As these introns are located in a gene portion encoding motif I and III respectively, a high selection pressure could be exerted on these parts, such as for instance the presence of regulatory elements that are important for gene expression. The i8l insertion site is only found in lepidoptera and some diptera, such as culicidae and psychodoidea. This site is very close to the chordate site i7c with only 24 nucleotides between these two sites forming a ����near intron pair���� as defined by. In contrast to i8l, i7c is conserved in many arthropods, cnidarians and nematodes. Thus, position i7c appeared before i8l and we hypothesize that the i8l site might have resulted from i7c sliding. Cinoxacin Similarly, the chordate insertion site i4c is found in the arthropod S. maritima. Comparison of the lepidoptera and chordata exon pairs 1-55-0/0-62-1 and 1-38-0/ 0-78-1, located between the conserved sites i3l and i5l, suggests the existence of a common ancestral exon. As hypothesized for i8l, position i4l could have BAY 80-6946 derived from the sliding of the ancestral position i4c. However, unlike i7c, i4c is not well conserved outside chordates. To conclude, we propose a model of fut8 gene evolution in animals. In this model, i7c and i9l are considered as ancestral intron insertion sites. Until the arthropoda-chordata split, intron gain seems to have been the most favored event with gain of i7l about 855MYA and of i3l, i5l, i4c about 783 MYA. After this divergence, intron loss seems to have become rather more common with the consecutive loss in arthropods of i3l, i7c, i5l and i9l. These intron losses could be accompanied by specific intron site gains or not. It has to be noted that the intron site gains and losses identified in arthropods are not observed in chordates. Indeed, insertions of spliceosomal introns are rarely observed during evolution of vertebrates. As fut8 intron-exon organization is order-specific, these intron losses and gains may be linked to evolutionary innovations, such as appearance of new orders. Finally, the gain of some intron sites, such as i8l in some diptera, i10l inC. intestinalis and i6l inT. spiralis, may be explained by convergent/parallel intron gains, as recently described by for hymenopteran paralogs, suggesting the presence of intron insertion hot spots.