Noncanonical GA and GG 5′ Intron Donor Splice Sites Are Common in the Copepod Eurytemora affinis

The noncanonical 5′ intron donor splice sites GA and GG are exceedingly rare in described eukaryotic genomes; however, they are present in ∼12% of introns in the genome of the copepod Eurytemora affinis. Failure to recognize the high frequency of these donor sites compromised the modeling of genes in this newly sequenced genome, including 10 conserved ionotropic glutamate receptor (GluR) family genes curated herein. These introns appear to have been acquired recently, along with many additional idiosyncratic introns. Their high frequency implies the evolution of modified intron donor splice site recognition in this copepod.