Exceptional Enlargement of the Mitochondrial Genome Results from Distinct Causes in Different Rain Frogs (Anura: Brevicipitidae: Breviceps)

The mitochondrial (mt) genome of the bushveld rain frog (Breviceps adspersus, Brevicipitidae, Afrobatrachia) is the largest (28.8 kbp) among the vertebrates investigated to date. The major cause of genome size enlargement in this species is the duplication of multiple genomic regions. To investigate the evolutionary lineage, timing, and process of mt genome enlargement, we sequenced the complete mtDNAs of two congeneric rain frogs, B. mossambicus and B. poweri. The mt genomic organization, gene content, and gene arrangements of these two rain frogs are very similar to each other but differ from those of B. adspersus. The B. mossambicus mt genome (22.5 kbp) does not differ significantly from that of most other afrobatrachians. In contrast, the B. poweri mtDNA (28.1 kbp) is considerably larger: currently the second largest among vertebrates, after B. adspersus. The main causes of genome enlargement differ among Breviceps species. Unusual elongation (12.5 kbp) of the control region (CR), a single major noncoding region of the vertebrate mt genome, is responsible for the extremely large mt genome in B. poweri. Based on the current Breviceps phylogeny and estimated divergence age, it can be concluded that the genome enlargements occurred independently in each species lineage within relatively short periods. Furthermore, a high nucleotide substitution rate and relaxation of selective pressures, which are considered to be involved in changes in genome size, were also detected in afrobatrachian lineages. Our results suggest that these factors were not direct causes but may have indirectly affected mt genome enlargements in Breviceps.

Primer retention owing to the absence of RNase H1 is catastrophic for mitochondrial DNA replication

Encoding ribonuclease H1 (RNase H1) degrades RNA hybridized to DNA, and its function is essential for mitochondrial DNA maintenance in the developing mouse. Here we define the role of RNase H1 in mitochondrial DNA replication. Analysis of replicating mitochondrial DNA in embryonic fibroblasts lacking RNase H1 reveals retention of three primers in the major noncoding region (NCR) and one at the prominent lagging-strand initiation site termed Ori-L. Primer retention does not lead immediately to depletion, as the persistent RNA is fully incorporated in mitochondrial DNA. However, the retained primers present an obstacle to the mitochondrial DNA polymerase γ in subsequent rounds of replication and lead to the catastrophic generation of a double-strand break at the origin when the resulting gapped molecules are copied. Hence, the essential role of RNase H1 in mitochondrial DNA replication is the removal of primers at the origin of replication.

Insect mitochondrial genomics 3: the complete mitochondrial genome sequences of representatives from two neuropteroid orders: a dobsonfly (order Megaloptera) and a giant lacewing and an owlfly (order Neuroptera)

We describe the complete mitochondrial genomes from representatives of two orders of the Neuropterida: a dobsonfly, Corydalus cornutus (Megaloptera: Corydalidae, GenBank Accession No. FJ171323), a giant lacewing Polystoechotes punctatus (Neuroptera: Polystoechotidae, FJ171325), and an owlfly, Ascaloptynx appendiculatus (Neuroptera: Ascalaphidae, FJ171324). The dobsonfly sequence is 15 687 base pairs with a major noncoding (A+T rich) region of approximately 967 bp. The gene content and organization of the dobsonfly is identical to that of most insects. The giant lacewing sequence is 16 036 bp with a major noncoding region of about 1123 bp, while the owlfly sequence is 15 877 bp with a major noncoding region of about 1066 bp. The two Neuroptera sequences include a transposition of two tRNA genes, tRNATrp and tRNACys. These tRNA genes are coded on opposite strands and overlap by seven residues in the standard insect mitochondrial gene arrangement. Thus, the transposition required a duplication of at least the region of overlap. It is likely that the transposition occurred by a duplication of both genes followed by deletion of one copy of each gene. Examination of this region in two other neuropteroid species, a snakefly, Agulla sp. (Raphidioptera: Raphidiidae), and an antlion, Myrmeleon immaculatus (Neuroptera: Myrmeleontidae), shows that the rearrangement is widespread in the order Neuroptera but not present in either of the other two orders of Neuropterida.

The Complete Mitochondrial DNA Sequence of the Bichir (Polypterus ornatipinnis), a Basal Ray-Finned Fish: Ancient Establishment of the Consensus Vertebrate Gene Order

The evolutionary position of bichirs is disputed, and they have been variously aligned with ray-finned fish (Actinopterygii) or lobe-finned fish (Sarcopterygii), which also include tetrapods. Alternatively, they have been placed into their own group, the Brachiopterygii. The phylogenetic position of bichirs as possibly the most primitive living bony fish (Osteichthyes) made knowledge about their mitochondrial genome of considerable evolutionary interest. We determined the complete nucleotide sequence (16,624 bp) of the mitochondrial genome of a bichir, Polypterus ornatipinnis. Its genome contains 13 protein-coding genes, 22 tRNAS, two rRNAs and one major noncoding region. The genome's structure and organization show that this is the most basal vertebrate that conforms to the consensus vertebrate mtDNA gene order. Bichir mitochondrial protein-coding and ribosomal RNA genes have greater sequence similarity to ray-finned fish than to either lamprey or lungfish. Phylogenetic analyses suggest the bichir's placement as the most basal living member of the ray-finned fish and rule out its classification as a lobe-finned fish. Hence, its lobe-fins are probably not a shared-derived trait with those of lobe-finned fish (Sarcopterygii).