Shared pattern in morph inheritance system and recessive gene conservation mechanisms in two polymorphic Hieraaetus species

Abstract

Disruptive selection, non-random transmission of alleles and sexual selection are considered one of the main mechanisms for the maintenance of genetic colour polymorphism in some bird species. We analyse the influence of these mechanisms and Mendelian inheritance on the maintenance and evolution of the colour polymorphism in two polymorphic raptor species: the booted eagle Hieraaetus pennatus and the little eagle Hieraaetus morphnoides. Using observational data from successful breeding events, we examine three populations in Spain (booted eagle) and New South Wales, Australia (little eagle). Our results showed that the dark offspring produced in breeding events involving mixed-morph pairs formed by a light male and a dark female far exceeds the expected value under the Hardy-Weinberg equilibrium. In addition, the low number of dark eaglets born from pairs formed by light individuals indicates that the number of breeding events of heterozygous light morph pairs was much less than expected. As the plausible existence of a transmission ratio distortion phenomenon in heterozygous light morph males does not, alone, explain the disproportionate number of dark eaglets observed, our results suggest that one or two selective mating phenomena may be occurring in this polymorphic system. Finally, we discuss our results suggesting that the two species share a similar mechanism for the maintenance of colour polymorphism since these would come from a common ancestor during the Pleistocene period.