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.

Keywords: asymmetrical introgression; extra-pair mating; plumage colour; sensory bias; sexual selection; speciation.

Figure 1.

The species range of the red-backed fairy-wren showing the distribution of the two differently coloured subspecies: the red M. m. cruentatus in the north, and the orange M. m. melanocephalus in the east. The intermediately coloured area in the northeast represents the contact zone between plumage types. The vertical line represents the genetic boundary between subspecies, the Carpentarian Barrier. The population where the study was conducted is marked with a star.

Figure 2.

(a) Comparisons of plumage hue between experimental groups (C, control; S, sham; R, reddened) pre- and post-manipulation, and comparison of experimentally reddened males with the natural hue of the red M. m. cruentatus subspecies. Values for plumage hue have been converted to absolute values for visualization purposes. (b–d) Photographs of (b) a red M. m. cruentatus male, (c) an orange M. m. melanocephalus male and (d) comparing a reddened male with a control male.

Figure 3.

The resulting effects of plumage manipulation on three components of reproductive success: (a) number of within-pair young, (b) number of extra-pair young and (c) total number of young. Error bars represent standard errors, and asterisks separate values that are significantly different.