For example, the main mitochondrial coding strand in Schistosoma mansoni (a flatworm) contains only 6.7% C, but it contains 25% G. Despite the contrasting GC and AT skew patterns, however, both mammals and flatworms contain essentially the same set of homologous mitochondrial genes and they span similar ranges of GC content.
The strand specific mutational bias in R. compacta is inverted on the main-coding strand compared to most other deuterostome genomes.
Vertebrate genomes present pronounced AT- and GC-skews on the two strands having an AC-rich main-coding strand (e.g. [ 25]).
The GT-rich main-coding strand of R. compacta exhibits all protein-coding genes showing an increase of GT-rich codons at the expense of AC-rich codons.
As most metazoan mitochondrial genomes show an AT-rich main-coding strand [ 23], the tRNAs preferentially have GNN anticodons for NNY codons, UNN for NNR codons, and also UNN for the four-fold degenerate codon families [ 19, 20].
The main-coding strand bears several oligoT-stretches with lengths of six (48 times), seven (38 times), eight (19 times) and nine nucleotides (10 times), while the longest stretch of 13 T's is found only once.
However, some mitochondrial genomes possess an inverse strand-specific mutational pattern showing a GT-rich main-coding strand, e.g. Branchiostomia (Cephalochordata) and Crinoidea (Echinodermata), which is usually explained by an inversion of the replication direction of the mtDNA [ 23, 29].
It shows one of the strongest strand-specific biases of metazoan genomes with high amounts of Thymine (48%) and Guanine (23%) but low amounts of Adenine (18%) and Cytosine (11%) on the main-coding strand causing strong negative AT- and positive GC-skews.
The mitochondrial DNA of Rhabdopleura compacta corresponds in size and gene content to typical mitochondrial genomes of metazoans, but shows the strongest known strand-specific mutational bias in the nucleotide composition among deuterostomes with a very GT-rich main-coding strand.
The strand biases of the main-coding strand, given as the AT-skew [ A-T /(A + T)] and the GC-skew [ A-T /(G + C)] [ 57], were cAlculated using an automatic tool developed for this purpose for both the complete genome of R. compacta and all complete mitogenomes available in NCBI RefSeq 41 [ 58].
Moreover, this sequence was present in the non-coding strand of all alleles at coding strand position 37 42 (Table 2).
