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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 theyare very ancient introns, potentially present in the last common ancestor of eukaryotes.    Conclusion  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.

A high density of ancient spliceosomal introns in oxymonad excavates.