Genetic architecture of novel ornamental traits and the establishment of sexual dimorphism: insights from domestic birds

Abstract

The evolution of sexually monomorphic ornamental traits is poorly understood and may require a shift in research focus away from strict sexual selection. To properly understand how selection affects such traits, we need to know to what extent they are constrained by their genetic architecture. Specifically, we need to understand how and in what state (monomorphic or dimorphic) ornamental traits first appear, how they are subsequently rendered sexually dimorphic, or not, and what mechanisms underlie their further elaboration and diversification. Here, I argue that recent developments in the genomic studies of domesticated birds provide insights into these questions. The causal mutations of several ornamental traits have been identified, particularly in chickens. I briefly review this literature and show that ornamental traits can be caused by a range of different types of mutations, including single-nucleotide polymorphisms and larger structural variations. Novel ornamental traits tend to be sexually monomorphic, suggesting that most cases of sexual dimorphism are the result of additional mutations, although we have no direct evidence for this in domestic birds. Several examples show how existing ornaments can be further elaborated and diversified by additional mutations. Ongoing research may also clarify what mutations cause the reversal to sexual monomorphism dull plumage in chickens. I conclude that current cases of sexually monomorphic ornamentation may consist of a mix of traits that are under selection for monomorphism and traits that are constrained to be monomorphic. Keywords Mutual ornamentation • Sexual monomorphism • Sexual selection • Genetic constraint • Chicken Zusammenfassung Die Evolution sexualmonorpher Ornamente ist bisher nur unzureichend verstanden. Es bedarf daher weiterer Erklärungsversuche, vor allem solcher, die nicht auf sexueller Selektion basieren. Um zu verstehen wie Selektion auf die Ausbildung sexualmonomorpher Ornamente wirkt, muss zunächst verstanden werden, in welchem Ausmaß sie durch ihre zu Grunde liegende genetische Architektur festgelegt sind. Insbesondere muss verstanden werden wie und in welchem Zustand (monomorph oder dimorph) Ornamente zunächst auftreten, ob und wie sie anschließend als sexualdimorph festgelegt werden und welche Mechanismen ihrer weitergehenden Ausschmückung und Diversifizierung zu Grunde liegen. Ich argumentiere an dieser Stelle, dass jüngste Entwicklungen im Bereich der Genomforschung domestizierter Vögel neue Einsichten in Bezug auf die genannten Fragestellungen eröffnen. Die der Ausbildung von Ornamenten zu Grunde liegenden Mutationen sind bereits bekannt, insbesondere in Hühnervögeln. Ich gebe hierzu eine kurze Literaturübersicht und zeige, dass die Ausbildung von Ornamenten durch verschiedene Typen von Mutationen verursacht werden kann, unter anderem durch Punktmutationen aber auch durch größere strukturelle Variationen. Neu auftretende Ornamente sind im Allgemeinen monomorph, was nahe legt, dass sexualdimorphe Ornamente ein Resultat zusätzlicher Muta...

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