Mate choice in the face of both inbreeding and outbreeding depression in the intertidal copepod Tigriopus californicus

 In species vulnerable to both inbreeding and outbreeding depression, individuals might be expected to choose mates at intermediate levels of genetic relatedness. Previous work on the intertidal copepod Tigriopus californicus has repeatedly shown that crosses between populations result in either no effect or hybrid vigor in the first generation, and hybrid breakdown in the second generation. Previous work also shows that mating between full siblings results in inbreeding depression. The present study again found inbreeding depression, with full sibling mating causing significant fitness declines in two of the three populations assayed. In the mate choice assays, a single female was combined with two males. Despite the costs of both inbreeding and outbreeding, mate choice showed clear inbreeding avoidance but no clear pattern of outbreeding avoidance. This lack of outbreeding avoidance may be attributed either to the temporary increase in fitness in the F₁ generation or to the absence of selection for premating isolation in wholly allopatric populations with infrequent migration. If this inability to avoid unwise matings is common to other taxa, it may contribute to the problem of outbreeding depression when allopatric populations are mixed together. Received: 18 May 1999 / Accepted: 25 January 2000