Age-specific fecundity under pathogenic threat in an insect: Terminal investment versus reproductive restraint

The terminal investment hypothesis predicts that as an organism's prospects for survival decrease, through age or when exposed to a pathogenic infection, it will invest more in reproduction, which should trade-off against somatic maintenance (including immunity) and therefore future survival. Attempts to test this hypothesis have produced mixed results, which, in addition, mainly rely on the assessment of changes in reproductive effort and often overlooking its impact on somatic defences and survival. Alternatively, animals may restrain current reproduction to sustain somatic protection, increasing the chance of surviving for additional reproductive opportunities. We tested both of these hypotheses in females of the yellow mealworm beetle, Tenebrio molitor, an iteroparous insect with reproductive tactics similar to that of long-lived organisms. To achieve this, we mimicked pathogenic bacterial infections early or late in the life of breeding females by injecting them with a suspension of inactivated Bacillus cereus, a known natural pathogen of T. molitor, and measured female age-specific fecundity, survival, body mass and immunity. Inconsistent with a terminal investment, females given either an early or late-life immune challenge did not exhibit reduced survival or enhance their reproductive output. Female fecundity declined with age and was reduced by the early but not the late immune challenge. Both early and late-life fecundity correlated positively with life expectancy. Finally, young and old females exhibited similar antibacterial immune responses, suggesting that they both restrained reproduction to sustain immunity. Our results clearly demonstrate that age-specific reproduction of T. molitor females under pathogenic threat is inconsistent with a terminal investment. In contrast, our results instead suggest that females used a reproductive restraint strategy to sustain immunity and therefore subsequent reproductive opportunities. However, as infections were mimicked only, the fitness benefit of this reproductive restraint could not be shown.

Late-life reproduction in an insect: Terminal investment, reproductive restraint or senescence

The terminal investment, reproductive restraint or senescence theories may explain individual late-life patterns of reproduction. The terminal investment hypothesis predicts that individuals increase reproductive allocation late in life as prospects for future survival decrease. The other two hypotheses predict reduced reproduction late in life, but for different reasons. Under the Reproductive Restraint hypothesis, individuals restrain their reproductive effort to sustain future survival and gain more time for reproducing, whereas under the Senescence process, reproduction is constrained because of somatic deterioration. While these hypotheses imply that reproduction is costly, they should have contrasted implications in terms of survival after late reproduction and somatic maintenance. Testing these hypotheses requires proper consideration of the effects of age-dependent reproductive effort on post-reproduction survival and age-related somatic functions. We experimentally tested these three hypotheses in females of the mealworm beetle, Tenebrio molitor, an iteroparous and income breeder insect. We manipulated their age-specific allocation into reproduction and observed the effects of this manipulation on their late-life fecundity, post-reproduction survival and immunocompetence as a measurement of somatic protection. We found that females exhibit age-related decline in fecundity and that this reproductive senescence is accelerated by a cost of early reproduction. The cost of reproduction had no significant effect on female longevity and their ability to survive a bacterial infection, despite that some immune cells were depleted by reproduction. We found that female post-infection survival deteriorated with age, which could be partly explained by a decline in some immune parameters. Importantly, females did not increase their reproductive effort late in life at the expense of their late-life post-reproduction survival. Late-life reproduction in T. molitor females is senescing and not consistent with a terminal investment strategy. Rather, our results suggest that females allocate resources according to a priority scheme favouring longevity at the expense of reproduction, which is in line with the reproductive restraint hypothesis. Such a priority scheme also shows that a relatively short-lived insect can evolve life-history strategies hitherto known only in long-lived animals. This puts in perspective the role of longevity in the evolution of life-history strategies.

Sex-specific patterns of senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

BACKGROUND

The disposable soma theory of ageing assumes that organisms optimally trade-off limited resources between reproduction and longevity to maximize fitness. Early reproduction should especially trade-off against late reproduction and longevity because of reduced investment into somatic protection, including immunity. Moreover, as optimal reproductive strategies of males and females differ, sexually dimorphic patterns of senescence may evolve. In particular, as males gain fitness through mating success, sexual competition should be a major factor accelerating male senescence. In a single experiment, we examined these possibilities by establishing artificial populations of the mealworm beetle, Tenebrio molitor, in which we manipulated the sex-ratio to generate variable levels of investment into reproductive effort and sexual competition in males and females.

