Novel host plant leads to the loss of sexual dimorphism in a sexually selected male weapon

Host Plant Effects on Sexual Selection Dynamics in Phytophagous Insects

Natural selection is notoriously dynamic in nature, and so, too, is sexual selection. The interactions between phytophagous insects and their host plants have provided valuable insights into the many ways in which ecological factors can influence sexual selection. In this review, we highlight recent discoveries and provide guidance for future work in this area. Importantly, host plants can affect both the agents of sexual selection (e.g., mate choice and male-male competition) and the traits under selection (e.g., ornaments and weapons). Furthermore, in our rapidly changing world, insects now routinely encounter new potential host plants. The process of adaptation to a new host may be hindered or accelerated by sexual selection, and the unexplored evolutionary trajectories that emerge from these dynamics are relevant to pest management and insect conservation strategies. Examining the effects of host plants on sexual selection has the potential to advance our fundamental understanding of sexual conflict, host range evolution, and speciation, with relevance across taxa.

Development of common leaf-footed bug pests depends on the presence and identity of their environmentally-acquired symbionts

Many beneficial symbioses between bacteria and their terrestrial arthropod hosts are vertically transmitted from mother to offspring, ensuring the progeny acquire necessary partners. Unusually, in several families of coreoid and lygeoid bugs (Hemiptera), nymphs must instead ingest the beneficial symbiont, Burkholderia (sensu lato), from the environment early in development. We studied the effects of Burkholderia on development of two species of leaf-footed bug (Coreidae) in the genus Leptoglossus, L. zonatus and L. phyllopus. We found no evidence for vertical transmission of the symbiont, but found stark differences in performance between symbiotic and aposymbiotic individuals. Symbiotic nymphs grew more rapidly, were approximately four times more likely to survive to adulthood than aposymbiotic bugs, and were two times larger. These findings suggest that Burkholderia is an obligate symbiont for Leptoglossus species. We also tested for variation in fitness effects conferred by four symbiont isolates representing different species within Burkholderia's insect-associated Stinkbug Beneficial and Environmental (SBE) clade. While three isolates conferred similar benefits to hosts, nymphs associated with the fourth isolate grew more slowly and weighed significantly less as adults. The effects of the four isolates were similar for both Leptoglossus species. This work indicates that both Burkholderia acquisition and isolate identity play critical roles in the growth and development of Leptoglossus. Importance Leptoglossus zonatus and L. phyllopus are important polyphagous pests and both species have been well-studied, but generally without regard to their dependance on a bacterial symbiont. Our results indicate that the central role of Burkholderia in the biology of these insects, as well as in other leaf-footed bugs, should be considered in future studies of coreid life history, ecology and pest management. Our work suggests acquisition of Burkholderia is critical for the growth and development of Leptoglossus species. Further, we found that there was variation in performance outcomes according to symbiont identity, even among members of the Stinkbug Beneficial and Environmental clade. This suggests that although environmental acquisition of a symbiont can provide extraordinary flexibility in partner associations, it also carries a risk if the partner is sub-optimal.

Developmental nutrition affects the structural integrity of a sexually selected weapon

Males in many species engage in physical combat over access to mates, and sexual selection has led to the evolution of weapons to enhance contest performance. The size of these often-elaborate structures is known to be exquisitely sensitive to nutrition. However, we know very little about the degree to which nutrition affects other attributes of animal weapons that can be crucial to fighting. In this study, we investigated the impact of natural dietary variation on weapon structural integrity in a fighting insect, Narnia femorata (Hemiptera: Coreidae). Males in this species display their enlarged, spiny hind legs to other males, and these legs serve as weapons in aggressive physical contests where they are used to strike and squeeze opponents. N. femorata feeds on the fruit of prickly pear cactus and sets up territories on this plant. In North Central Florida the prickly pear Opuntia mesacantha spp. lata blooms and begins to produce fruits in April and May. N. femorata has multiple, overlapping generations while the green fruits slowly ripen over the next several months. We examined insects reaching adulthood at two nearby time points in this range, June and July, to test the influence of the nutrition provided by ripening green cactus fruit on weapon size and its ability to resist puncture. We also raised insects on cactus with red, ripe fruit for comparison. We found a striking effect of cactus fruit phenology on weapons. Insects raised with the more mature green fruit (those in the second cohort) had 71% larger weapon area and 4.4 times greater puncture resistance than those raised on the early green fruit (those in the first cohort). In contrast, insects raised on red, ripe fruit were moderate in size, had high puncture resistance, and they changed little phenotypically from the first to second cohort. Increased structural integrity of the hind femur weapon was associated with the increased body size that came with better nutrition. This pattern highlights that cuticle thickness increased or its material properties changed when weapons were larger. Importantly, effects of nutrition on puncture resistance also transcended size. Insects of the same size had greater structural integrity if they received superior nutrition. Sexually selected weapons are often used as visual signals to conspecifics before fights, and this work hints that the size of the weapons may be a poor signal of weapon performance when nutrition is variable.

