Polyandry and female control: the red flour beetle Tribolium castaneum as a case study

Anatomical changes of Tenebrio molitor and Tribolium castaneum during complete metamorphosis

In holometabolous insects, extensive reorganisation of tissues and cells occurs at the pupal stage. The remodelling of the external exoskeleton and internal organs that intervenes during metamorphosis has been traditionally studied in many insect species based on histological or ultrastructural methods. This study demonstrates the use of synchrotron X-ray phase-contrast micro-computed tomography as a powerful, non-destructive tool for in situ morphological observation of anatomical structures at the pupal stage in two Tenebrionid beetles, i.e. Tribolium castaneum and Tenebrio molitor, known as important pests, as well as emerging and promising models in experimental biology. Virtual sections and three-dimensional reconstructions were performed on both males and females at early, intermediate, and late pupal stage. The dataset allowed us to observe the remodelling of the gut and nervous system as well as the shaping of the female and male reproductive system at different pupal ages in both mealworm and red flour beetles. Moreover, we observed that the timing and duration pattern of organ development varied between the species analysed, likely related to the species-specific adaptations of the pre-imaginal stages to environmental conditions, which ultimately affect their life cycle. This research provides new knowledge on the morphological modifications that occur during the pupal stage of holometabolous insects and provides a baseline set of information on beetle metamorphosis that may support future research in forensics, physiology, and ecology as well as an image atlas for educational purposes.

Bias in sex ratios and polyandry rate in reproduction of Leptinotarsa decemlineata

The Colorado potato beetle (CPB, Leptinotarsa decemlineata Slechtd.) is an invasive pest with economic importance worldwide. Sex ratios during egg-hatching and a frequency of polyandry in single-female families were analysed to clarify the reproduction strategy of CPB, which was still known only in fragments. 1296 just hatching 1st instar CPB larvae were collected from 19 single-female families, of which 13 were random families collected from potato fields and 6 were families produced by laboratory farming of naturally fertilised females. All larvae were analysed to detect a sex using a qPCR-based method and to detect polymorphisms in genotypes of 9 microsatellite (SSR) markers. The bias in sex ratio in favour of females was confirmed using linear mixed-effects model in both experimental groups of families: field collections (F = 36.39; P = 0.0001) and laboratory farming (F = 13.74; P = 0.0139). The analysis of diversity in microsatellites proved the polyandry in all progenies as 73% of analysed segregation patterns did not match with the patterns expected for full-sib progenies; on average per locus, 46% of allelic and 49.7% of genotype ratios showed irregular segregation. Both findings contribute toward understanding CPB success rate as an invasive species, as the preferential bearing of females with polyandry has a great potential to keep fitness of progenies, to maintain and operate population diversity, and to accelerate the reproduction of the pest.

Tribolium castaneum: A Model Insect for Fundamental and Applied Research

Tribolium castaneum has a long history as a model species in many distinct subject areas, but improved connections among the genetics, genomics, behavioral, ecological, and pest management fields are needed to fully realize this species' potential as a model. Tribolium castaneum was the first beetle whose genome was sequenced, and a new genome assembly and enhanced annotation, combined with readily available genomic research tools, have facilitated its increased use in a wide range of functional genomics research. Research into T. castaneum's sensory systems, response to pheromones and kairomones, and patterns of movement and landscape utilization has improved our understanding of behavioral and ecological processes. Tribolium castaneum has also been a model in the development of pest monitoring and management tactics, including evaluation of insecticide resistance mechanisms. Application of functional genomics approaches to behavioral, ecological, and pest management research is in its infancy but offers a powerful tool that can link mechanism with function and facilitate exploitation of these relationships to better manage this important food pest.

