Facultative social insects can provide insights into the reversal of the longevity/fecundity trade-off across the eusocial insects

Costs of reproduction are present but latent in eusocial bumblebee queens

BACKGROUND

The standard evolutionary theory of ageing proposes that ageing occurs because of a trade-off between reproduction and longevity. Eusocial insect queens exhibit positive fecundity-longevity associations and so have been suggested to be counter-examples through not expressing costs of reproduction and through remodelling conserved genetic and endocrine networks regulating ageing and reproduction. If so, eusocial evolution from solitary ancestors with negative fecundity-longevity associations must have involved a stage at which costs of reproduction were suppressed and fecundity and longevity became positively associated. Using the bumblebee (Bombus terrestris), we experimentally tested whether queens in annual eusocial insects at an intermediate level of eusocial complexity experience costs of reproduction, and, using mRNA-seq, the extent to which they exhibit a remodelling of relevant genetic and endocrine networks. Specifically, we tested whether costs of reproduction are present but latent, or whether a remodelling of relevant genetic and endocrine networks has already occurred allowing queens to reproduce without costs.

RESULTS

We experimentally increased queens' costs of reproduction by removing their eggs, which caused queens to increase their egg-laying rate. Treatment queens had significantly reduced longevity relative to control queens whose egg-laying rate was not increased. Reduced longevity in treatment queens was not caused by increased worker-to-queen aggression or by increased overall activity in queens. In addition, treatment and control queens differed in age-related gene expression based on mRNA-seq in both their overall expression profiles and the expression of ageing-related genes. Remarkably, these differences appeared to occur principally with respect to relative age, not chronological age.

CONCLUSIONS

This study represents the first simultaneously phenotypic and transcriptomic experimental test for a longevity cost of reproduction in eusocial insect queens. The results support the occurrence of costs of reproduction in annual eusocial insects of intermediate social complexity and suggest that reproductive costs are present but latent in queens of such species, i.e. that these queens exhibit condition-dependent positive fecundity-longevity associations. They also raise the possibility that a partial remodelling of genetic and endocrine networks underpinning ageing may have occurred in intermediately eusocial species such that, in unmanipulated conditions, age-related gene expression depends more on chronological than relative age.

Population growth of Tetranychus truncatus (Acari: Tetranychidae) on different drought-tolerant potato cultivars

Tetranychus truncatus Ehara (Acari: Tetranychidae) has become one of the major phytophagous pests in China in recent years, and is found on a wide range of host plants. However, little information is available on the population performance of this arthropod pest on potatoes. In this study, we explored the population growth of T. truncatus on two drought-tolerant potato (Solanum tuberosum L.) cultivars under laboratory conditions using the age-stage, two-sex life table. Tetranychus truncatus completed its entire life history on both potato cultivars tested, Holland 15 and Longshu 10. There was no significant difference between two potato cultivars in developmental duration. Tetranychus truncatus had shorter adult longevity (20.61 days), adult female longevity (20.41 days), and total female longevity (33.66 days) on Longshu 10 than Holland 15 (21.16 days, 21.19 days, and 34.38 days, respectively). However, it exhibited a higher preadult survival rate, higher fecundity (F = 88.32 eggs per female), and relatively higher population parameters when reared on Longshu 10 than on Holland 15 (F = 75.70 eggs per female). Growth projection also showed that the population size of T. truncatus on Longshu 10 (expand 750-fold) was larger than that on Holland 15 (expand 273-fold) after 60 days. Our results demonstrate that the drought-sensitive potato variety, Holland 15, is relatively resistant to T. truncatus compared with the drought-tolerant variety, Longshu 10, and suggest that T. truncatus exhibited a trade-off between longevity and reproduction on both potato cultivars. Our findings provide information on population prediction, which may aid the management of this pest mite species of potatoes.