RESULTS

As predicted, variation in sex-ratio affected male and female reproductive efforts, with contrasted sex-specific trade-offs between lifetime reproduction, survival and immunity. High effort of reproduction accelerated mortality in females, without affecting immunity, but high early reproductive success was observed only in balanced sex-ratio condition. Male reproduction was costly on longevity and immunity, mainly because of their investment into copulations rather than in sexual competition.

CONCLUSIONS

Our results suggest that T. molitor males, like females, maximize fitness through enhanced longevity, partly explaining their comparable longevity.

Immune Defenses of a Beneficial Pest: The Mealworm Beetle, Tenebrio molitor

The mealworm beetle, Tenebrio molitor, is currently considered as a pest when infesting stored grains or grain products. However, mealworms are now being promoted as a beneficial insect because their high nutrient content makes them a viable food source and because they are capable of degrading polystyrene and plastic waste. These attributes make T. molitor attractive for mass rearing, which may promote disease transmission within the insect colonies. Disease resistance is of paramount importance for both the control and the culture of mealworms, and several biotic and abiotic environmental factors affect the success of their anti-parasitic defenses, both positively and negatively. After providing a detailed description of T. molitor’s anti-parasitic defenses, we review the main biotic and abiotic environmental factors that alter their presentation, and we discuss their implications for the purpose of controlling the development and health of this insect.

Fluctuating versus constant temperatures: effects on metabolic rate and oxidative damages in freshwater crustacean embryos

Rising temperatures will pose a major threat, notably for freshwater ecosystems, in the decades to come. Temperature, a major environmental factor, affects organisms’ physiology and metabolism. Most studies of temperature effect address constant thermal regime (CTR), whereas organisms are exposed to fluctuating thermal regime (FTR) in their natural environments. In addition, previous works have predominantly addressed issues of thermal tolerance in adults rather than in early life stages. Therefore, for the first time to our knowledge, we aimed to investigate the influence of thermal conditions, either FTR or CTR, on the physiology of the crustacean amphipod Gammarus roeseli Gervais, 1835 at different embryonic stages. We measured the metabolic rate and the TBARS (thiobarbituric acid reactive substances) body content (to assess the level of oxidative damage). Oxygen consumption rate strongly increased throughout embryo development, whereas oxidative damages did not clearly change. In addition, the embryos tended to consume oxygen equally but displayed less oxidative damage when developing under FTR compared with developing under CTR. Moreover, our results revealed that fluctuating temperatures (and especially the existence of a colder (nonstressful) period during the day) could allow cell-damage repairs, and therefore, allow G. roeseli embryos to ensure good development by implementing an efficient protection response against oxidative stress.

[Mesenteric vein and inferior vena cava thrombosis: disseminated intravascular coagulation and/or G 20210 A mutation of the prothrombin gene?]

The authors report the case of a 30-year-old man who died from pulmonary embolism and multiorgan failure caused by mesenteric and inferior vena cava thrombosis. The patient was found heterozygous for the prothrombin gene variant (G 20210 A). The family study showed the same asymptomatic anomaly in his brother. This recently described mutation is associated with an increased risk for venous thrombosis. The investigations and treatment of mesenteric venous thrombosis are discussed.

Non-penetrating subclavian artery trauma: management by selective transluminally placed stent device

Non-penetrating injury to the subclavian artery has not often been reported. The limited experience of surgeons with this type of trauma and the difficult vascular control required for its management make it a surgical challenge. We report on two cases, one after blunt trauma and the other with a subclavian artery aneurysm following anterior dislocation of the shoulder. Percutaneous stent implantation in the subclavian artery was successfully performed with, in the second case, coil embolization of the aneurysm. Follow-up Doppler sonography and angiogram demonstrated patency and luminal integrity of the involved artery. This less invasive procedure may be a significant advance and a new approach in the conservative management of traumatic subclavian injury for selected cases.