Evidence of a rapid and adaptive response of hemipteran mouthparts to a physical barrier

Animals have encountered novel foods at points throughout history, due to factors such as range expansions and niche shifts driven by competition. One of the first challenges presented by novel foods is how to eat them. Mouthpart morphology is thus critical during the process of host shifts. Developmental plasticity in mouthparts is one potential mechanism that may allow animals to tolerate new foods and eventually to thrive upon them. Here, we investigated the extent to which insect mouthparts from two geographically distant populations can converge in morphology when feeding on common resources. We conducted a common garden/reciprocal transplant experiment using two populations of the cactus bug, Narnia femorata, that differ in mouthpart length. This insect uses straw-like mouthparts (hereafter 'beak') to get through the cactus fruit wall to reach the pulp inside. Our experimental results revealed clear developmental plasticity in beak length. Insects from both populations grew longer beaks when they fed on the cactus fruit with the thicker walls, and they grew shorter beaks when they fed on the cactus fruit with the thinner walls. Thus, insects from distant populations exhibited immediate developmental responses to a new food, and in the predicted directions. These results suggest that some fauna may be able to respond more rapidly than predicted when they encounter novel plants.

Sex differences in local adaptation: what can we learn from reciprocal transplant experiments?

Local adaptation is of fundamental interest to evolutionary biologists. Traditionally, local adaptation has been studied using reciprocal transplant experiments to quantify fitness differences between residents and immigrants in pairwise transplants between study populations. Previous studies have detected local adaptation in some cases, but others have shown lack of adaptation or even maladaptation. Recently, the importance of different fitness components, such as survival and fecundity, to local adaptation have been emphasized. Here, we address another neglected aspect in studies of local adaptation: sex differences. Given the ubiquity of sexual dimorphism in life histories and phenotypic traits, this neglect is surprising, but may be partly explained by differences in research traditions and terminology in the fields of local adaptation and sexual selection. Studies that investigate differences in mating success between resident and immigrants across populations tend to be framed in terms of reproductive and behavioural isolation, rather than local adaptation. We briefly review the published literature that bridges these areas and suggest that reciprocal transplant experiments could benefit from quantifying both male and female fitness components. Such a more integrative research approach could clarify the role of sex differences in the evolution of local adaptations.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

On the Reciprocally Causal and Constructive Nature of Developmental Plasticity and Robustness

Exposure to environmental variation is a characteristic feature of normal development, one that organisms can respond to during their lifetimes by actively adjusting or maintaining their phenotype in order to maximize fitness. Plasticity and robustness have historically been studied by evolutionary biologists through quantitative genetic and reaction norm approaches, while more recent efforts emerging from evolutionary developmental biology have begun to characterize the molecular and developmental genetic underpinnings of both plastic and robust trait formation. In this review, we explore how our growing mechanistic understanding of plasticity and robustness is beginning to force a revision of our perception of both phenomena, away from our conventional view of plasticity and robustness as opposites along a continuum and toward a framework that emphasizes their reciprocal, constructive, and integrative nature. We do so in three sections. Following an introduction, the first section looks inward and reviews the genetic, epigenetic, and developmental mechanisms that enable organisms to sense and respond to environmental conditions, maintaining and adjusting trait formation in the process. In the second section, we change perspective and look outward, exploring the ways in which organisms reciprocally shape their environments in ways that influence trait formation, and do so through the lens of behavioral plasticity, niche construction, and host-microbiota interactions. In the final section, we revisit established plasticity and robustness concepts in light of these findings, and highlight research opportunities to further advance our understanding of the causes, mechanisms, and consequences of these ubiquitous, and interrelated, phenomena.

Novel host plant leads to the loss of sexual dimorphism in a sexually selected male weapon

In this time of massive global change, species are now frequently interacting with novel players. Greater insight into the impact of these novel interactions on traits linked to fitness is essential, because effects on these traits can hinder population existence or promote rapid adaptation. Sexually selected weapons and ornaments frequently influence fitness and often have heightened condition-dependence in response to nutrition. Condition-dependence in response to different ecological conditions, a form of developmental plasticity, may be responsible for much of the intraspecific variation in sexually selected ornaments and weapons in wild populations. Here we examined the consequences of developing on a novel plant for the expression of size and shape in the leaf-footed cactus bug Narnia femorata (Hemiptera: Coreidae). The males of this species possess enlarged, sexually dimorphic femurs on their hind legs. These legs are used as weapons in male-male contests. Females are typically larger in overall body size. Our study revealed that developing upon a novel host can lead to pronounced phenotypically plastic change in sexually dimorphic traits. Male hind femurs were greatly impacted by the novel diet to the extent that the sexual dimorphism in hind femurs was lost. Further, dimorphism in body size increased, as males became tiny adults while females better maintained their body size. These patterns underscore the complex effects that novel species interactions may have on sexual phenotypes.

Males that drop a sexually selected weapon grow larger testes

Costly sexually selected weapons are predicted to trade off with postcopulatory traits, such as testes. Although weapons can be important for achieving access to females, individuals of some species can permanently drop (i.e. autotomize) their weapons, without regeneration, to escape danger. We capitalized on this natural behavior to experimentally address whether the loss of a sexually selected weapon leads to increased testes investment in the leaf-footed cactus bug, Narnia femorata Stål (Hemiptera: Coreidae). In a second experiment, we measured offspring production for males that lost a weapon during development. As predicted, males that dropped a hind limb during development grew significantly larger testes than the control treatments. Hind-limb autotomy did not result in the enlargement of other nearby traits. Our results are the first to experimentally demonstrate that males compensate for natural weapon loss by investing more in testes. In a second experiment we found that females paired with males that lost a hind limb had 40% lower egg hatching success than females paired with intact males, perhaps because of lower mating receptivity to males with a lost limb. Importantly, in those cases where viable offspring were produced, males missing a hind limb produced 42% more offspring than males with intact limbs. These results suggest that the loss of a hind-limb weapon can, in some cases, lead to greater fertilization success.