Hybrids that maintain the parental chromosome number (homoploid hybrid speciation) and are fertile, are uncommon in plants and unknown in animals. In November 29 Science, Duncan Greig and colleagues at The Galton Laboratory, University College London, UK, show that homoploid hybrid speciation can easily occur between two experimental populations of yeast and the resulting hybrids are fertile (Science, 298:1773-1775, November 29, 2002).

Greig et al. used the ability of Saccharomyces gametes to divide and switch mating type to test the potential for instantaneous homoploid hybrid speciation. They crossed S. cerevisiae and S. paradoxus and obtained 80 independent viable haploid gametes from their F1 hybrid offspring. They showed that the resulting hybrids had high self-fertility (about 82%), low fertility when backcrossed to either parental species (about 7.5%), and vigorous growth under different thermal environments. In addition, they observed extensive karyotypic changes (tetrasomy) in the hybrids,...