Coevolution in action: disruptive selection on egg colour in an avian brood parasite and its host.
BACKGROUND
Trait 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.


RESULTS
Here, 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
The 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.
School of Life Sciences, Sun Yat-sen University, Guangzhou, People's Republic of China.
Canchao
C
Yang
Wei
W
Liang
Yan
Y
Cai
Suhua
S
Shi
Fugo
F
Takasu
Anders P
AP
Møller
Anton
A
Antonov
Frode
F
Fossøy
Arne
A
Moksnes
Eivin
E
Røskaft
Bård G
BG
Stokke
D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000469:parasitology*; D052284:Clutch Size; D015350:Contrast Sensitivity; D006790:Host-Parasite Interactions; D009425:Nesting Behavior / Q000502:physiology*; D010063:Ovum / Q000502:physiology*; D010271:Parasites / Q000502:physiology*; D010858:Pigmentation / Q000502:physiology*; D011235:Predatory Behavior / Q000502:physiology; D013997:Time Factors