A cancer-causing gene is positively correlated with male aggression in Xiphophorus cortezi

Abstract

The persistence of seemingly maladaptive genes in organisms challenges evolutionary biological thought. In Xiphophorus fishes, certain melanin patterns form malignant melanomas because of a cancer-causing gene (Xiphophorus melanoma receptor kinase; Xmrk), which arose several millions years ago from unequal meiotic recombination. Xiphophorus melanomas are male biased and induced by androgens however male behaviour and Xmrk genotype has not been investigated. This study found that male X. cortezi with the spotted caudal (Sc) pattern, from which melanomas originate, displayed increased aggression in mirror image trials. Furthermore, Xmrk males (regardless of Sc phenotype) bit and performed more agonistic displays than Xmrk deficient males. Male aggressive response decreased when males viewed their Sc image as compared with their non-Sc image. Collectively, these results indicate that Xmrk males experience a competitive advantage over wild-type males and that intrasexual selection could be an important component in the evolutionary maintenance of this oncogene within Xiphophorus.

Figure 1

These X. cortezi males (Panels A–D) were all collected on the same day from the Conchita collection site (San Luís Potosí, Mexico). This amount of variation in pattern size and saturation is typical across X. cortezi populations. Scale bar: 5 mm.

Figure 2

The number of bites (A) and lateral displays (B) performed by X. cortezi males during the five minute MIS trials. The results of analysis 1 (aggression and Sc phenotype) are presented on the left side of each bar graph (Sc/no Sc) and the results of analysis 2 (aggression and the Xmrk genotype) are presented on the right side of each graphic (Xmrk/no Xmrk). As a reminder in both analyses 1 and 2, males were responding to the same stimulus, their mirror image with the Sc phenotype (i.e., no Sc males had the Sc phenotype painted on). Bars represent the mean ± SE.

Figure 3

The number of bites (A) and lateral displays (B) performed by X. cortezi males towards either their non-Sc image (white bars) or their painted Sc image (grey bars) during the five minute MIS trials. Bars represent the mean ± SE.