidieae genomes, suggesting that the intrinsic genomic features underlying the extent of diploidization are shared among genera and species within one clade. Nevertheless, even congeneric species may exert considerable morphological disparity (e.g. in fruit shape), whereas some species within different clades experience extensive morphological convergence despite the different pace of their genome diploidization. We showed that faster genome diploidization is positively associated with mean morphological disparity and evolution of chloroplast genes (plastid-nuclear genome coevolution). Higher speciation rates in perennials than in annual species were observed. Altogether, our results confirm the potential of Microlepidieae as a promising subject for the analysis of postpolyploid genome diploidization in Brassicaceae.

A temporal gradient of cytonuclear coordination of chaperonins and chaperones during RuBisCo biogenesis in allopolyploid plants

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo), consisting of subunits encoded by nuclear and cytoplasmic genes, is a model for cytonuclear evolution in plant allopolyploids. To date, coordinated cytonuclear evolutionary responses of auxiliary cofactors involved in RuBisCo biogenesis remain unexplored. This study characterized and compared genomic and transcriptional cytonuclear coevolutionary responses of chaperonin/chaperones in RuBisCo folding and assembly processes across different allopolyploids. We discovered significant cytonuclear evolutionary responses in folding cofactors, with diminishing or attenuated responses later during assembly. Our results have general significance for understanding the unrecognized cytonuclear evolution of chaperonin/chaperone genes, structural and functional features of intermediate complexes, and the functioning stage of the Raf2 cofactor. Generally, the results reveal a hitherto unexplored dimension of allopolyploidy in plants.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) has long been studied from many perspectives. As a multisubunit (large subunits [LSUs] and small subunits[SSUs]) protein encoded by genes residing in the chloroplast (rbcL) and nuclear (rbcS) genomes, RuBisCo also is a model for cytonuclear coevolution following allopolyploid speciation in plants. Here, we studied the genomic and transcriptional cytonuclear coordination of auxiliary chaperonin and chaperones that facilitate RuBisCo biogenesis across multiple natural and artificially synthesized plant allopolyploids. We found similar genomic and transcriptional cytonuclear responses, including respective paternal-to-maternal conversions and maternal homeologous biased expression, in chaperonin/chaperon-assisted folding and assembly of RuBisCo in different allopolyploids. One observation is about the temporally attenuated genomic and transcriptional cytonuclear evolutionary responses during early folding and later assembly process of RuBisCo biogenesis, which were established by long-term evolution and immediate onset of allopolyploidy, respectively. Our study not only points to the potential widespread and hitherto unrecognized features of cytonuclear evolution but also bears implications for the structural interaction interface between LSU and Cpn60 chaperonin and the functioning stage of the Raf2 chaperone.

Genome-Wide Identification and Characterization of CPR5 Genes in Gossypium Reveals Their Potential Role in Trichome Development

Trichomes protect plants against insects, microbes, herbivores, and abiotic damages and assist seed dispersal. The function of CPR5 genes have been found to be involved in the trichome development but the research on the underlying genetic and molecular mechanisms are extremely limited. Herein, genome wide identification and characterization of CPR5 genes was performed. In total, 26 CPR5 family members were identified in Gossypium species. Phylogenetic analysis, structural characteristics, and synteny analysis of CPR5s showed the conserved evolution relationships of CPR5. The promoter analysis of CPR5 genes revealed hormone, stress, and development-related cis-elements. Gene ontology (GO) enrichment analysis showed that the CPR5 genes were largely related to biological regulation, developmental process, multicellular organismal process. Protein-protein interaction analysis predicted several trichome development related proteins (SIM, LGO, and GRL) directly interacting with CPR5 genes. Further, nine putative Gossypium-miRNAs were also identified, targeting Gossypium CPR5 genes. RNA-Seq data of G. arboreum (with trichomes) and G. herbaceum (with no trichomes) was used to perform the co-expression network analysis. GheCPR5.1 was identified as a hub gene in a co-expression network analysis. RT-qPCR of GheCPR5.1 gene in different tissues suggests that this gene has higher expressions in the petiole and might be a key candidate involved in the trichome development. Virus induced gene silencing of GheCPR5.1 (Ghe02G17590) confirms its role in trichome development and elongation. Current results provide proofs of the possible role of CPR5 genes and provide preliminary information for further studies of GheCPR5.1 functions in trichome development.

