Dataset Information

Cutoffs and k-mers: implications from a transcriptome study in allopolyploid plants.

ABSTRACT:

Background

Transcriptome analysis is increasingly being used to study the evolutionary origins and ecology of non-model plants. One issue for both transcriptome assembly and differential gene expression analyses is the common occurrence in plants of hybridisation and whole genome duplication (WGD) and hybridization resulting in allopolyploidy. The divergence of duplicated genes following WGD creates near identical homeologues that can be problematic for de novo assembly and also reference based assembly protocols that use short reads (35 - 100 bp).

Results

Here we report a successful strategy for the assembly of two transcriptomes made using 75 bp Illumina reads from Pachycladon fastigiatum and Pachycladon cheesemanii. Both are allopolyploid plant species (2n = 20) that originated in the New Zealand Alps about 0.8 million years ago. In a systematic analysis of 19 different coverage cutoffs and 20 different k-mer sizes we showed that i) none of the genes could be assembled across all of the parameter space ii) assembly of each gene required an optimal set of parameter values and iii) these parameter values could be explained in part by different gene expression levels and different degrees of similarity between genes.

Conclusions

To obtain optimal transcriptome assemblies for allopolyploid plants, k-mer size and k-mer coverage need to be considered simultaneously across a broad parameter space. This is important for assembling a maximum number of full length ESTs and for avoiding chimeric assemblies of homeologous and paralogous gene copies.

SUBMITTER: Gruenheit N

PROVIDER: S-EPMC3378427 | biostudies-literature | 2012 Mar

REPOSITORIES: biostudies-literature

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Publications

Cutoffs and k-mers: implications from a transcriptome study in allopolyploid plants.

Gruenheit Nicole N Deusch Oliver O Esser Christian C Becker Matthias M Voelckel Claudia C Lockhart Peter P

BMC genomics 20120314

<h4>Background</h4>Transcriptome analysis is increasingly being used to study the evolutionary origins and ecology of non-model plants. One issue for both transcriptome assembly and differential gene expression analyses is the common occurrence in plants of hybridisation and whole genome duplication (WGD) and hybridization resulting in allopolyploidy. The divergence of duplicated genes following WGD creates near identical homeologues that can be problematic for de novo assembly and also referenc ...[more]

PMID: 22417298

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Project description:BackgroundModern allotetraploid cotton contains an "A" and "D" genome from an ancestral polyploidy event that occurred approximately 1-2 million years ago. Diploid A- and D-genome species can be compared to the A- and D-genomes found within these allotetraploids to make evolutionary inferences about polyploidy. In this paper we present a comprehensive EST assembly derived from diploid and model allotetraploid cottons and demonstrate several evolutionary inferences regarding genic evolution that can be drawn from these data.ResultsWe generated a set of cotton expressed sequence tags (ESTs), comprising approximately 4.4 million Sanger and next-generation (454) transcripts supplemented by approximately 152 million Illumina reads from diploid and allotetraploid cottons. From the EST alignments we inferred 259,192 genome-specific single nucleotide polymorphisms (SNPs). Molecular evolutionary analyses of protein-coding regions demonstrate that the rate of nucleotide substitution has increased among both allotetraploid genomes relative to the diploids, and that the ratio of nonsynonymous to synonymous substitutions has increased in one of the two polyploid lineages we sampled. We also use these SNPs to show that a surprisingly high percentage of duplicate genes (~7 %) show a signature of non-independent evolution in the allotetraploid nucleus, having experienced one or more episodes of nonreciprocal homoeologous recombination (NRHR).ConclusionsIn this study we characterize the functional and mutational properties of the cotton transcriptome, produce a large genome-specific SNP database, and detect illegitimate genetic exchanges between duplicate genomes sharing a common allotetraploid nucleus. Our findings have important implications for our understanding of the consequences of polyploidy and duplicate gene evolution. We demonstrate that cotton genes have experienced an increased rate of molecular evolution following duplication by polyploidy, and that polyploidy has enabled considerable levels of nonreciprocal exchange between homoeologous genes.

Project description:Background and aimsNatural hybridization was investigated between two predominantly allohexaploid wheatgrasses, weedy Elytrigia repens and steppic E. intermedia, with respect to habitats characterized by different degrees of anthropogenic disturbance.MethodsUsing flow cytometry (relative DNA content), 269 plants from three localities were analysed. Hybrids were further analysed using nuclear ribosomal (ITS1-5.8S-ITS2 region) and chloroplast (trnT-F region) DNA markers in addition to absolute DNA content and chromosome numbers.Key resultsWeedy E. repens was rare in a steppic locality whereas E. intermedia was almost absent at two sites of agricultural land-use. Nevertheless, hybrids were common there whereas none were found at the steppic locality, underlining the importance of different ecological conditions for hybrid formation or establishment. At one highly disturbed site, > 16 % of randomly collected plants were hybrids. Hexaploid hybrids showed intermediate genome size compared with the parents and additive patterns of parental ITS copies. Some evidence of backcrosses was found. The direction of hybridization was highly asymmetric as cpDNA identified E. intermedia as the maternal parent in 61 out of 63 cases. Out of nine nonaploid cytotypes (2n = 9x = 63) which likely originated by fusion of unreduced and reduced gametes of hexaploids, eight were hybrids whereas one was a nonaploid cytotype of E. repens. The progeny of one nonaploid hybrid demonstrated gene flow between hexaploid and nonaploid cytotypes.ConclusionsThe results show that E. repens and E. intermedia frequently cross at places where they co-occur. Hybrid frequency is likely influenced by habitat type; sites disturbed by human influence sustain hybrid formation and/or establishment. Hexaploid and nonaploid hybrid fertility is not negligible, backcrossing is possible, and the progeny is variable. The frequent production of new at least partially fertile cyto- and genotypes provides ample raw material for evolution and adaptation.

Project description: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.

Dataset Information

Cutoffs and k-mers: implications from a transcriptome study in allopolyploid plants.

Background

Results

Conclusions

Publications

Cutoffs and k-mers: implications from a transcriptome study in allopolyploid plants.

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