Yû Suzaki

Effect of weapon-supportive traits on fighting success in armed insects

Male fighting frequently results in the evolution of traits used as weapons. These are often coupled with correlated modifications in other somatic traits (hereafter referred to as supportive traits), which are thought to support the weapon functionally. No previous studies have investigated whether supportive traits themselves affect fighting success. We evaluated this possibility in two armed insects, the beetle Gnatocerus cornutus and the bug Riptortus pedestris. Using a principal components analysis, we identified a subset of supportive traits that were associated with weapons in both species. The patterns of supportive structures differed between the two species, probably reflecting differences in the use of weapons during fights. In both species, fighting success was higher in males with enhanced supportive traits. It appears that weapon-supportive structures develop as the result of selection that favours coordination among multiple traits associated with fighting behaviour. Thus, our results provide rare evidence of the benefit of a supportive trait that is a clear example of phenotypic integration.

Male courtship behavior and weapon trait as indicators of indirect benefit in the bean bug, Riptortus pedestris.

Females prefer male traits that are associated with direct and/or indirect benefits to themselves. Male–male competition also drives evolution of male traits that represent competitive ability. Because female choice and male–male competition rarely act independently, exploring how these two mechanisms interact is necessary for integrative understanding of the evolution of sexually selected traits. Here, we focused on direct and indirect benefits to females from male attractiveness, courtship, and weapon characters in the armed bug Riptortus pedestris. The males use their hind legs to fight other males over territory and perform courtship displays for successful copulation. Females of R. pedestris receive no direct benefit from mating with attractive males. On the other hand, we found that male attractiveness, courtship rate, and weapon size were significantly heritable and that male attractiveness had positive genetic covariances with both courtship rate and weapon traits. Thus, females obtain indirect benefits from mating with attractive males by producing sons with high courtship success rates and high competitive ability. Moreover, it is evident that courtship rate and hind leg length act as evaluative cues of female choice. Therefore, female mate choice and male–male competition may facilitate each other in R. pedestris. This is consistent with current basic concepts of sexual selection.

Male Aggressive Behavior and Exaggerated Hindlegs of the Bean Bug Riptortus pedestris

Males of the bean bug species Riptortus pedestris possess larger hindlegs than females. Observations of male-male interactions showed that the enlarged hindlegs are used as weapons in male fights, and that males with larger hindlegs win fights more frequently. Morphological analysis based on the positive allometry test showed that the femora of larger males are relatively bigger than those of smaller males, but femora of larger females are not relatively larger than those of smaller females. These results suggest that sexual selection in R. pedestris favors larger hindlegs for male fighting. In addition, the thorax and abdomen lengths were larger in the male than in the female. The males often lift their abdomen with their back to the opponent for displays against an opponent. As a result, abdominal size may be under stronger selection in the male than in the female, as for the exaggerated hindlegs.

Relationships among male sexually selected traits in the bean bug, Riptortus pedestris (Heteroptera: Alydidae)

Current concepts of sexual selection suggest that male reproductive success is determined by multiple sexual traits. As expression and production of multiple sexual traits are frequently associated with each other, positive or negative correlations among multiple sexual traits ensue. These relationships among traits associated with male reproductive success may be crucial in the evolution of male reproductive strategies. Here, we investigate phenotypic relationships among sexually selected traits in the armed bean bug Riptortus pedestris. In this insect, males with a larger body and weapon are more likely to win male–male competitions, and males with a larger weapon or higher courtship rate are more attractive to females. There was a significant positive correlation between body size and weapon size, whereas the courtship rate had significant negative correlations with body size and weapon size. Our results suggested that there was a phenotypic trade‐off between courtship rate and male morphology. In this insect, smaller males may make more effort in courtship behavior as an alternative mating tactic.

Relationships between male attractiveness, female remating, and sperm competition in the cigarette beetle

Polyandry creates the opportunity for post-mating sexual selection, and pre- and post-mating sexual selection affects male traits. Investigation of selection pressures in both pre- and post-mating stages is necessary to understand sexual selection. In the cigarette beetle Lasioderma serricorne, we found previously that males that mated faster are thought to be more attractive in the pre-mating process. However, whether the attractive males are favored by the post-mating process remains unclear and thus we set out to investigate this. The attractive males (judged by the pre-mating process) were able to mate sooner with both virgin and non-virgin females. However, attractive males invested less in ejaculation and have difficulty keeping mated females from remating with other males. Thus, under the polyandrous condition, attractive males have a disadvantage in reproductive success due to the risk of sperm competition because they cannot prevent female remating. Therefore, whether a female remates or not would be an important factor in the reproductive success of an attractive male. On the other hand, when a female mated with two males, the last male always sires more offspring, and males who were attractive in pre-mating process did not sire more offspring. These findings suggest that attractive males are at least not favored by the post-mating process itself, and the association between pre- and post-mating sexual selection in L. serricorne is not as simple as reinforcing or undermining.Significance statementBecause females generally mate with multiple males, sexual selection occurs not only before copulation but also after. Thus, investigating the interplay between pre- and post-mating sexual selections is needed to understand sexual selection, but it is still relatively understudied. In the cigarette beetle, males that courted females more were favored by pre-mating processes. Such attractive males had more success mating with virgin and non-virgin females. However, females that have mated with attractive males readily remate with other males, suggesting that the attractive males cannot prevent sperm competition. Thus, the reproductive success of attractive males may be lower under the higher frequency of female remating. Furthermore, non-virgin females were unlikely to bias paternity toward attractive males. These results suggest that males that are favored by pre-mating processes are not favored by post-mating processes in this beetle.