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.

The contributions from the progenitor genomes of the mesopolyploid Brassiceae are evolutionarily distinct but functionally compatible

The members of the tribe Brassiceae share a whole-genome triplication (WGT), and one proposed model for its formation is a two-step pair of hybridizations producing hexaploid descendants. However, evidence for this model is incomplete, and the evolutionary and functional constraints that drove evolution after the hexaploidy are even less understood. Here, we report a new genome sequence of Crambe hispanica, a species sister to most sequenced Brassiceae. Using this new genome and three others that share the hexaploidy, we traced the history of gene loss after the WGT using the Polyploidy Orthology Inference Tool (POInT). We confirm the two-step formation model and infer that there was a significant temporal gap between those two allopolyploidizations, with about a third of the gene losses from the first two subgenomes occurring before the arrival of the third. We also, for the 90,000 individual genes in our study, make parental subgenome assignments, inferring, with measured uncertainty, from which of the progenitor genomes of the allohexaploidy each gene derives. We further show that each subgenome has a statistically distinguishable rate of homoeolog losses. There is little indication of functional distinction between the three subgenomes: the individual subgenomes show no patterns of functional enrichment, no excess of shared protein-protein or metabolic interactions between their members, and no biases in their likelihood of having experienced a recent selective sweep. We propose a "mix and match" model of allopolyploidy, in which subgenome origin drives homoeolog loss propensities but where genes from different subgenomes function together without difficulty.

Imprints of independent allopolyploid formations on patterns of gene expression in two sibling yarrow species (Achillea, Asteraceae)

BACKGROUND

Polyploid species often originate recurrently. While this is well known, there is little information on the extent to which distinct allotetraploid species formed from the same parent species differ in gene expression. The tetraploid yarrow species Achillea alpina and A. wilsoniana arose independently from allopolyploidization between diploid A. acuminata and A. asiatica. The genetics and geography of these origins are clear from previous studies, providing a solid basis for comparing gene expression patterns of sibling allopolyploid species that arose independently.

RESULTS

We conducted comparative RNA-sequencing analyses on the two Achillea tetraploid species and their diploid progenitors to evaluate: 1) species-specific gene expression and coexpression across the four species; 2) patterns of inheritance of parental gene expression; 3) parental contributions to gene expression in the allotetraploid species, and homeolog expression bias. Diploid A. asiatica showed a higher contribution than diploid A. acuminata to the transcriptomes of both tetraploids and also greater homeolog bias in these transcriptomes, possibly reflecting a maternal effect. Comparing expressed genes in the two allotetraploids, we found expression of ca. 30% genes were species-specific in each, which were most enriched for GO terms pertaining to "defense response". Despite species-specific and differentially expressed genes between the two allotetraploids, they display similar transcriptome changes in comparison to their diploid progenitors.

CONCLUSION

Two independently originated Achillea allotetraploid species exhibited difference in gene expression, some of which must be related to differential adaptation during their post-speciation evolution. On the other hand, they showed similar expression profiles when compared to their progenitors. This similarity might be expected when pairs of merged diploid genomes in tetraploids are similar, as is the case in these two particular allotetraploids.

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.

