Recombination of Chlamydomonas chloroplast DNA occurs more frequently in the large inverted repeat sequence than in the single-copy regions

Interspecific Plastome Recombination Reflects Ancient Reticulate Evolution in Picea (Pinaceae)

Plastid sequences are a cornerstone in plant systematic studies and key aspects of their evolution, such as uniparental inheritance and absent recombination, are often treated as axioms. While exceptions to these assumptions can profoundly influence evolutionary inference, detecting them can require extensive sampling, abundant sequence data, and detailed testing. Using advancements in high-throughput sequencing, we analyzed the whole plastomes of 65 accessions of Picea, a genus of ∼35 coniferous forest tree species, to test for deviations from canonical plastome evolution. Using complementary hypothesis and data-driven tests, we found evidence for chimeric plastomes generated by interspecific hybridization and recombination in the clade comprising Norway spruce (P. abies) and 10 other species. Support for interspecific recombination remained after controlling for sequence saturation, positive selection, and potential alignment artifacts. These results reconcile previous conflicting plastid-based phylogenies and strengthen the mounting evidence of reticulate evolution in Picea. Given the relatively high frequency of hybridization and biparental plastid inheritance in plants, we suggest interspecific plastome recombination may be more widespread than currently appreciated and could underlie reported cases of discordant plastid phylogenies.

A six-mRNA signature model for the prognosis of head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC), one of the most common cancers with high morbidity and mortality rates worldwide, has a poor prognosis. The transcriptome sequencing data of 500 patients with HNSCC in the TCGA dataset were assessed to find biomarkers associated with HNSCC prognosis so as to improve the prognosis of patients with HNSCC. The patients were divided into the training and testing sets. A model of six mRNAs (FRMD5, PCMT1, PDGFA, TMC8, YIPF4, ZNF324B) that could predict patient prognosis was identified in the training set using the Cox regression analysis. According to this model, the patients were divided into high-risk and low-risk groups. The Kaplan-Meier analysis showed that the high-risk group showed significantly shorter overall survival time compared with the low-risk group in both training and testing sets. The receiver operating characteristic analysis further confirmed high sensitivity and specificity for the model, which was more accurate compared with some known biomarkers in predicting HNSCC prognosis. Moreover, the model was applicable to patients of different ages, genders, clinical stages, tumor locations, smoking history, and human papillomavirus (HPV) status, as well as to microarray dataset. This model could be used as a novel biomarker for the prognosis of HNSCC and a significant tool for guiding the clinical treatment of HNSCC. The risk score acquired from the model might contribute to improving outcome prediction and management for patients with HNSCC, indicating its clinical significance.

Plastidic trnFUUC pseudogenes in North American genus Boechera (Brassicaceae): mechanistic aspects of evolution

The origin and maintenance of a plastidic tandem repeat next to the TRNF (UUC) gene were analyzed in the genus BOECHERA in a phylogenetic context and were compared to published analogous examples that emerged in parallel in the Asteraceae and Juncaceae, respectively. Although we identified some features common to these taxonomic groups with respect to structure and origin of the region, obvious differences were encountered, which argue against a specific mechanism or evolutionary principle underlying the parallel origin and maintenance of the TRNF-tandem repeats in those families. In contrast to the situation in the Asteraceae, no reciprocal recombinant repeat types have been observed in the Brassicaceae. Forty copy types, classified into three groups, were isolated from 103 chloroplast haplotypes of BOECHERA and it was demonstrated that they are composed of four subregions of various origins. We discuss various mutation mechanisms such as DNA replication slippage, and inter- and intrachromosomal recombination which were reported to mediate variation in copy numbers and other types of observed sequence length polymorphism. It is shown that the observed molecular structure of the tandem repeat region did not fully fit the particular patterns expected under a scenario of evolution including any of the known mechanisms. Nevertheless, it appeared that intermolecular unequal crossing-over is most likely the driving force in the evolution of this tandem repeat. However, it remains to be explained, why no reciprocal recombinant copy types have been observed. The reconstructed phylogenetic relationships among copies reflected different evolutionary scenarios as follows: (1) A single and ancient origin of copies pre-dates the radiation of BOECHERA. (2) Parallel expansion and shortening of the tandem repeat within different BOECHERA lineages. (3) Conservation of the first copy, as it was the only one present in all chloroplast haplotypes.