Living with relatives offsets the harm caused by pathogens in natural populations

Living with relatives can be highly beneficial, enhancing reproduction and survival. High relatedness can, however, increase susceptibility to pathogens. Here, we examine whether the benefits of living with relatives offset the harm caused by pathogens, and if this depends on whether species typically live with kin. Using comparative meta-analysis of plants, animals, and a bacterium (nspecies = 56), we show that high within-group relatedness increases mortality when pathogens are present. In contrast, mortality decreased with relatedness when pathogens were rare, particularly in species that live with kin. Furthermore, across groups variation in mortality was lower when relatedness was high, but abundances of pathogens were more variable. The effects of within-group relatedness were only evident when pathogens were experimentally manipulated, suggesting that the harm caused by pathogens is masked by the benefits of living with relatives in nature. These results highlight the importance of kin selection for understanding disease spread in natural populations.

Tribolium beetles as a model system in evolution and ecology

Flour beetles of the genus Tribolium have been utilised as informative study systems for over a century and contributed to major advances across many fields. This review serves to highlight the significant historical contribution that Tribolium study systems have made to the fields of ecology and evolution, and to promote their use as contemporary research models. We review the broad range of studies employing Tribolium to make significant advances in ecology and evolution. We show that research using Tribolium beetles has contributed a substantial amount to evolutionary and ecological understanding, especially in the fields of population dynamics, reproduction and sexual selection, population and quantitative genetics, and behaviour, physiology and life history. We propose a number of future research opportunities using Tribolium, with particular focus on how their amenability to forward and reverse genetic manipulation may provide a valuable complement to other insect models.

How biases in sperm storage relate to sperm use during oviposition in female yellow dung flies

Precise mechanisms underlying sperm storage and utilization are largely unknown, and data directly linking stored sperm to paternity remain scarce. We used competitive microsatellite PCR to study the effects of female morphology, copula duration and oviposition on the proportion of stored sperm provided by the second of two copulating males (S2) in Scathophaga stercoraria (Diptera: Scathophagidae), the classic model for sperm competition studies. We genotyped all offspring from potentially mixed-paternity clutches to establish the relationship between a second male’s stored sperm (S2) and paternity success (P2). We found consistent skew in sperm storage across the three female spermathecae, with relatively more second-male sperm stored in the singlet spermatheca than in the doublet spermathecae. S2 generally decreased with increasing spermathecal size, consistent with either heightened first-male storage in larger spermathecae, or less efficient sperm displacement in them. Additionally, copula duration and several two-way interactions influenced S2, highlighting the complexity of postcopulatory processes and sperm storage. Importantly, S2 and P2 were strongly correlated. Manipulation of the timing of oviposition strongly influenced observed sperm-storage patterns, with higher S2 when females laid no eggs before being sacrificed than when they oviposited between copulations, an observation consistent with adaptive plasticity in insemination. Our results identified multiple factors influencing sperm storage, nevertheless suggesting that the proportion of stored sperm is strongly linked to paternity (i.e., a fair raffle). Even more detailed data in this vein are needed to evaluate the general importance of sperm competition relative to cryptic female choice in postcopulatory sexual selection.

A screen for bacterial endosymbionts in the model organisms Tribolium castaneum, T. confusum, Callosobruchus maculatus, and related species

Reproductive parasites such as Wolbachia are extremely widespread amongst the arthropods and can have a large influence over the reproduction and fitness of their hosts. Undetected infections could thus confound the results of a wide range of studies that focus on aspects of host behavior, reproduction, fitness, and degrees of reproductive isolation. This potential problem has already been underlined by work investigating the incidence of Wolbachia infections in stocks of the model system Drosophila melanogaster. Here we survey a range of lab stocks of further commonly used model arthropods, focusing especially on the flour beetles Tribolium castaneum and Tribolium confusum, the cowpea weevil Callosobruchus maculatus and related species (Coleoptera: Tenebrionidae and Bruchidae). These species are widespread stored product pests so knowledge of infections with symbionts further has potential use in informing biocontrol measures. Beetles were assessed for infection with 3 known microbial reproductive parasites: Wolbachia, Rickettsia, Spiroplasma. Infections with some of these microbes were found in some of the lab stocks studied, although overall infections were relatively rare. The consequences of finding infections in these or other species and the type of previous studies likely to be affected most are discussed.

