Well prepared: It is well known that species radiate into new niches by adapting to novel environments. But why do some species radiate in this way, while other, related, species do not. Ishikawa et al. looked across sticklebacks to determine why some, originally marine, lineages were able to colonize postglacial freshwater environments (see the Perspective by Weber and Tong). They found that a gene involved in fatty acid desaturation was duplicated in freshwater lineages. Transgenic manipulation of this gene allowed marine lineages to synthesize fatty acids and thus survive on fatty acid–deficient freshwater diets. Science , this issue p. 886 ; see also p. 831

Two ecologically distinct forms, fresh- and brackish-water types, of ninespine stickleback co-exist in several freshwater systems on the coast of eastern Hokkaido. Recent genetic analyses of 13 allozyme loci revealed genetic separation between the two types even though their spawning grounds were in close proximity. On the other hand, there is only a small difference in mitochondrial DNA (mtDNA) sequence between the two types suggesting that they diverged quite recently or that mtDNA introgression occurred between them. To test for postzygotic reproductive isolating mechanisms and hybrid mediated gene flow, we examined the viability and reproductive performance of reciprocal F1 hybrids. The hybrids grew to the adult size normally and both sexes expressed secondary sexual characters in the reciprocal crosses. The female hybrids were reciprocally fertile, while the male hybrids were reciprocally sterile. Histological and flow-cytometric analyses of the hybrid testis revealed that the sterility pattern was classified as `gametic sterility,' with gonads of normal size but abnormal spermatogenesis. To our knowledge, the present finding is a novel example of one sex hybrid sterility in the stickleback family (Gasterosteidae)