Interorganellar DNA transfer in wheat: dynamics and phylogenetic origin

A homology search of wheat chloroplast (ct) and mitochondrial (mt) genomes identified 54 ctDNA segments that have homology with 66 mtDNA segments. The mtDNA segments were classified according to their origin: orthologs (prokaryotic origin), xenologs (interorganellar DNA transfer origin) and paralogs (intraorganellar DNA amplification origin). The 66 mtDNA sequences with homology to ctDNA segments included 14 paralogs, 18 orthologs and 34 xenologs. Analysis of the xenologs indicated that the DNA transfer occurred unidirectionally from the ct genome to the mt genome. The evolutionary timing of each interorganellar DNA transfer that generated a xenolog was estimated. This analysis showed that 2 xenologs originated early in green plant evolution, 4 in angiosperm evolution, 3 in monocotyledon evolution, 9 during cereal diversification and 8 in the evolution of wheat. Six other xenologs showed recurrent transfer from the ct to mt genomes in more than one taxon. The two remaining xenologs were uninformative on the evolutionary timing of their transfer. The wheat mt nad9 gene was found to be chimeric, consisting of the cereal nad9 gene and its 291 bp 5′-flanking region that included a 58 bp xenolog of the ct-ndhC origin

Evolutionary significance of inversions in legume chloroplast DNAs

Cloned genes from tobacco, spinach, and pea were used as hybridization probes to localize 36 protein genes on the chloroplast chromosomes of four legumes — mung bean, common bean, soybean, and pea. The first three chloroplast DNAs (cpDNAs), all of which retain a large inverted repeat, have an identical gene order with but one exception. A 78 kb segment encompassing nearly the entire large single copy region is inverted in mung bean and common bean relative to soybean and non-legumes. The simplest evolutionary explanation for this difference is a 78 kb inversion, with one endpoint between rps8 and inf A and the second between psb A and rpl2 . However, we can not rule out a two-step re-arrangement (consisting of successive expansion and contraction of the inverted repeat) leading to the relocation of a block of six ribosomal protein genes ( rps 19- rps 8) from one end of the large single copy region to the other. Analysis of gene locations in pea cpDNA, which lacks the large inverted repeat, combined with cross-hybridization studies using 59 clones covering the mung bean genome, leads to a refined picture of the position and nature of the numerous rearrangements previously described in the pea genome. A minimum of eight large inversions are postulated to account for these rearrangements. None of these inversions disrupt groups of genes that are transcriptionally linked in angiosperm cpDNA. Rather, the end-points of inversions are associated with relatively spacer-rich segments of the genome, many of which contain tRNA genes. All of the pea-specific inversions are shown to be positionally distinct from those recently described in a closely related legume, broad bean.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46965/1/294_2004_Article_BF00405856.pd