Complete chloroplast genome sequencing and comparative analysis reveals changes to the chloroplast genome after allopolyploidization in Cucumis

Allopolyploids undergo "genomic shock" leading to significant genetic and epigenetic modifications. Previous studies have mainly focused on nuclear changes, while little is known about the inheritance and changes of organelle genome in allopolyploidization. The synthetic allotetraploid Cucumis ×hytivus, which is generated via hybridization between C. hystrix and C. sativus, is a useful model system for studying cytonuclear variation. Here, we report the chloroplast genome of allotetraploid C. ×hytivus and its diploid parents via sequencing and comparative analysis. The size of the obtained chloroplast genomes ranged from 154 673 to 155 760 bp, while their gene contents, gene orders, and GC contents were similar to each other. Comparative genome analysis supports chloroplast maternal inheritance. However, we identified 51 indels and 292 SNP genetic variants in the chloroplast genome of the allopolyploid C. ×hytivus relative to its female parent C. hystrix. Nine intergenic regions with rich variation were identified through comparative analysis of the chloroplast genomes within the subgenus Cucumis. The phylogenetic network based on the chloroplast genome sequences clarified the evolution and taxonomic position of the synthetic allotetraploid C. ×hytivus. The results of this study provide us with an insight into the changes of organelle genome after allopolyploidization, and a new understanding of the cytonuclear evolution.

Coevolution in Hybrid Genomes: Nuclear-Encoded Rubisco Small Subunits and Their Plastid-Targeting Translocons Accompanying Sequential Allopolyploidy Events in Triticum

The Triticum/Aegilops complex includes hybrid species resulting from homoploid hybrid speciation and allopolyploid speciation. Sequential allotetra- and allohexaploidy events presumably result in two challenges for the hybrids, which involve 1) cytonuclear stoichiometric disruptions caused by combining two diverged nuclear genomes with the maternal inheritance of the cytoplasmic organellar donor; and 2) incompatibility of chimeric protein complexes with diverged subunits from nuclear and cytoplasmic genomes. Here, we describe coevolution of nuclear rbcS genes encoding the small subunits of Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) and nuclear genes encoding plastid translocons, which mediate recognition and translocation of nuclear-encoded proteins into plastids, in allopolyploid wheat species. We demonstrate that intergenomic paternal-to-maternal gene conversion specifically occurred in the genic region of the homoeologous rbcS3 gene from the D-genome progenitor of wheat (abbreviated as rbcS3D) such that it encodes a maternal-like or B-subgenome-like SSU3D transit peptide in allohexaploid wheat but not in allotetraploid wheat. Divergent and limited interaction between SSU3D and the D-subgenomic TOC90D translocon subunit is implicated to underpin SSU3D targeting into the chloroplast of hexaploid wheat. This implicates early selection favoring individuals harboring optimal maternal-like organellar SSU3D targeting in hexaploid wheat. These data represent a novel dimension of cytonuclear evolution mediated by organellar targeting and transportation of nuclear proteins.

Transcriptome Dynamics of the Inflorescence in Reciprocally Formed Allopolyploid Tragopogon miscellus (Asteraceae)

Polyploidy is an important evolutionary mechanism and is prevalent among land plants. Most polyploid species examined have multiple origins, which provide genetic diversity and may enhance the success of polyploids. In some polyploids, recurrent origins can result from reciprocal crosses between the same diploid progenitors. Although great progress has been made in understanding the genetic consequences of polyploidy, the genetic implications of reciprocal polyploidization remain poorly understood, especially in natural polyploids. Tragopogon (Asteraceae) has become an evolutionary model system for studies of recent and recurrent polyploidy. Allotetraploid T. miscellus has formed reciprocally in nature with resultant distinctive floral and inflorescence morphologies (i.e., short- vs. long-liguled forms). In this study, we performed comparative inflorescence transcriptome analyses of reciprocally formed T. miscellus and its diploid parents, T. dubius and T. pratensis. In both forms of T. miscellus, homeolog expression of ∼70% of the loci showed vertical transmission of the parental expression patterns (i.e., parental legacy), and ∼20% of the loci showed biased homeolog expression, which was unbalanced toward T. pratensis. However, 17.9% of orthologous pairs showed different homeolog expression patterns between the two forms of T. miscellus. No clear effect of cytonuclear interaction on biased expression of the maternal homeolog was found. In terms of the total expression level of the homeologs studied, 22.6% and 16.2% of the loci displayed non-additive expression in short- and long-liguled T. miscellus, respectively. Unbalanced expression level dominance toward T. pratensis was observed in both forms of T. miscellus. Significantly, genes annotated as being involved in pectin catabolic processes were highly expressed in long-liguled T. miscellus relative to the short-liguled form, and the majority of these differentially expressed genes were transgressively down-regulated in short-liguled T. miscellus. Given the known role of these genes in cell expansion, they may play a role in the differing floral and inflorescence morphologies of the two forms. In summary, the overall inflorescence transcriptome profiles are highly similar between reciprocal origins of T. miscellus. However, the dynamic homeolog-specific expression and non-additive expression patterns observed in T. miscellus emphasize the importance of reciprocal origins in promoting the genetic diversity of polyploids.

