How fish find brothers and sisters.

Young Arctic char, which respond to the scent of siblings they've never met, may accomplish the trick by tracking clues controlled by immune-system genes.

Normal char, a salmon relative, prefer a sibling's scent over a stranger's even if the youngsters have been separated since fertilization, note K. Hakan Olsen of Uppsala University in Sweden and his colleagues. To recognize kin, the fish do need some early exposure to their own species, though not to their own family. Char reared in complete isolation don't seem able to pick out siblings.

To begin unraveling how fish recognize kin, Olsen's group set up an artificial stream offering fish a choice of water sources. The researchers checked a segment of the fishes' DNA in one of the major histocompatibility complex (MHC) genes, whose human versions are key to the immune system. These genes have been implicated in kin recognition among a variety of animals.

The fish preferred water scented by a sibling with an identical DNA segment rather than water from a sibling with a different segment. Moreover, the nonmatching sibling won out over an unrelated fish, which also had a nonmatching DNA segment. In the August Animal Behaviour, the researchers argue that MHC "has a significant influence" on cues for recognizing fishy kin.