Abstract
BackgroundTrait polymorphism can evolve as a consequence of frequency-dependent selection. Coevolutionary interactions between hosts and parasites may lead to selection on both to evolve extreme phenotypes deviating from the norm, through disruptive selection.Methodology/Principal findingHere, we show through detailed field studies and experimental procedures that the ashy-throated parrotbill (Paradoxornis alphonsianus) and its avian brood parasite, the common cuckoo (Cuculus canorus), have both evolved egg polymorphism manifested in discrete immaculate white, pale blue, and blue egg phenotypes within a single population. In this host-parasite system the most common egg colours were white and blue, with no significant difference in parasitism rates between hosts laying eggs of either colour. Furthermore, selection on parasites for countering the evolution of host egg types appears to be strong, since ashy-throated parrotbills have evolved rejection abilities for even partially mimetic eggs.Conclusions/SignificanceThe parrotbill-cuckoo system constitutes a clear outcome of disruptive selection on both host and parasite egg phenotypes driven by coevolution, due to the cost of parasitism in the host and by host defences in the parasite. The present study is to our knowledge the first to report the influence of disruptive selection on evolution of discrete phenotypes in both parasite and host traits in an avian brood parasitism system.
Highlights
Polymorphism in natural populations can evolve and be maintained as a consequence of frequency-dependent predation [1,2], with the textbook example being industrial melanism in the peppered moth (Biston betularia) [1,3]
We show that a passerine host of the common cuckoo (Cuculus canorus) has evolved three discrete egg colour morphs, and that its brood parasite has evolved three discrete egg morphs that perfectly match those of its host
Quantification of egg colour and size During the course of the study we discovered that parrotbills laid immaculate eggs which could be classified based on human vision in three discrete morphs: white, pale blue and blue eggs
Summary
Polymorphism in natural populations can evolve and be maintained as a consequence of frequency-dependent predation [1,2], with the textbook example being industrial melanism in the peppered moth (Biston betularia) [1,3]. Other interspecific interactions acting in a frequency-dependent manner should likewise be able to produce polymorphisms in the interacting parties. We describe such an example of egg colour polymorphism in a brood parasite and its host. Coevolutionary interactions between hosts and parasites may lead to selection on both to evolve extreme phenotypes deviating from the norm, through disruptive selection
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