‘Out of Africa’ out of the window?

Abstract

One of the dogmas frequently quoted in (mammalian) mitochondrial genetics is the absence of recombination. The matrilineal inheritance of mtDNA in humans has allowed the construction of a detailed phylogeny of human populations, in which the entire mtDNA sequence of an individual is regarded as a haplotype that never exchanges information with any other haplotype. The data to date strongly supports an African origin for modern humans, but this has been thrown into doubt by a recent comparison of mtDNA sequences both in humans and in chimpanzees.Awadalla et al.1xLinkage disequilibrium and recombination in hominid mitochondrial DNA. Awadalla, P. et al. Science. 1999; 286: 2524–2525Crossref | PubMed | Scopus (194)See all References1 looked at the question of linkage disequilibrium: essentially, the chance that two different polymorphisms are found together in different populations. They found a straightforward inverse relationship between the physical distance separating polymorphic sites in mtDNA, and the probability that they were co-inherited: exactly what one would predict if recombination between mtDNAs were permitted during the reproductive cycle.This could take place only if paternal mtDNA can interact and recombine with the mtDNA stored in the oocyte, although it does not mean that this need take place routinely, and it can still accommodate the view that paternal mtDNA is usually eliminated.However, it does mean that novel mtDNA haplogroups can, in principle, be created by a rare mating between two individuals from different populations. Such an event might be deemed relatively likely during a migration in which a founder female from a foreign population arrived on a distant shore. Any of her male companions on the journey are likely to have been eliminated as efficiently as their mtDNA, leading to a repeated backcross of her maternal line to the indigenous male population, maximizing the chance that any mitochondrial recombination would create a new haplogroup.Thus, we might need to ask again all of the anthropological questions that mtDNA analysis was thought to have settled; the answers could prove to be far more complex than was originally imagined. Alternatively, believers in the ‘no recombination’ theory will need a testable alternative hypothesis to explain mitochondrial linkage disequilibrium.