plant mitochondria

Palmer JD & Herbon LA 1988 Plant mitochondrial DNA evolves rapidly in structure, but slowly in sequence. J Mol Evol 28:87-97.

  • the rate of rearrangements is extraordinarily faster in plant mtDNA than in cpDNA and animal mtDNA
  • we shall use "Brassica" to refer to all five species compared herein that are classified in the genus Brassica and also for Raphanus sativus (radish)
  • because cytoplasmically it too belongs in Brassica
  • the mitochondrial genomes of three of the six species examined here, B. campestris, B. nigra, and Raphanus sativa, exist in a tripartite organization as the result of intragenomic recombination across a pair of direct repeats
  • this recombination interconverts a master chromosome with two subgenomic circles
  • B. hirta mtDNA lacks any recombination repeats
  • so exists as a single circular chromosome
  • rearrangements has been most extensive in the lineage leading to radish R. sativa
  • 10 rearrangements ... have been shown (Makaroff and Palmer 1988) to distinguish the CMS and fertile mtDNAs of R. sativa
  • the large size of plant mitochondrial genomes and their abundance of noncoding sequences suggest that a high proportion of possible rearrangements is likely to be tolerated
  • i.e., will not disrupt gene function
  • at most only about one-third of the 218-kb mitochondrial genome of B. campestris is likely to have coding functions and that almost all genes are transcribed singly
  • land plant cpDNAs are densely packed with genes
  • many of which are cotranscribed
  • animal mtDNA lacks spacer sequences almost entirely and in mammals at least is transcribed into a single genome-sized RNA
  • plant mitochondrial genomes seem able to tolerate at least certain rearrangements that actually do affect gene structure
  • these generate novel, recombinant genes that appear to be involved in cytoplasmic male sterility
  • a second major factor promoting rearrangement in plant mtDNAs is the prevalence of short dispersed repeats that could serve as the points of crossover for homologous recombination events
  • such repeats are virtually absent from animal mtDNA and from most cpDNAs
  • these repeats are located near or at the precise breakpoints of the three inversions (Fig. 3) that distinguish mtDNAs of B. campestris and B. oleracea
  • the fact that the genomes are mostly noncoding might cause one to expect high levels of sequence variation
  • yet, plant mtDNA accumulates substitutions 3-4 and 100 times more slowly than the densely gene-packed DNAs of chloroplasts and animal mitochondria, respectively