Fitness costs of polyandry to female cigarette beetle Lasioderma serricorne

Although polyandry is common, it is often unclear why females mate with multiple males. While polyandry may provide females with direct or indirect fitness benefits, it can also be costly. Thus, investigating both the costs and benefits of polyandry is needed to understand the evolution of female polyandry. Here, we investigated the potential benefits and costs of polyandry to females of the cigarette beetle Lasioderma serricorne. We established two treatments: single mating and remating treatments. In the remating treatment, females had the opportunity to remate with a second male. This treatment had two outcomes, that is, acceptance or rejection of mating with the second male. Adult lifespans were shorter in females that accepted and rejected remating than the singly mating female, and there was no difference in lifetime fecundity. This suggests that polyandry is costly to the female and that the cost is due to excessive courting by second mates. Indeed, the direct cost was greater when the second mate was an attractive male. Moreover, we found no difference in offspring quality between females that mated once, accepted, or rejected an additional mating, indicating no indirect benefit of polyandry. Thus, polyandry is thought to carry fitness costs but not benefits to females in L. serricorne.Significance statementPolyandry, in which females mate with multiple males, often provides females with fitness benefits, but it can sometimes be costly. Because the adaptive significance of polyandry remains controversial, investigating a cost-benefit balance of polyandry is needed to understand the evolution of female multiple mating. Thus, relationships between fitness consequences and polyandry have to be carefully investigated, and we focused on fitness consequences of females that mated singly, accepted, or rejected remating. When females of the cigarette beetle L. serricorne were courted by two males, the female lifespan decreased compared with singly mated female, even if the females did not experience multiple mating. This indicates a direct fitness cost to females due to contact with two males. Additionally, we found no difference in offspring quality between monandrous and polyandrous females. This suggests no indirect benefit of polyandry. In conclusion, polyandry is thought to carry female fitness costs in the cigarette beetle.

Seasonal abundance and reproductive season of Chauliops fallax (Heteroptera: Malcidae) on kudzu Pueraria lobata

We investigated the seasonal abundance and seasonal dynamics of reproduction in the stalk-eyed bug Chauliops fallax Scott, which is described as a minor pest of bean plants such as soybean and a minor cause of pecky rice. We counted the number of adult males and females, mating pairs, and nymphs of C. fallax on kudzu plants, Pueraria lobata (Willd.), in the southern part of Okayama City, Japan, from 2005 to 2007 and in 2009. Two peaks in abundance for mating pairs were found in late May and August. Both male and female solitary adults appeared from late April, and two peaks of solitary adults were found in May and September. Two clearly separate peaks of nymphs were found in late June and late August. Therefore, it seems that C. fallax produces two generations a year in the southern Okayama area. These data may provide essential information for pest control, and therefore, we discuss them from an applied entomological standpoint to predict invasions by C. fallax into agriculture fields from weeds that grow around and/or in fields.

Attractive males produce high-quality daughters in the bean bug Riptortus pedestris

There are numerous studies on the genetic benefit of female mate choice. Fisherian benefits are detected frequently, in which attractive males benefit females by increasing the mating success of sons. In contrast, good‐genes benefits are relatively small or undetectable, especially as males often face a trade‐off between the expression of secondary sexual traits and viability. In this situation, the effects of good genes might be masked in their sons and, therefore, should be investigated in daughters. A previous study has shown that attractive males produce attractive sons (i.e., Fisherian benefit); the present study aimed to verify the existence of good‐genes benefits by revealing whether attractive males of the bean bug Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) produced high‐quality daughters. Male attractiveness was measured using courtship latency, and the fitness of the females and their offspring was measured based on their lifetime reproductive success as well as the longevity of the daughters. There was no evidence that females directly benefited from mating with attractive males. Whereas male attractiveness (i.e., courtship latency) did not affect nymphal viability or the longevity of daughters, the attractive males with lower courtship latency could produce the daughters with higher lifetime reproductive success. These results suggest that female mating preference in R. pedestris evolved via Fisherian and good‐genes benefits.

Diurnal rhythm of male–male combat behavior in the bean bug Riptortus pedestris (Heteroptera: Alydidae)

In many insects, mating is affected by the day–night cycle, i.e., diurnal rhythm. Although there are many reports that mating and other reproductive behaviors are controlled by daily rhythms in various taxonomic insect species, little attention has been paid to the effect of daily rhythms on male fighting behavior. Here, we investigate whether the frequency and escalation of male–male aggressive interaction exhibit diurnal rhythms under a long‐day condition in the bean bug Riptortus pedestris. Despite the fact that male aggressive behaviors were most often observed in the middle of the later half of light periods, no interaction was found between escalation of fighting and the time period. The results, at least, suggest that male aggressive behaviors are influenced by diurnal rhythms like other reproductive behaviors in R. pedestris.