Re: Introns in mitochondria and the endosymbiotic theory

With the advent of automated, high-throughput sequencing methods there has been a great deal of recent evolutionary research based the comparison of large amount of genomic data, including complete genomes. Among the genome sequencing efforts being performed around the world there is a fungal mitochondrial genome sequencing project (Paquin et al. Current Genetics, vol 31, pp. 380-395, 1997). Data from this project has illuminated many aspects of the evolution of fungal mitochondria, including the evolution of mitochondrial introns.

Because of genomic similarities it is believed that mitochondria evolved from a prokaryotic ancestor in an ancient unicellular lineage. Comparisons of mitochondrial DNA sequences with prokaryotic genomes indicates that the best candidate for this event is in the alpha-proteobacteria. In these prokaryotes one species has one intron in a tRNA sequence. However, this intron has no similarity (in either location or sequence) with introns found in yeast mitochondria. Though there is evidence that some fungal mitochondria introns have an ancient origin, the evidence does not support the presence of any fungal mitochondrial introns in the mitochondrial prokaryotic ancestor. There is a great deal more data on fungal mitochondrial genomes available from the website for the Fungal Mitochondrial Genome Project at
http://www.bch.umontreal.ca/People/lang/FMGP/FMGP.html

Comparison of fungal mitochondrial genomes has found introns with similar sequence at different genomic locations among different species. Among those who believe introns are a recent genomic acquisition, transposable elements are thought to be one source of these intervening sequences. The pattern of fungal intron location mentioned above is consistent with a transposition scenario for intron evolution. Other potential sources of mitochondrial introns are the exchange of genetic material with the nucleus (Blanchard and Lynch, 2000, Trends in Genetics, vol 16, pp. 315-320) and horizontal transfer across lineages.