Genomics of Evolutionary Novelty in Hybrids and Polyploids

It has long been recognized that hybridization and polyploidy are prominent processes in plant evolution. Although classically recognized as significant in speciation and adaptation, recognition of the importance of interspecific gene flow has dramatically increased during the genomics era, concomitant with an unending flood of empirical examples, with or without genome doubling. Interspecific gene flow is thus increasingly thought to lead to evolutionary innovation and diversification, via adaptive introgression, homoploid hybrid speciation and allopolyploid speciation. Less well understood, however, are the suite of genetic and genomic mechanisms set in motion by the merger of differentiated genomes, and the temporal scale over which recombinational complexity mediated by gene flow might be expressed and exposed to natural selection. We focus on these issues here, considering the types of molecular genetic and genomic processes that might be set in motion by the saltational event of genome merger between two diverged species, either with or without genome doubling, and how these various processes can contribute to novel phenotypes. Genetic mechanisms include the infusion of new alleles and the genesis of novel structural variation including translocations and inversions, homoeologous exchanges, transposable element mobilization and novel insertional effects, presence-absence variation and copy number variation. Polyploidy generates massive transcriptomic and regulatory alteration, presumably set in motion by disrupted stoichiometries of regulatory factors, small RNAs and other genome interactions that cascade from single-gene expression change up through entire networks of transformed regulatory modules. We highlight both these novel combinatorial possibilities and the range of temporal scales over which such complexity might be generated, and thus exposed to natural selection and drift.

Robust Cytonuclear Coordination of Transcription in Nascent Arabidopsis thaliana Autopolyploids

Polyploidy is hypothesized to cause dosage imbalances between the nucleus and the other genome-containing organelles (mitochondria and plastids), but the evidence for this is limited. We performed RNA-seq on Arabidopsis thaliana diploids and their derived autopolyploids to quantify the degree of inter-genome coordination of transcriptional responses to nuclear whole genome duplication in two different organs (sepals and rosette leaves). We show that nuclear and organellar genomes exhibit highly coordinated responses in both organs. First, organelle genome copy number increased in response to nuclear whole genome duplication (WGD), at least partially compensating for altered nuclear genome dosage. Second, transcriptional output of the different cellular compartments is tuned to maintain diploid-like levels of relative expression among interacting genes. In particular, plastid genes and nuclear genes whose products are plastid-targeted show coordinated down-regulation, such that their expression levels relative to each other remain constant across ploidy levels. Conversely, mitochondrial genes and nuclear genes with mitochondrial targeting show either constant or coordinated up-regulation of expression relative to other nuclear genes. Thus, cytonuclear coordination is robust to changes in nuclear ploidy level, with diploid-like balance in transcript abundances achieved within three generations after nuclear whole genome duplication.