The suppressive potential of a gene drive in populations of invasive social wasps is currently limited

Social insects are very successful invasive species, and the continued increase of global trade and transportation has exacerbated this problem. The yellow-legged hornet, Vespa velutina nigrithorax (henceforth Asian hornet), is drastically expanding its range in Western Europe. As an apex insect predator, this hornet poses a serious threat to the honey bee industry and endemic pollinators. Current suppression methods have proven too inefficient and expensive to limit its spread. Gene drives might be an effective tool to control this species, but their use has not yet been thoroughly investigated in social insects. Here, we built a model that matches the hornet's life history and modelled the effect of different gene drive scenarios on an established invasive population. To test the broader applicability and sensitivity of the model, we also incorporated the invasive European paper wasp Polistes dominula. We find that, due to the haplodiploidy of social hymenopterans, only a gene drive targeting female fertility is promising for population control. Our results show that although a gene drive can suppress a social wasp population, it can only do so under fairly stringent gene drive-specific conditions. This is due to a combination of two factors: first, the large number of surviving offspring that social wasp colonies produce make it possible that, even with very limited formation of resistance alleles, such alleles can quickly spread and rescue the population. Second, due to social wasp life history, infertile individuals do not compete with fertile ones, allowing fertile individuals to maintain a large population size even when drive alleles are widespread. Nevertheless, continued improvements in gene drive technology may make it a promising method for the control of invasive social insects in the future.

The evolution of ageing in cooperative breeders

Cooperatively breeding animals live longer than their solitary counterparts. This has been suggested for birds, mole rats, and social insects. A common explanation for these long lifespans is that cooperative breeding evolves more readily in long-lived species because lower mortality reduces the rate of territory turnover and thus leads to a limitation of breeding territories. Here, we reverse this argument and show that-rather than being a cause for its evolution-long lifespans are an evolutionary consequence of cooperative breeding. In evolutionary individual-based simulations, we show that natural selection favors a delayed onset of senescence in cooperative breeders, relative to solitary breeders, because cooperative breeders have a delayed age of first reproduction as helpers wait in a reproductive queue to obtain breeder status. Especially long lifespans evolve in cooperative breeders in which queue positions depend on the helpers' age rank among the helpers within the breeding territory. Furthermore, we show that lower genetic relatedness among group members leads to the evolution of longer lifespans. This is because selection against higher mortality is weaker when mortality reduces competition for breeding between relatives. Our results link the evolutionary theory of ageing with kin selection theory, demonstrating that the evolution of ageing in cooperative breeders is driven by the timing of reproduction and kin structure within breeding territories.

Transcriptomic Signatures of Ageing Vary in Solitary and Social Forms of an Orchid Bee

Eusocial insect queens are remarkable in their ability to maximize both fecundity and longevity, thus escaping the typical trade-off between these two traits. Several mechanisms have been proposed to underlie the remolding of the trade-off, such as reshaping of the juvenile hormone (JH) pathway, or caste-specific susceptibility to oxidative stress. However, it remains a challenge to disentangle the molecular mechanisms underlying the remolding of the trade-off in eusocial insects from caste-specific physiological attributes that have subsequently arisen. The socially polymorphic orchid bee Euglossa viridissima represents an excellent model to address the role of sociality per se in longevity as it allows direct comparisons of solitary and social individuals within a common genetic background. We investigated gene expression and JH levels in young and old bees from both solitary and social nests. We found 902 genes to be differentially expressed with age in solitary females, including genes involved in oxidative stress, versus only 100 genes in social dominant females, and 13 genes in subordinate females. A weighted gene coexpression network analysis further highlights pathways related to ageing in this species, including the target of rapamycin pathway. Eleven genes involved in translation, apoptosis, and DNA repair show concurrent age-related expression changes in solitary but not in social females, representing potential differences based on social status. JH titers did not vary with age or social status. Our results represent an important step in understanding the proximate mechanisms underlying the remodeling of the fecundity/longevity trade-off that accompanies the evolutionary transition from solitary life to eusociality.

Does Larval Rearing Diet Lead to Premating Isolation in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)?

