title: A high density of ancient spliceosomal introns in oxymonad excavates.
abstract: BACKGROUND
  Certain eukaryotic genomes, such as those of the amitochondriate parasites Giardia and Trichomonas, have very low intron densities, so low that canonical spliceosomal introns have only recently been discovered through genome sequencing. These organisms were formerly thought to be ancient eukaryotes that diverged before introns originated, or at least became common. Now however, they are thought to be members of a supergroup known as excavates, whose members generally appear to have low densities of canonical introns. Here we have used environmental expressed sequence tag (EST) sequencing to identify 17 genes from the uncultivable oxymonad Streblomastix strix, to survey intron densities in this most poorly studied excavate group.
  
  
  RESULTS
  We find that Streblomastix genes contain an unexpectedly high intron density of about 1.1 introns per gene. Moreover, over 50% of these are at positions shared between a broad spectrum of eukaryotes, suggesting they are very ancient introns, potentially present in the last common ancestor of eukaryotes.
  
  
  CONCLUSIONS
  The Streblomastix data show that the genome of the ancestor of excavates likely contained many introns and the subsequent evolution of introns has proceeded very differently in different excavate lineages: in Streblomastix there has been much stasis while in Trichomonas and Giardia most introns have been lost.
authors.affiliation: Canadian Institute for Advanced Research, Botany Department, University of British Columbia, 3529-6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada. claudio@mail.botany.ubc.ca
authors.forename: Claudio H
authors.initials: CH
authors.lastname: Slamovits
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Keeling
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005057:Eukaryotic Cells / Q000145:classification / Q000378:metabolism; D019143:Evolution, Molecular; D020224:Expressed Sequence Tags / Q000737:chemistry; D005872:Giardia / Q000235:genetics; D007438:Introns; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D017381:Spliceosomes / Q000378:metabolism; D014244:Trichomonas / Q000235:genetics