Gene Expression Changes During the Allo-/Deallopolyploidization Process of Brassica napus

Gene expression changes due to allopolyploidization have been extensively studied in plants over the past few decades. Nearly all these studies focused on comparing the changes before and after genome merger. In this study, we used the uniquely restituted Brassica rapa (RBR, A_eA_e, 2n = 20) obtained from Brassica napus (A_nA_nC_nC_n, 2n = 38) to analyze the gene expression changes and its potential mechanism during the process of allo-/deallopolyploidization. RNA-seq-based transcriptome profiling identified a large number of differentially expressed genes (DEGs) between RBR and natural B. rapa (A_rA_r), suggesting potential effects of allopolyploidization/domestication of AA component of B. napus at the tetrapolyploid level. Meanwhile, it was revealed that up to 20% of gene expressions were immediately altered when compared with those in the A_n-subgenome. Interestingly, one fifth of these changes are in fact indicative of the recovery of antecedent gene expression alternations occurring since the origin of B. napus and showed association with homoeologous expression bias between A_n and C_n subgenomes. Enrichment of distinct gene ontology (GO) categories of the above sets of genes further indicated potential functional cooperation of the A_n and C_n subgenome of B. napus. Whole genome methylation analysis revealed a small number of DEGs were identified in the differentially methylated regions.

Nuclear-Cytoplasmic Coevolution Analysis of RuBisCO in Synthesized Cucumis Allopolyploid

Allopolyploids are often faced with the challenge of maintaining well-coordination between nuclear and cytoplasmic genes inherited from different species. The synthetic allotetraploid Cucumis × hytivus is a useful model to explore cytonuclear coevolution. In this study, the sequences and expression of cytonuclear enzyme complex RuBisCO as well as its content and activity in C. × hytivus were compared to its parents to explore plastid–nuclear coevolution. The plastome-coded rbcL gene sequence was confirmed to be stable maternal inheritance, and parental copy of nuclear rbcS genes were both preserved in C. × hytivus. Thus, the maternal plastid may interact with the biparentally inherited rbcS alleles. The expression of the rbcS gene of C-homoeologs (paternal) was significantly higher than that of H-homoeologs (maternal) in C. × hytivus (HHCC). Protein interaction prediction analysis showed that the rbcL protein has stronger binding affinity to the paternal copy of rbcS protein than that of maternal copy in C. × hytivus, which might explain the transcriptional bias of the rbcS homoeologs. Moreover, both the activity and content of RuBisCO in C. × hytivus showed mid-parent heterosis. In summary, our results indicate a paternal transcriptional bias of the rbcS genes in C. × hytivus, and we found new nuclear–cytoplasmic combination may be one of the reasons for allopolyploids heterosis.

Cytonuclear Coevolution following Homoploid Hybrid Speciation in Aegilops tauschii

The diploid D-genome lineage of the Triticum/Aegilops complex has an evolutionary history involving genomic contributions from ancient A- and B/S-genome species. We explored here the possible cytonuclear evolutionary responses to this history of hybridization. Phylogenetic analysis of chloroplast DNAs indicates that the D-genome lineage has a maternal origin of the A-genome or some other closely allied lineage. Analyses of the nuclear genome in the D-genome species Aegilops tauschii indicate that accompanying and/or following this ancient hybridization, there has been biased maintenance of maternal A-genome ancestry in nuclear genes encoding cytonuclear enzyme complexes (CECs). Our study provides insights into mechanisms of cytonuclear coevolution accompanying the evolution and eventual stabilization of homoploid hybrid species. We suggest that this coevolutionary process includes likely rapid fixation of A-genome CEC orthologs as well as biased retention of A-genome nucleotides in CEC homologs following population level recombination during the initial generations.