Adaptive evolution of chloroplast genome structure inferred using a parametric bootstrap approach

Background

Genome rearrangements influence gene order and configuration of gene clusters in all genomes. Most land plant chloroplast DNAs (cpDNAs) share a highly conserved gene content and with notable exceptions, a largely co-linear gene order. Conserved gene orders may reflect a slow intrinsic rate of neutral chromosomal rearrangements, or selective constraint. It is unknown to what extent observed changes in gene order are random or adaptive. We investigate the influence of natural selection on gene order in association with increased rate of chromosomal rearrangement. We use a novel parametric bootstrap approach to test if directional selection is responsible for the clustering of functionally related genes observed in the highly rearranged chloroplast genome of the unicellular green alga Chlamydomonas reinhardtii, relative to ancestral chloroplast genomes.

Results

Ancestral gene orders were inferred and then subjected to simulated rearrangement events under the random breakage model with varying ratios of inversions and transpositions. We found that adjacent chloroplast genes in C. reinhardtii were located on the same strand much more frequently than in simulated genomes that were generated under a random rearrangement processes (increased sidedness; p < 0.0001). In addition, functionally related genes were found to be more clustered than those evolved under random rearrangements (p < 0.0001). We report evidence of co-transcription of neighboring genes, which may be responsible for the observed gene clusters in C. reinhardtii cpDNA.

Conclusion

Simulations and experimental evidence suggest that both selective maintenance and directional selection for gene clusters are determinants of chloroplast gene order.

Homeologous plastid DNA transformation in tobacco is mediated by multiple recombination events

Efficient plastid transformation has been achieved in Nicotiana tabacum using cloned plastid DNA of Solanum nigrum carrying mutations conferring spectinomycin and streptomycin resistance. The use of the incompletely homologous (homeologous) Solanum plastid DNA as donor resulted in a Nicotiana plastid transformation frequency comparable with that of other experiments where completely homologous plastid DNA was introduced. Physical mapping and nucleotide sequence analysis of the targeted plastid DNA region in the transformants demonstrated efficient site-specific integration of the 7.8-kb Solanum plastid DNA and the exclusion of the vector DNA. The integration of the cloned Solanum plastid DNA into the Nicotiana plastid genome involved multiple recombination events as revealed by the presence of discontinuous tracts of Solanum-specific sequences that were interspersed between Nicotiana-specific markers. Marked position effects resulted in very frequent cointegration of the nonselected peripheral donor markers located adjacent to the vector DNA. Data presented here on the efficiency and features of homeologous plastid DNA recombination are consistent with the existence of an active RecA-mediated, but a diminished mismatch, recombination/repair system in higher-plant plastids.

Analysis of a hotspot for deletion formation within the intron of the chloroplast trnI gene

The chloroplast genomes of higher plants encode several tRNA genes that contain highly conserved type II introns. Using primers specific to conserved 5' and 3' regions within the introns of the genes trnA (tRNA-ala) and trnI (tRNA-ile) we have PCR amplified parts of these introns from 36 plant species representing a wide range of plant families. Deletions were found in the introns of both tRNA genes. Fourteen species had detectable deletions within the intron of trnI and four species within the intron of trnA. The occurrence of these deletions among the various plant families suggests that the events leading to the formation of these deletions occurred independently many times during the evolution of higher plants. Analysis of the amplified PCR products from the trnI intron suggests that these independent deletions may not be random but appear to fall into two size classes. Several members of each class were cloned and sequenced and the end points of the deletions were mapped. The 3' ends of all deletions studied terminate within the same short region. The 5' ends of the deletions map to two different regions, giving rise to the two size classes. These two 5' deletion endpoint regions show some sequence similarity. Only two of the identified deletions contain directly repeated sequences at the deletion endpoints, a feature associated with homologous recombination. Our results suggest that within the trnI intron, there are preferred sites or "hotspots" for deletion formation involving a novel imprecise recombination mechanism. The significance of these sequences and possible mechanisms for deletion formation are discussed.

Chloroplast DNA variation in Populus. III. Novel chloroplast DNA variants in natural Populus x canadensis hybrids

A rare phenomenon of the occurrence of novel non-parental chloroplast DNA (cpDNA) variants in natural sexual interspecific hybrids between Populus deltoides var deltoides and P. nigra, P. x canadensis is described. Restriction fragment variation of cpDNA in 17 P. x canadensis cultivars was examined and compared with that of representative samples of P. deltoides and P. nigra using 83 combinations of 16 restriction enzymes and six Petunia hybrida cpDNA probes. Twelve cultivars had one to five novel non-parental cpDNA fragments in the chloroplast genome region homologous to the 9.0-kb PstI cpDNA fragment of Petunia from the large single-copy region.