Simple Summary

Spodoptera litura Fabricius (Lepidoptera: Noctuidae) is a serious polyphagous pest. Most studies focus on the effects of natural hosts on S. litura. However, progressively more laboratory studies S. litura involve feeding the larvae with an artificial diet. We compared the life performance and observed mating choice of S. litura reared on tobacco, Chinese cabbage, and an artificial diet. The results revealed that diet had a significant effect on the duration of each stage of development. In the multiple-choice test with individual males consuming tobacco, Chinese cabbage, or an artificial diet, females fed on the artificial diet preferred to mate with males that were fed on the same diet and rarely mated with males fed on tobacco or Chinese cabbage. We suggest that the diet of S. litura has a potential impact on mate choice and sexual isolation.

Abstract

Host plant preference during the larval stage may help shape not only phenotypic plasticity but also behavioral isolation. We assessed the effects of diet on population parameters and mate choice in Spodoptera litura. We raised larvae fed on tobacco, Chinese cabbage, or an artificial diet, and we observed the shortest developmental time and highest fecundity in individuals fed the artificial diet. However, survival rates were higher for larvae on either of the natural diets. Population parameters including intrinsic rate of increase and finite rate of increase were significantly higher with the artificial diet, but this diet led to a lower mean generation time. Copulation duration, copulation time, and number of eggs reared significantly differed between diets. In terms of mate choice, females on the artificial diet rarely mated with males fed on a natural host. Our results support the hypothesis that different diets may promote behavioral isolation, affecting mating outcomes. Thus, findings for populations fed an artificial diet may not reflect findings for populations in the field.

Alteration of insulin and nutrition signal gene expression or depletion of Met reduce both lifespan and reproduction in the German cockroach

In insects, nutrition and hormones coordinately regulate lifespan and reproduction, which might affect each other. We here investigated how nutrition, insulin, and juvenile hormone (JH) signal genes affect lifespan and reproduction in the German cockroach, Blattella germanica, a serious urban pest throughout the world. Starvation as well as altering insulin and nutrition signal genes by RNA interference (RNAi) knockdown of the expression of either positive or negative components in the two pathways simultaneously reduced lifespan and ootheca number of the mated female cockroaches. Meanwhile, depletion of the JH receptor Methoprene-tolerant (Met), but not kruppel homolog 1 (Kr-h1) that predominately transduces JH signaling to prevent metamorphosis, significantly reduced the two parameters. Moreover, suppressing the expression of several reproduction-related genes, including doublesex (Dsx), vitellogenin (Vg), and the Vg receptor (VgR), had little yet various effects on lifespan; nevertheless, it is likely that there are some reproduction-independent mating factors reducing lifespan. In conclusion, through blocking lifespan and reproduction in a simultaneous manner, the alteration of insulin and nutrient signal gene expression or the depletion of Met might provide a great potential for controlling the German cockroach.

Social behavior and aging: A fly model

The field of behavioral genetics has recently begun to explore the effect of age on social behaviors. Such studies are particularly important, as certain neuropsychiatric disorders with abnormal social interactions, like autism and schizophrenia, have been linked to older parents. Appropriate social interaction can also have a positive impact on longevity, and is associated with successful aging in humans. Currently, there are few genetic models for understanding the effect of aging on social behavior and its potential transgenerational inheritance. The fly is emerging as a powerful model for identifying the basic molecular mechanisms underlying neurological and neuropsychiatric disorders. In this review, we discuss these recent advancements, with a focus on how studies in Drosophila melanogaster have provided insight into the effect of aging on aspects of social behavior, including across generations.

How to make a sterile helper

The sterile worker castes found in the colonies of social insects are often cited as archetypal examples of altruism in nature. The challenge is to explain why losing the ability to mate has evolved as a superior strategy for transmitting genes into future generations. We propose that two conditions are necessary for the evolution of sterility: completely overlapping generations and monogamy. A review of the literature indicates that when these two conditions are met we consistently observe the evolution of sterile helpers. We explain the theory and evidence behind these ideas, and discuss the importance of ecology in predicting whether sterility will evolve using examples from social birds, mammals, and insects. In doing so, we offer an explanation for the extraordinary lifespans of some cooperative species which hint at ways in which we can unlock the secrets of long life.