Cytonuclear interactions remain stable during allopolyploid evolution despite repeated whole-genome duplications in Brassica

Several plastid macromolecular protein complexes are encoded by both nuclear and plastid genes. Therefore, cytonuclear interactions are held in place to prevent genomic conflicts that may lead to incompatibilities. Allopolyploidy resulting from hybridization and genome doubling of two divergent species can disrupt these fine-tuned interactions, as newly formed allopolyploid species confront biparental nuclear chromosomes with a uniparentally inherited plastid genome. To avoid any deleterious effects of unequal genome inheritance, preferential transcription of the plastid donor over the other donor has been hypothesized to occur in allopolyploids. We used Brassica as a model to study the effects of paleopolyploidy in diploid parental species, as well as the effects of recent and ancient allopolyploidy in Brassica napus, on genes implicated in plastid protein complexes. We first identified redundant nuclear copies involved in those complexes. Compared with cytosolic protein complexes and with genome-wide retention rates, genes involved in plastid protein complexes show a higher retention of genes in duplicated and triplicated copies. Those redundant copies are functional and are undergoing strong purifying selection. We then compared transcription patterns and sequences of those redundant gene copies between resynthesized allopolyploids and their diploid parents. The neopolyploids showed no biased subgenome expression or maternal homogenization via gene conversion, despite the presence of some non-synonymous substitutions between plastid genomes of parental progenitors. Instead, subgenome dominance was observed regardless of the maternal progenitor. Our results provide new insights on the evolution of plastid protein complexes that could be tested and generalized in other allopolyploid species.

Homoeolog expression bias in allopolyploid oleaginous marine diatom Fistulifera solaris

Background

Allopolyploidy is a genomic structure wherein two or more sets of chromosomes derived from divergent parental species coexist within an organism. It is a prevalent genomic configuration in plants, as an important source of genetic variation, and also frequently confers environmental adaptability and increased crop productivity. We previously reported the oleaginous marine diatom Fistulifera solaris JPCC DA0580 to be a promising host for biofuel production and that its genome is allopolyploid, which had never previously been reported in eukaryotic microalgae. However, the study of allopolyploidy in F. solaris was hindered by the difficulty in classifying the homoeologous genes based on their progenitor origins, owing to the shortage of diatom genomic references.

Results

In this study, the allopolyploid genome of F. solaris was tentatively classified into two pseudo-parental subgenomes using sequence analysis based on GC content and codon frequency in each homoeologous gene pair. This approach clearly separated the genome into two distinct fractions, subgenome Fso_h and Fso_l, which also showed the potency of codon usage analysis to differentiate the allopolyploid subgenome. Subsequent homoeolog expression bias analysis revealed that, although both subgenomes appear to contribute to global transcription, there were subgenomic preferences in approximately 61% of homoeologous gene pairs, and the majority of these genes showed continuous bias towards a specific subgenome during lipid accumulation. Additional promoter analysis indicated the possibility of promoter motifs involved in biased transcription of homoeologous genes. Among these subgenomic preferences, genes involved in lipid metabolic pathways showed interesting patterns in that biosynthetic and degradative pathways showed opposite subgenomic preferences, suggesting the possibility that the oleaginous characteristics of F. solaris derived from one of its progenitors.

Conclusions

We report the detailed genomic structure and expression patterns in the allopolyploid eukaryotic microalga F. solaris. The allele-specific patterns reported may contribute to the oleaginous characteristics of F. solaris and also suggest the robust oleaginous characteristics of one of its progenitors. Our data reveal novel aspects of allopolyploidy in a diatom that is not only important for evolutionary studies but may also be advantageous for biofuel production in microalgae.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4691-0) contains supplementary material, which is available to authorized users.

Positive Selection Driving Cytoplasmic Genome Evolution of the Medicinally Important Ginseng Plant Genus Panax

