Why do crosses between closely related species fail to produce fertile hybrids? A new study led by Professor Axel Imhof of Ludwig-Maximilians-Universitaet (LMU) in Munich shows that differences in the levels - not necessarily the sequences - of certain key proteins are crucial in mediating reproductive isolation.
Two individuals are defined as belonging to the same biological species, if matings between them give rise to viable and fertile offspring. Crosses between closely related, but already distinct, species produce hybrid offspring that are either inviable or sterile, and thus cannot give rise to a self-propagating hybrid lineage. In the early 20th century, geneticists and evolutionary biologists developed a theoretical model to explain why distinct species that share the same common ancestor soon diverge to such an extent that their hybrids are unable to reproduce. This model postulates that certain genes evolve more rapidly than others and in a manner which ensures that they function well within each of the diverging populations, but interfere with one another when brought together in hybrid genomes.
Such genes are referred to as Dobzhansky-Muller gene pairs, or hybrid-incompatibility genes. "Although several Dobzhansky-Muller gene pairs have been isolated in the last five years, their function remained essentially unclear prior to our study," says Professor Axel Imhof of LMU's Adolf Butenandt Institute. In their new study, he and his team have now characterized the functions of the genes Lmr (Lethal male rescue) and Hmr (Hybrid male rescue), which form a Dobzhansky-Muller gene pair in hybrid matings between fruitflies belonging to the closely related Drosophila species D. melanogaster and D. simulans.
Hybrid males produced by crosses between these two species are inviable, while the female hybrids are viable but sterile. Imhof's group has now shown that the proteins encoded by the genes Hmr and Lhr form a
|Contact: Luise Dirscherl|