Similar evolutionary patterning among repeated and single copy nematode mitochondrial genes.

Ten of 12 mitochondrial protein-coding genes and the large (16S) mitochondrial rRNA gene have been identified and mapped within the Romanomermis culicivorax mitochondrial genome. This transcriptional map differs from other nematode mitochondrial DNAs (mtDNAs) with respect to gene order and transcriptional orientation of some genes. Several of these coding regions are components of a 3.0-kilobase mtDNA repeating unit, allowing a direct comparison of nucleotide and amino acid sequence composition for repeated and single copy genes. Analysis of protein-coding regions representing repeated (ND3, ND6) and single copy genes (ATPase 6, cyt.b, COI, COIII, ND1, ND4, ND5), and four repeat-associated open reading frames (ORFs) with unassigned function have revealed striking similarities in nucleotide composition, amino acid frequencies, and codon biases. Although we anticipated that reiterated protein coding regions might be evolving under relaxed selection, our results indicate that both repeated and unique mitochondrial genes appear subject to similar functional constraints.