Panax L. (the ginseng genus) is a shade-demanding group within the family Araliaceae and all of its species are of crucial significance in traditional Chinese medicine. Phylogenetic and biogeographic analyses demonstrated that two rounds of whole genome duplications accompanying with geographic and ecological isolations promoted the diversification of Panax species. However, contributions of the cytoplasmic genomes to the adaptive evolution of Panax species remained largely uninvestigated. In this study, we sequenced the chloroplast and mitochondrial genomes of 11 accessions belonging to seven Panax species. Our results show that heterogeneity in nucleotide substitution rate is abundant in both of the two cytoplasmic genomes, with the mitochondrial genome possessing more variants at the total level but the chloroplast showing higher sequence polymorphisms at the genic regions. Genome-wide scanning of positive selection identified five and 12 genes from the chloroplast and mitochondrial genomes, respectively. Functional analyses further revealed that these selected genes play important roles in plant development, cellular metabolism and adaptation. We therefore conclude that positive selection might be one of the potential evolutionary forces that shaped nucleotide variation pattern of these Panax species. In particular, the mitochondrial genes evolved under stronger selective pressure compared to the chloroplast genes.

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.

The on-again, off-again relationship between mitochondrial genomes and species boundaries

The study of reproductive isolation and species barriers frequently focuses on mitochondrial genomes and has produced two alternative and almost diametrically opposed narratives. On one hand, mtDNA may be at the forefront of speciation events, with co-evolved mitonuclear interactions responsible for some of the earliest genetic incompatibilities arising among isolated populations. On the other hand, there are numerous cases of introgression of mtDNA across species boundaries even when nuclear gene flow is restricted. We argue that these seemingly contradictory patterns can result from a single underlying cause. Specifically, the accumulation of deleterious mutations in mtDNA creates a problem with two alternative evolutionary solutions. In some cases, compensatory or epistatic changes in the nuclear genome may ameliorate the effects of mitochondrial mutations, thereby establishing coadapted mitonuclear genotypes within populations and forming the basis of reproductive incompatibilities between populations. Alternatively, populations with high mitochondrial mutation loads may be rescued by replacement with a more fit, foreign mitochondrial haplotype. Coupled with many nonadaptive mechanisms of introgression that can preferentially affect cytoplasmic genomes, this form of adaptive introgression may contribute to the widespread discordance between mitochondrial and nuclear genealogies. Here, we review recent advances related to mitochondrial introgression and mitonuclear incompatibilities, including the potential for cointrogression of mtDNA and interacting nuclear genes. We also address an emerging controversy over the classic assumption that selection on mitochondrial genomes is inefficient and discuss the mechanisms that lead lineages down alternative evolutionary paths in response to mitochondrial mutation accumulation.

Cytonuclear Coordination Is Not Immediate upon Allopolyploid Formation in Tragopogon miscellus (Asteraceae) Allopolyploids

Allopolyploids, formed by hybridization and chromosome doubling, face the immediate challenge of having duplicated nuclear genomes that interact with the haploid and maternally inherited cytoplasmic (plastid and mitochondrial) genomes. Most of our knowledge of the genomic consequences of allopolyploidy has focused on the fate of the duplicated nuclear genes without regard to their potential interactions with cytoplasmic genomes. As a step toward understanding the fates of nuclear-encoded subunits that are plastid-targeted, here we examine the retention and expression of the gene encoding the small subunit of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco; rbcS) in multiple populations of allotetraploid Tragopogon miscellus (Asteraceae). These polyploids formed recently (~80 years ago) and repeatedly from T. dubius and T. pratensis in the northwestern United States. Examination of 79 T. miscellus individuals from 10 natural populations, as well as 25 synthetic allotetraploids, including reciprocally formed plants, revealed a low percentage of naturally occurring individuals that show a bias in either gene (homeolog) loss (12%) or expression (16%), usually toward maintaining the maternal nuclear copy of rbcS. For individuals showing loss, seven retained the maternally derived rbcS homeolog only, while three had the paternally derived copy. All of the synthetic polyploid individuals examined (S0 and S1 generations) retained and expressed both parental homeologs. These results demonstrate that cytonuclear coordination does not happen immediately upon polyploid formation in Tragopogon miscellus.

