Chromosome races with Pliocene origins: evidence from mtDNA

There are eight distinct chromosomal races of the New Zealand weta Hemidein a thoracica. We used mtDNA sequence data to test the hypothesis that these races originated on islands during the early Pliocene (7-4 million years ag o). Nine major mitochondrial lineages were identified from 65 cytochrome ox idase I sequences. Phylogenetic analysis of these lineages suggests that th ey arose at approximately the same time. The geographical distribution of s ome lineages coincides with areas that were islands during the Pliocene. Ov erall, hierarchical AMOVA analysis shows that chromosomal races and Pliocen e islands describe only 28% and 24%, respectively, of the total current mtD NA variation. However, removing one widespread (A) and one putatively intro gressed (F) lineage increases these estimates to 65% and 80%, respectively. Intraspecific sequence divergence was very high, reaching a maximum of 9.5 % (uncorrected distance) and GC content was high compared to other insect m tDNA sequences. Average corrected distance among mtDNA lineages supports th e Pliocene origins of this level of genetic diversity. In the southern part of the species range there is reduced mtDNA variation, probably related to local extinction of H. thoracica populations from recent volcanic activity and subsequent re-colonization from a leading edge. In contrast, in this s outhern part there are five chromosome races, suggesting that chromosome ra ces here may be younger than those in the north.