Unprecedented frequency of mitochondrial introns in colonial bilaterians

Animal mitogenomes are typically devoid of introns. Here, we report the largest number of mitochondrial introns ever recorded from bilaterian animals. Group II introns were identified in four species of bryozoans from three families (Order Cheilostomatida). A total of eight introns were found in the complete mitogenome of Exechonella vieirai, and five, 18 and 19 introns were found in the partial mitogenomes of Parantropora penelope, Discoporella sp. and Cupuladria cf. biporosa, respectively. Intron-encoded protein reverse transcriptase and intron maturase open-reading frames (RVT-IM ORFs) were identified in all species. Introns in E. vieirai and P. penelope had conserved group IIA intron ribozyme domains V and VI. Conserved domains were lacking from introns in Discoporella sp. and C. cf. bicornis, preventing their further categorization. Putative origins of metazoan introns were explored in a phylogenetic context, using an up-to-date alignment of mitochondrial RVT-IM ORFs. Results confirmed previous findings of multiple origins of annelid, placozoan and sponge RVT-IM ORFs and provided evidence for common intron donor sources across metazoan phyla. Our results corroborate growing evidence that some metazoans with regenerative abilities (i.e. placozoans, sponges, annelids and bryozoans) are susceptible to intron integration, most likely via horizontal gene transfer.