Nonadditive gene expression in polyploids

Allopolyploidy involves hybridization and duplication of divergent parental genomes and provides new avenues for gene expression. The expression levels of duplicated genes in polyploids can show deviation from parental additivity (the arithmetic average of the parental expression levels). Nonadditive expression has been widely observed in diverse polyploids and comprises at least three possible scenarios: (a) The total gene expression level in a polyploid is similar to that of one of its parents (expression-level dominance); (b) total gene expression is lower or higher than in both parents (transgressive expression); and (c) the relative contribution of the parental copies (homeologs) to the total gene expression is unequal (homeolog expression bias). Several factors may result in expression nonadditivity in polyploids, including maternal-paternal influence, gene dosage balance, cis- and/or trans-regulatory networks, and epigenetic regulation. As our understanding of nonadditive gene expression in polyploids remains limited, a new generation of investigators should explore additional phenomena (i.e., alternative splicing) and use other high-throughput "omics" technologies to measure the impact of nonadditive expression on phenotype, proteome, and metabolome.

Evolutionary origins and dynamics of octoploid strawberry subgenomes revealed by dense targeted capture linkage maps

Whole-genome duplications are radical evolutionary events that have driven speciation and adaptation in many taxa. Higher-order polyploids have complex histories often including interspecific hybridization and dynamic genomic changes. This chromosomal reshuffling is poorly understood for most polyploid species, despite their evolutionary and agricultural importance, due to the challenge of distinguishing homologous sequences from each other. Here, we use dense linkage maps generated with targeted sequence capture to improve the diploid strawberry (Fragaria vesca) reference genome and to disentangle the subgenomes of the wild octoploid progenitors of cultivated strawberry, Fragaria virginiana and Fragaria chiloensis. Our novel approach, POLiMAPS (Phylogenetics Of Linkage-Map-Anchored Polyploid Subgenomes), leverages sequence reads to associate informative interhomeolog phylogenetic markers with linkage groups and reference genome positions. In contrast to a widely accepted model, we find that one of the four subgenomes originates with the diploid cytoplasm donor F. vesca, one with the diploid Fragaria iinumae, and two with an unknown ancestor close to F. iinumae. Extensive unidirectional introgression has converted F. iinumae-like subgenomes to be more F. vesca-like, but never the reverse, due either to homoploid hybridization in the F. iinumae-like diploid ancestors or else strong selection spreading F. vesca-like sequence among subgenomes through homeologous exchange. In addition, divergence between homeologous chromosomes has been substantially augmented by interchromosomal rearrangements. Our phylogenetic approach reveals novel aspects of the complicated web of genetic exchanges that occur during polyploid evolution and suggests a path forward for unraveling other agriculturally and ecologically important polyploid genomes.

Cytonuclear evolution of rubisco in four allopolyploid lineages

Allopolyploidization in plants entails the merger of two divergent nuclear genomes, typically with only one set (usually maternal) of parental plastidial and mitochondrial genomes and with an altered cytonuclear stoichiometry. Thus, we might expect cytonuclear coevolution to be an important dimension of allopolyploid evolution. Here, we investigate cytonuclear coordination for the key chloroplast protein rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase), which is composed of nuclear-encoded, small subunits (SSUs) and plastid-encoded, large subunits. By studying gene composition and diversity as well as gene expression in four model allopolyploid lineages, Arabidopsis, Arachis, Brassica, and Nicotiana, we demonstrate that paralogous nuclear-encoded rbcS genes within diploids are subject to homogenization via gene conversion and that such concerted evolution via gene conversion characterizes duplicated genes (homoeologs) at the polyploid level. Many gene conversions in the polyploids are intergenomic with respect to the diploid progenitor genomes, occur in functional domains of the homoeologous SSUs, and are directionally biased, such that the maternal amino acid states are favored. This consistent preferential maternal-to-paternal gene conversion is mirrored at the transcriptional level, with a uniform transcriptional bias of the maternal-like rbcS homoeologs. These data, repeated among multiple diverse angiosperm genera for an important photosynthetic enzyme, suggest that cytonuclear coevolution may be mediated by intergenomic gene conversion and altered transcription of duplicated, now homoeologous nuclear genes.