Experimental evidence that extra-pair mating drives asymmetrical introgression of a sexual trait

Abstract

Theory suggests that traits under positive selection may introgress asymmetrically across a hybrid zone, potentially driven by sexual selection. Two subspecies of the red-backed fairy-wren (Malurus melanocephalus) differ primarily in a sexual signal used in mate choice-red versus orange male back plumage colour-but phylogeographic analyses suggest asymmetrical introgression of red plumage into the genetic background of the orange subspecies. We hypothesized that this asymmetrical introgression may be facilitated by sexual selection if red males have a mating advantage over orange males. We tested this hypothesis with correlational data and a plumage manipulation experiment where we reddened the back plumage of a subset of orange males to mimic males of the red subspecies. There was no correlational evidence of a mating advantage to naturally redder males in this population. Experimentally reddened males sired a similar amount of within-pair young and lost paternity at the same rate as orange males, but they sired significantly more extra-pair young, leading to substantially higher total reproductive success. Thus, we conclude that sexual selection via extra-pair mating is a likely mechanism responsible for the asymmetrical introgression of plumage colour in this system, and is potentially driven by a sensory bias for the red plumage signal.

Genomic and Morphological Analysis of a Semipermeable Avian Hybrid Zone Suggests Asymmetrical Introgression of a Sexual Signal

“…Five SNP clines had centers located east of the plumage cline, supporting the idea that eastward genetic introgression in some parts of the genome could have been substantial enough to recombine “red alleles” from the cruentatus subspecies into the genomic background of the orange melanocephalus subspecies. Previous work has shown that experimentally reddened males in an allopatric population of the melanocephalus subspecies sire significantly more extra‐pair offspring than orange males (Baldassarre and Webster ). The greater reproductive success of red males may provide the selective force driving the observed introgression, eroding one potential barrier between the subspecies.…”

“…TetraColorSpace produces several values that quantify the hue, saturation, and brightness of a color. For this study, we focused on one measure of hue, theta ( θ ), because previous analyses have shown that it best captures the variation in red to orange that characterizes the subspecies (Baldassarre et al ; Baldassarre and Webster ).…”

“…). Previously, we suggested that this lack of coincidence was due to asymmetrical introgression of alleles for red back plumage color across the hybrid zone, likely facilitated by sexual selection via extra‐pair mating (Baldassarre et al ; Baldassarre and Webster ).…”

“…The red‐backed fairy‐wren ( Malurus melanocephalus ), a small insectivorous passerine bird endemic to Australia, appears to exhibit asymmetrical introgression of a sexual trait. This species is highly sexually dichromatic (Schodde ; Rowley and Russell ), exhibits very high rates of extra‐pair paternity (Karubian ; Webster et al ; Varian‐Ramos and Webster ; Varian‐Ramos et al ; Baldassarre and Webster ), and has large testes relative to body size (Rowe and Pruett‐Jones , ), all of which suggest strong sexual selection on males. Moreover, two distinct subspecies are recognized based on variation in a sexual signal, male nuptial plumage color: the western, red‐backed M. m. cruentatus and the eastern, orange‐backed M. m. melanocephalus (Schodde ; Rowley and Russell ; Fig.…”

Crossing species boundaries : the hybrid histories of the true geese

“…For instance, hybridization between Red-backed Fairy-wren (Malurus melanocephalus cruentatus) and Orange-backed Fairy-wren (Malurus m. melanocephalus) across the Carpentarian Barrier in Australia has led to the introgression of red plumage colour into the Orange-backed subspecies (Figure 4.1B, Baldassarre et al, 2014). An experimental study showed that artificially reddened males have significantly more extra-pair offspring compared to orange males, which suggests that greater reproductive success of red males drives the observed introgression (Baldassarre and Webster, 2013). A similar situation has been described in Panama, where yellow plumage colour of Golden-collared Manakin (Manacus vitellinus) has spread into the populations of the White-collared Manakin (Manacus candei) by means of strong sexual selection at mixed leks, where males of both species gather to attract mates , Parsons et al, 1993, Stein and Uy, 2006.…”