Insertion of a self-splicing intron into the mtDNA of atriploblastic animal
Nephtys longosetosa is a carnivorous polychaete worm that lives in the intertidal and subtidal zones with worldwide distribution (pleijel&rouse2001). Its mitochondrial genome has the characteristics typical of most metazoans: 37 genes; circular molecule; almost no intergenic sequence; and no significant gene rearrangements when compared to other annelid mtDNAs (booremoritz19981995). Ubiquitous features as small intergenic regions and lack of introns suggested that metazoan mtDNAs are under strong selective pressures to reduce their genome size allowing for faster replication requirements (booremoritz19981995Lynch2005). Yet, in 1996 two type I introns were found in the mtDNA of the basal metazoan Metridium senile (FigureX). Breaking a long-standing rule (absence of introns in metazoan mtDNA), this finding was later supported by the further presence of group I introns in other cnidarians. Interestingly, only the class Anthozoa within cnidarians seems to harbor such introns. Although several hundreds of triploblastic metazoan mtDNAs have been sequenced, this study is the first evidence of mitochondrial introns in triploblastic metazoans. The cox1 gene of N. longosetosa has an intron of almost 2 kbs in length. This finding represents as well the first instance of a group II intron (anthozoans harbor group I introns) in all metazoan lineages. Opposite trends are observed within plants, fungi and protist mtDNAs, where introns (both group I and II) and other non-coding sequences are widespread. Plant, fungal and protist mtDNA structure and organization differ enormously from that of metazoan mtDNA. Both, plant and fungal mtDNA are dynamic molecules that undergo high rates of recombination, contain long intergenic spacer regions and harbor both group I and group II introns. However, as metazoans they have a conserved gene content. Protists, on the other hand have a striking variation of gene content and introns that account for the genome size variation. In contrast to this mtDNA structure and organization diversity, current genome level studies point to a monophyletic origin of the mitochondria (REFS), raising questions such as: what are the pressures at work shaping the evolution of the mitochondrial genome at 'higher' levels? What drives the absence of introns and other non-coding spacers in metazoan mtDNA? What characteristics must have an intron to be maintained in an environment where 'extra chromosomes' are usually selected against?
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director, Office of Science; National ScienceFoundation
- DOE Contract Number:
- DE-AC02-05CH11231; NSF:EAR-0342392
- OSTI ID:
- 932678
- Report Number(s):
- LBNL-60476; R&D Project: Y00010; BnR: 600305000; TRN: US200813%%460
- Journal Information:
- Public Library of Science One, Vol. 3, Issue 1; Related Information: Journal Publication Date: 2008
- Country of Publication:
- United States
- Language:
- English
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