A recent study made by German scientists revealed that
Takifugu rubripes, Tetraodon nigroviridis, Oryzias latipes, Gasterosteus
aculeatus and Danio rerio genomes are respectively comprised of 2.83%,
3.42%, 4.49%, 4.35% and 4.02% single exon genes (SEGs). These SEGs encode for a variety of
family proteins including claudins, olfactory receptors and histones
that are essential for various biological functions. Annotation features of three Dicentrarchus labrax chromosomes revealed 78 (5.30%)
intronless genes, comparisons with G. aculeatus showed that SEG
composition and their order varied significantly among corresponding
chromosomes, even for those with nearly complete synteny. More than half
of SEGs identified in most of the species have at least one ortholog
multiple exon gene in the same genome, which provides insight to their
possible origin by retrotransposition. In spite of the fact that they
belong to the same lineage, the fraction of predicted SEGs varied
significantly between the genomes analyzed, and only a low fraction of
proteins (4.1%) is conserved between all five species. Furthermore, the
inter-specific distribution of SEGs as well as the functional categories
shared by species did not reflect their phylogenetic relationships.
These results indicate that new SEGs are continuously and independently
generated after species divergence over evolutionary time as evidenced
by the phylogenetic results of single exon claudins genes. Results of this study provide strong support for the idea that retrotransposition
followed by tandem duplications is the most probable event that can
explain the expansion of SEGs in eukaryotic organisms.
Study was published in Marine Genomics. 2011 4(2):109-19.
