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Christiane M.-R. Fauron

Research Assistant Professor of Human Genetics

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Research

Mitochondrial function is essential for normal growth. Our research is to understand the organization, the function and the evolution of the plant mitochondrial genomes (mtDNAs). Higher p lant mitochondrial genomes are unusual in their diversity of structure and rapidity of change. They are substantially larger (200 kb to 2500 kb) and more variable than those of mammalian (~16 kb) or fungi mtDNAs. The high frequency of recombination is perhaps the most unique feature of higher plant mtDNAs and the recombination contributes to the extensive intraspecies polymorphism. Beside the fluidity of the genome structure due to the high frequency of recombination, higher plant mtDNAs presents many other unusual and interesting features. The genome has integrated foreign DNA (chloroplast, nuclear, viral, and plasmid DNA) which carry some functional genes.   Almost all the protein gene transcripts are edited and the production of mature transcripts of some protein genes require transplicing. Comparison of the complete sequences of plant mitochondrial genomes showed that, although most of the known coding regions are very conserved, large intergenic regions show no sequence similarities. These "non-coding" regions have important roles in the rapid structural evolution and have been found to become functional as components of   "chimeric" genes, some of which have been described as being responsible for several cases of cytoplasmic male sterility (cms). Cms is a useful agronomic trait usually associated with expression of chimeric genes that cause a specific toxicity during pollen development. In order to elucidate the events that contribute to the rapid structural evolution of plant mitochondrial genomes, it is necessary to examine closely related species.

Our project is focused on the analysis and comparison of the mitochondrial genomes of maize, its closely related species (teosintes) as well as more distantly related species (sorghum and Trypsacum). Complete sequencing data generated circular maps for the fertile maize NB and NA mtDNAs (569,630 base pairs (bp) and 701,046 bp respectively), for male sterile maize genotypes cmsT and cmsC (535,825 bp and 739,719 bp respectively). The map resulting from the sequencing data of the 569,553 bp maize cmsS mtDNA is linear. MtDNA sequences have also been completed for three teosintes : Zea parviglumis (the probable progenitor of maize), Zea luxurians  and Zea perennis ( the most distant section of the teosintes ) which generated circular maps of 584,546 bp, 539,368 bp and 570,354 bp long respectively. The complete sequences for distantly related grasses, Tripsacum dactyloides and Sorghum bicolor mitochondrial genome sequences are being finalized.

Sequence comparisons reveal almost no differences in known gene content among all the grasses mitochondrial genomes, but do reveal major rearrangements, large duplications, many indels and insertions of foreign DNA of unknown origin. As phylogenetic distance increases, non-genic regions having no identifiable similarity to one another rapidly increase. Between the two fertile maize mitochondrial genomes, 4.5% of the NA genome is not present in NB, nor is it found in any other sequence in GenBank. In comparison, 72.1% of the maize NB mitochondrial genomic sequences are not found in another grass, the rice mtDNA. Most of the increase in size of NA (701 kb) and cmsC (740 kb) relative to NB (570 kb) are due to sequence duplication and not to small dispersed repeats. The sequencing approach is complemented with the mapping of several other Zea mitochondrial genotypes using an efficient cosmid fingerprinting strategy to construct maps and identify regions of rearrangement and divergence. We have identified all the potential genes and are looking at how they are preferentially expressed during specific tissues and stages of plant development.

 

References