Pattern of conservation between species for one of the C. briggsae genes with a newly acquired intron, and a hypothesis for intron acquisition. (a) Intron presence and absence, and sequence similarity, for C. briggsae gene CBG18597 and orthologs. Introns 1 and 5 are common to all orthologs; intron 3 is unique to C. briggsae. Black boxes represent exons and lines represent introns. The gray boxes represent the common palindromic sequence within introns. Other introns in the genes are omitted for simplicity (indicated by breaks in the boxes). Not drawn to scale. (b) Probable origin of the sequence similarity between introns 1 and 5. The absence of the common palindromic sequence in introns of orthologs suggests that both C. briggsae introns 1 and 5 acquired the sequence through independent recent transposon insertions. Intron 3 may contain the common sequence as a result of transposition of another intron (interpretation of Coghlan and Wolfe ), by creation of the intron by a third transposon-insertion event (argued here) or via a third transposon insertion into a previously existing intron 3.