Fig. 1.

Identifying novel introns. To ensure that a putative novel intron was almost certainly caused by insertion rather than by deletion, we drew phylogenetic trees of the gene and its animal and nematode orthologs. We required that there be at least three nodes between the gene and the outgroup. We also required that, in a protein alignment of the gene and its orthologs, ≥5/10-aa residues on either side of the intron be identical or well conserved among the animal genomes.