bims-plasge Biomed News
on Plastid genes
Issue of 2019‒09‒01
three papers selected by
Vera S. Bogdanova
Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences

  1. Ecol Evol. 2019 Aug;9(16): 9100-9110
      Whole-genome duplications (WGD) through polyploid speciation are associated with disruptions of well-tuned relationships among the three plant cell genomes. Key metabolic processes comprising multi-subunit enzyme complexes, for which partner proteins are both nuclear- and plastid-encoded, are dependent on maintenance of stoichiometric ratios among the subunits to avoid cytonuclear imbalances after WGDs. By using qPCR for gene copy and transcript number quantification, we have studied the relationship of subunit expression in the two gene pairs rbcL/rbcS (the two subunits of RuBisCO) and psbA/psbO (two members of photosystem II) in closely related members of Leucanthemum (Compositae, Anthemideae), comprising a diploid, a tetraploid, and a hexaploid species. While gene copy numbers exhibit the expected pattern of an increase in the nuclear-encoded partner gene relative to the plastid-encoded one, we find that the two partner gene systems behave differently after WGD: While in the psbA/psbO partner gene system, shifts in the gene copy balance caused by polyploidization are not accommodated for through changes in transcription intensities of the two partner genes, the rbcL/rbcS system even shows an unexpected reversed dosage effect with up-regulated transcription intensities on both the nuclear and the plastidal side. We interpret the behavior of the psbA/psbO partner gene system as being due to the stoichiometrically relaxed relationship between the two gene products caused by a fast, damage-provoked combustion of the psbA gene product (the D1 core protein of PSII). Conversely, the finely tuned expression dependencies of the rbcL/rbcS system may be the reason for the observed positive feedback runaway signal as reaction to gene copy imbalances caused by a polyploidization shock.
    Keywords:  Calvin cycle; gene expression; hybridization; photosynthesis; photosystem II; polyploidy
  2. Genomics. 2019 Aug 25. pii: S0888-7543(19)30394-5. [Epub ahead of print]
      Recombination is a crucial component of evolution and breeding. New combinations of variation on chromosomes are shaped by recombination. Recombination is also involved in chromosomal rearrangements. However, recombination rates vary tremendously among chromosome segments. Genome-wide genetic maps are one of the best tools to study variation of recombination. Here, we describe high density genetic maps of Glycine max and Glycine soja constructed from four segregating populations. The maps were used to identify chromosomal rearrangements and find the highly predictable pattern of cross-overs on the broad scale in soybean. Markers on these genetic maps were used to evaluate assembly quality of the current soybean reference genome sequence. We find a strong inversion candidate larger than 3 Mb based on patterns of cross-overs. We also identify quantitative trait loci (QTL) that control number of cross-overs. This study provides fundamental insights relevant to practical strategy for breeding programs and for pan-genome researches.
    Keywords:  Genetic map; Inversion; Recombination; Soybean
  3. PLoS Genet. 2019 Aug 30. 15(8): e1008373
      Plant mitochondrial genomes are usually assembled and displayed as circular maps based on the widely-held view across the broad community of life scientists that circular genome-sized molecules are the primary form of plant mitochondrial DNA, despite the understanding by plant mitochondrial researchers that this is an inaccurate and outdated concept. Many plant mitochondrial genomes have one or more pairs of large repeats that can act as sites for inter- or intramolecular recombination, leading to multiple alternative arrangements (isoforms). Most mitochondrial genomes have been assembled using methods unable to capture the complete spectrum of isoforms within a species, leading to an incomplete inference of their structure and recombinational activity. To document and investigate underlying reasons for structural diversity in plant mitochondrial DNA, we used long-read (PacBio) and short-read (Illumina) sequencing data to assemble and compare mitochondrial genomes of domesticated (Lactuca sativa) and wild (L. saligna and L. serriola) lettuce species. We characterized a comprehensive, complex set of isoforms within each species and compared genome structures between species. Physical analysis of L. sativa mtDNA molecules by fluorescence microscopy revealed a variety of linear, branched, and circular structures. The mitochondrial genomes for L. sativa and L. serriola were identical in sequence and arrangement and differed substantially from L. saligna, indicating that the mitochondrial genome structure did not change during domestication. From the isoforms in our data, we infer that recombination occurs at repeats of all sizes at variable frequencies. The differences in genome structure between L. saligna and the two other Lactuca species can be largely explained by rare recombination events that rearranged the structure. Our data demonstrate that representations of plant mitochondrial genomes as simple, circular molecules are not accurate descriptions of their true nature and that in reality plant mitochondrial DNA is a complex, dynamic mixture of forms.