Unprecedented frequency of mitochondrial introns in colonial bilaterians

Abstract

Animal mitogenomes are typically devoid of introns. Here, we report the largest number of mitochondrial introns ever recorded from bilaterian animals. Mitochondrial introns were identified for the first time from the phylum Bryozoa. They were found in four species from three families (Order Cheilostomatida). A total of eight introns were found in the complete mitogenome of Exechonella vieirai, and five, 17 and 18 introns were found in the partial mitogenomes of Parantropora penelope, Discoporella cookae and Cupuladria biporosa, respectively. Intron-encoded protein domains reverse transcriptase and intron maturase (RVT-IM) were identified in all species. Introns in E. vieirai and P. penelope had conserved Group II intron ribozyme domains V and VI. Conserved domains were lacking from introns in D. cookae and C. biporosa, 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 domains. Results confirmed previous findings of multiple origins of annelid, placozoan and sponge RVT-IM domains 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.

Rediscovering the unusual, solitary bryozoan Monobryozoon ambulans Remane, 1936: first molecular and new morphological data clarify its phylogenetic position

“…The mitochondrial genome of M. ambulans shows typical characteristics of metazoan mitogenomes, consisting of 13 PCGs., two rRNAs and 22 tRNAs. The size of M. ambulans mitochondrial genomes is 17,386 bp in length which falls within the size range of other studied mitogenomes of gymnolaemate bryozoans that range from 13,026 bp in F. hispida [ 75 ] to 23,057 bp Exechonella vieirai [ 76 ]. The GC content of M. ambulans mitogenome was 33.3% which is somewhat higher than the GC content of B. neritina (30%) [ 44 ] (Jang et al 2009) and lower than F. hispida (41.6%) [ 75 ].…”

The invasive land flatworm Arthurdendyus triangulatus has repeated sequences in the mitogenome, extra-long cox2 gene and paralogous nuclear rRNA clusters

“…5 ), we could identify which conserved residues are present in the extension segment of the cox2 -encoded protein of Rhynchodeminae. Introns in the mitogenomes of Metazoa are rather scarce, especially among bilaterians 70 , and we could not detect introns in the cox2 gene. One of the explanations for the presence of this extra segment was that it could corresponds to an intein, i.e.…”