The genetic mating system and tests for cuckoldry in a pipefish species inwhich males fertilize eggs and brood offspring externally

Highly variable microsatellite loci were used to study the mating system of Nerophis ophidion, a species of pipefish in which pregnant males carry emb ryos on the outside of their body rather than in an enclosed brood pouch. D espite this mode of external fertilization and brooding, otherwise rare in the family Syngnathidae, the genotypes of all embryos proved to be consiste nt with paternity by the tending male, thus indicating that cuckoldry by sn eaker males is rare or nonexistent in this species. N. ophidion is a phylog enetic outlier within the Syngnathidae and its reproductive morphology is t hought to be close to the presumed ancestral condition for pipefishes and s eahorses. Thus, our genetic results suggest that the evolutionary elaborati on of the enclosed brood pouch elsewhere in the family was probably not in response to selection pressures on pregnant males to avoid fertilization th ievery. With regard to maternity assignments, our genotypic data are consis tent with behavioural observations indicating that females sometimes mate w ith more than one male during a breeding episode, and that each male carrie s eggs from a single female. Thus, the polyandrous genetic mating system in this species parallels the social mating system, and both are consistent w ith a more intense sexual selection operating on females, and the elaborati on of secondary sexual characters in that gender.