Lack of host specialization despite selective host use in brood parasitic cuckoo catfish

Host-parasite dynamics involves coevolutionary arms races, commonly leading to host specialization. General understanding of evolutionary trajectories of specialization in brood parasites is compromised by restricted focus on bird and insect lineages. We studied host utilization and host specificity in a natural population of the cuckoo catfish (Synodontis multipunctatus) which is an obligate parasite of parental care of mouthbrooding cichlids in Lake Tanganyika. On a sample of 779 host broods from 20 cichlid species, we detected four host species (with prevalence of parasitism of 2-18%). Phylogenetic analysis based on genomic (ddRAD sequencing) and mitochondrial (Dloop) data from cuckoo catfish embryos showed an absence of host-specific lineages, despite former indications of two morphological forms of the cuckoo catfish. This was corroborated by analyses of genetic structure and co-ancestry matrix. All host species were from the tribe Tropheini, maternal mouthbrooders that spawn over a substrate (rather than in open water). Parasitized host individuals carried smaller clutches (as cuckoo catfish prey on cichlid eggs), but did not differ in their body size or habitat use from non-parasitized conspecifics. We conclude that the cuckoo catfish is an intermediate generalist, selecting a subset of available cichlid species as hosts but not forming host-specific lineages. Brood parasitism in the cuckoo catfish arose in a lineage which lacks any parental care and we discuss costs and benefits of host specialization in this species and brood parasites in general.