Mitochondrial DNA in the sea urchin Arbacia lixula: Nucleotide sequence differences between two polymorphic molecules indicate asymmetry of mutations

Two polymorphic forms of mitochondrial DNA (mtDNA) extracted from Arbacia lixula eggs were cloned and the nucleotide sequences of specific regions determined. A comparison of the sequences of the sense strand of the two molecules demonstrates that all the differences are transitions and only of the A?G type. A change such as G->A (or A->G) on the sense mtDNA strand results from either a direct G->A (or A->G) mutation on that strand or a C->T (or T->C) on the complementary strand. None of the C->T (or T->C) changes were detected on the sense strand, which implies that the A?G mutation bias on the sense strand is not reversed for the other strand. Our observation indicates the existence of mechanisms acting asymmetrically on the two mtDNA strands, possibly during mtDNA replication.