Population genetics

The genetic legacy of paleolithic Homo sapiens sapiens in extant Europeans: a Y chromosome perspective. Semino, O. et al. Science 290 , 1155–1159 (2000) [PubMed]

By typing 1007 Y chromosomes from 25 European and Middle Eastern regions for 22 biallelic markers, this study found that 95% of these Y chromosomes could be classified by just 10 key mutations, which reveal the history and origins of European populations. Two mutations, for example, have been present in Europe since Paleolithic times, and the remaining mutations probably entered Europe during independent migrations from the Middle East and the Urals. The study also shows that Y-chromosome variation patterns have been less influenced by natural selection than by migration, particularly during the last ice age.

Evolution

Adaptive amplification: an inducible chromosomal instability mechanism. Hastings, P. J. et al. Cell 103 , 723–731 (2000) [PubMed]

According to the Darwinian view of genetic change, natural selection selects the fittest organism among pre-existing variants. However, it has also been shown that some organisms undergo adaptive mutation, whereby mutations arise in response to, rather than before, exposure to a changing environment. The traditional explanation for adaptive mutation invokes a rise in genetic mutation rates, which increase an organism's adaptability. The amplification of the lac operon in E. coli under selective conditions, as shown here, provides evidence that DNA amplification, like hypermutation, can be an adaptive process. Cells use adaptive point mutation and amplification as two independent mechanisms, to confer a permanent or reversible response to stress, respectively.

Genome manipulation

Illegitimate Cre-dependent chromosome rearrangements in transgenic mouse spermatids. Schmidt, E. E. et al. Proc. Natl Acad. Sci. USA 97 , 13702–13707 (2000) [PubMed]

The Cre–Lox system is a popular way to generate conditional mutations in the mouse (see Highlight on page 7 ). Activating the expression of the Cre recombinase gene at a particular time or place causes mutations by catalysing recombination between LoxP sites introduced into the genome. But there might be a problem. Schmidt et al. show that Cre can induce recombination in the mouse in the absence of LoxP sites. In their study, the expression of Cre in the male germ line caused 100% male sterility as a result of chromosomal rearrangements that produced inviable embryos. It is not known whether Cre can mediate such recombination in somatic tissue but, to limit unwanted recombination in conditional transgenesis experiments, it might be wise to switch Cre off once its job is done.