Resource quality affects weapon and testis size and the ability of these traits to respond to selection in the leaf-footed cactus bug, Narnia femorata

Abstract

The size of weapons and testes can be central to male reproductive success. Yet, the expression of these traits is often extremely variable. Studies are needed that take a more complete organism perspective, investigating the sources of variation in both traits simultaneously and using developmental conditions that mimic those in nature. In this study, we investigated the components of variation in weapon and testis sizes using the leaf-footed cactus bug, Narnia femorata (Hemiptera: Coreidae) on three natural developmental diets. We show that the developmental diet has profound effects on both weapon and testis expression and scaling. Intriguingly, males in the medium-quality diet express large weapons but have relatively tiny testes, suggesting complex allocation decisions. We also find that heritability, evolvability, and additive genetic variation are highest in the high-quality diet for testis and body mass. This result suggests that these traits may have an enhanced ability to respond to selection during a small window of time each year when this diet is available. Taken together, these results illustrate that normal, seasonal fluctuations in the nutritional environment may play a large role in the expression of sexually selected traits and the ability of these traits to respond to selection.

Keywords: Development; Narnia femorata; evolvability; heritability; nutrition; sexual selection; testes; weapons.

Figure 1

A pair of N. femorata mating on ripe fruit of the cactus O. humifusa.

Figure 2

Opuntia humifusa fruit availability and quality from April to December 2010. Data collected from a 160‐m‐long, one‐meter‐wide transects at the Ordway‐Swisher Biological Station. Figure modified with permission from Gillespie et al. (2014).

Figure 3

Schematic of the half‐sib split‐brood design. This schematic represents one sire family. Thirty‐four sire families were used in the experiment. Each male mated with three virgin females. The offspring of these three females were placed into the developmental environments with ripe fruit, unripe fruit, or without fruit at beginning of their 4th instar, and F₁ males were the focal males in the experiment.

Figure 4

The effects of diet on body mass (A), femur area (B), and testis mass (C). For every trait, males raised on cactus with fruit were larger than males raised on cactus with unripe or without fruit. Lowercase letters indicate significant differences across the developmental environments based on Tukey's HSD (α = 0.05). Significant differences based on analyses using transformed data; untransformed data shown here.

Figure 5

The scaling relationship between body mass and femur area. Males from the unripe fruit environment had the largest femurs relative to body size. Males raised without fruit had the smallest femurs at all body sizes. The solid regression line represents the ripe fruit environment, the long‐dashed regression line represents the unripe fruit environment, and the short‐dashed regression line represents the without fruit environment. Regression lines shown here were calculated from the transformed data rather than from the ANCOVA model.

Figure 6

The scaling relationship between femur area and testis mass. We found no significant differences in the slopes of males raised in the three environments. Males raised on cactus with unripe fruit had smaller testes for their femur area than males raised on cactus with ripe fruit or without fruit. Males raised on cactus with ripe fruit had the largest testes relative to their femur area. The solid regression line represents the ripe fruit environment, the long‐dashed regression line represents the unripe fruit environment, and the short‐dashed regression line represents the without fruit environment. Regression lines shown here were calculated from the transformed data rather than from the ANCOVA model.