Uncovering historical signature of mitochondrial DNA hidden in the nuclear genome: the biogeography of Schistocerca revisited.

Inadvertent coamplification of nuclear mitochondrial pseudogenes (numts) is a serious problem in mitochondrial systematics, but numts can also be a valuable source of information because they represent ancient forms of mtDNA. We present a conceptual framework of numt accumulation, which states that in a given species there can be two types of numts, synaponumts and autaponumts, resulting from integration occurring respectively before and after a speciation event. In a given clade, a species that diverged early can only have its own autaponumts as well as synaponumts that were already present in the genome of the last common ancestor. A species that diverged more recently may, however, have many different synaponumts integrated at each different divergence as well as its own autaponumts. Therefore it is possible to decipher the evolutionary history of a species based on the phylogenetic distribution of numts in a simultaneous analysis of numts and extant mtDNA. In this study, we test this idea empirically in the context of addressing a controversial question regarding the biogeography of the grasshopper genus Schistocerca Stål (Orthoptera: Acrididae), based on numts of the cytochrome c oxidase subunit I (COI) gene. We find that our empirical data can be explained adequately by our conceptual framework, and that the phylogenetic distribution of COI numts reveals intricate evolutionary histories about past speciation events that are otherwise difficult to detect using conventional markers. Our study strongly favours the Old World origin of the desert locust, Schistocerca gregaria and the New World Schistocerca species are descendants from an ancestral gregaria-like species that colonized the New World via westward transatlantic flight. However, the phylogenetic distribution of S. gregaria numts raises a distinct possibility that there might have been multiple founding events from Africa to America to give rise to the present-day diversity of the genus. This is a case study for a creative use of numts as molecular fossils, and we demonstrate that numts provide an interesting and powerful phylogenetic signal, much more than what extant mtDNA or nuclear gene sequences might be able to provide.