Origins and evolution of trans-splicing of bursicon in mosquitos

Broad transcriptomic sequencing of eukaryotes has revealed the ubiquity of splicing of nuclear genes. While the vast majority of splicing events join segments of the same RNA transcript, various studies have found a few intriguing cases of trans-splicing of introns, in which splicing events within protein coding regions join segments of different RNA transcripts. The most structurally intricate case known involves the bursicon gene in mosquitos, in which an internal exon is encoded at a distinct locus, requiring multiple trans-splicing events form the mature mRNA. This arrangement is known to be ancestral to mosquitos, however the exact timing of the origin of trans-splicing and the history of the bursicon gene within mosquitos is unknown. Taking advantage of the recent availability of genomes from various Anopheles mosquitos and from relatives of mosquitos, I determined trans versus cis encoding of bursicon across Culicomorpha. I conclude that trans-splicing emerged in the last common ancestor of mosquitos, and that trans-splicing has been retained in all 19 studied Anopheles species. The retention of trans-splicing could indicate functional importance of this arrangement, or could alternatively reflect the rarity of mutations giving rise to viable allelic alternatives.