A possible breakage of linkage disequilibrium between mitochondrial and chloroplast genomes during Emmer and Dinkel wheat evolution

Introduction Chloroplast and mitochondria contain their own genomes, collectively called plasmon. Genetic information stored in the plasmon is a rich source of study for revealing phylogenetic relationships among taxa at [...]
In wheat (Triticum) and Aegilops, chloroplast and mitochondrial genomes have been studied for over three decades to clarify the phylogenetic relationships among species, and most of the maternal lineages of polyploid species have been clarified. Mitochondrial genomes of Emmer (tetraploid with nuclear genome AABB) and Dinkel (hexaploid with AABBDD) wheat are classified into two different types, VIIa and VIIb, by the presence-absence of the third largest HindIII fragment (named H3) in the mitochondrial DNA. Although the mitochondrial genome in the genera often provides useful information to clarify the phylogenetic relationship among closely related species, the phylogenetic significance of this dimorphism has yet not been clarified. In this study, to facilitate analysis using a large number of accessions, a sequence characterized amplified region (SCAR) marker that distinguishes the type VIIb mitochondrial genome from type Vila was first developed. Mitochondrial genome type was determined for each of 30 accessions of wild and cultivated Emmer wheat and 25 accessions of Dinkel wheat. The mitochondrial genome type for each accession was compared with the plastogroup that had been determined using chloroplast microsatellite markers. Unexpectedly, the distribution of mitochondrial genome type was not in accordance with that of the plastogroups, suggesting occasional paternal leakage of either the mitochondrial or chloroplast genome during speciation and differentiation of Emmer and Dinkel wheat. An alternative possibility that substoichiometric shifting is involved in the observed dimorphism of the mitochondrial genome is also discussed. Key words: wheat, mitochondrial DNA, SCAR, linkage disequilibrium, substoichiometric shifting. Chez le ble (Triticum)etl'Aegilops, les genomes chloroplastiques et mitochondriaux ont ete etudies depuis plus de trois decennies pour eclairer les relations phylogenetiques entre les especes et la plupart des lignages maternels des especes polyploides ont ete determines. Les genomes mitochondriaux du ble amidonnier (tetraploide avec un genome nucleaire AABB) et de l'epeautre (hexaploide, AABBDD) appartiennent aux types Vila et VlIb sur la base de la presence-absence du troisieme plus grand fragment HindIII (H3) au sein de l'ADN mitochondrial. Bien que le genome mitochondrial procure souvent des informations utiles pour eclairer les relations phylogenetiques au niveau du genre entre especes proches, la signification phylogenetique de ce dimorphisme n'a pas encore ete determinee. Dans ce travail, afin de faciliter l'analyse d'une grand nombre d'accessions, un marqueur PCR de type SCAR (<< sequence characterized amplified region >>) permettant de distinguer les genomes mitochondriaux des types VIIb et VIIa a ete developpe. Le type de genome mitochondrial a ete determine pour chacune de 30 accessions de ble amidonnier sauvage ou cultive ainsi que pour 25 accessions d'epeautre. Le type de genome mitochondrial rencontre chez chaque accession a ete compare au groupe plastidique qui avait ete determine a l'aide de marqueurs microsatellites chloroplastiques. Contrairement aux attentes, la distribution du type de genome mitochondrial ne concordait pas avec celui des groupes plastidiques, ce qui suggere une transmission paternelle occasionnelle de l'un ou l'autre genome au cours de la speciation et de la differenciation des bles amidonniers et des epeautres. Les auteurs discutent de la possibilite alternative que des fluctuations substoechiometriques soient responsables du dimorphisme observe au sein du genome mitochondrial. [Traduit par la Redaction] Mots-cles : ble, ADN mitochondrial, SCAR, desequilibre de liaison, fluctuations substoechiometriques.