Differences in sperm storage and remating propensity between adult females of two morphotypes of the Anastrepha fraterculus (Diptera: Tephritidae) cryptic species complex

The South American fruit fly, Anastrepha fraterculus, is a complex of cryptic species composed of at least seven morphotypes. Some of them, such as the Peruvian and Brazilian 1 morphotypes (which include Argentinean populations), exhibit strong pre-copulatory isolation, yet it is possible to obtain heterotypic crosses when forcing copulation of adults under laboratory conditions. The cross involving Peruvian males and Argentinean females produces F1 offspring with reduced viability in terms of egg hatch. This low hatchability could be caused by a reduced amount of sperm transferred to and stored by females mated with heterotypic males, which in turn could affect their post-copulatory behaviour. To test these hypotheses, we investigated sperm transfer and female mating and remating behaviour for homotypic and heterotypic crosses between adults of two morphotypes (Brazilian 1 [Argentina] and Peruvian [Peru]) of the A. fraterculus cryptic species complex. As reported before, Argentinean males and females mated earlier in the day than the other three mating combinations. Peruvian females engaged in shorter copulation times than Argentinean females. Peruvian females tended to store smaller quantities of sperm than Argentinean females, and almost a half of the crosses involving Argentinean males and Peruvian females were unsuccessful (no sperm transfer). However, there was no evidence that the cross between Peruvian males and Argentinean females resulted in storage of a critically small amount of sperm (posing risk of sperm shortage). Argentinean females were more willing to remate than Peruvian females, irrespective of male morphotype, but latency to remating was not affected by male or female morphotype. This study shows that mating behaviour differs between some of the A. fraterculus complex morphotypes, with female but not male morphotype determining female likelihood to remate.

Identification of nuclear receptors involved in regulation of male reproduction in the red flour beetle, Tribolium castaneum

Nineteen canonical and two Knirps-like family nuclear receptors (NRs) were identified in the genome of Tribolium castaneum. The current study was conducted to identify NRs involved in regulation of male reproduction. RNA interference (RNAi)-aided knockdown in the expression of genes coding for all 21 NRs showed that reduction in the levels of 11 NRs (E75, E78, FTZ-F1, HR38, HR4, Knirps-like, HNF4, Tailless, HR51, Dsf and HR39) in the male beetles caused more than 50% reduction in the eggs laid by the female beetles mated with RNAi male beetles. Among these 11 NRs that are required for male reproduction, knockdown in the expression of genes coding for E78 and HR39 in the male beetles resulted in a reduction in the number of sperm produced and transferred to the female when compared to the sperms produced and transferred by the control male beetles injected with bacterial malE dsRNA. In contrast, knockdown in the expression of genes coding for E75 and HR38 caused a reduction in the size of male accessory glands (MAG), the amount of protein produced by the MAG and the expression of genes coding for accessory gland proteins. These data suggest that NRs such as E78 and HR39 regulate sperm production and their transfer to the females and the other NRs such as E75 and HR38 regulate the development of MAG and the production of accessory gland proteins.

The spatiotemporal dynamics of Tribolium castaneum (Herbst): adult flight and gene flow

Tribolium castaneum (Herbst) has been used as a model organism to develop and test important ecological and evolutionary concepts and is also a major pest of grain and grain products globally. This beetle species is assumed to be a good colonizer of grain storages through anthropogenic movement of grain, and active dispersal by flight is considered unlikely. Studies using T. castaneum have therefore used confined or walking insects. We combine an ecological study of dispersal with an analysis of gene flow using microsatellites to investigate the spatiotemporal dynamics and adult flight of T. castaneum in an ecological landscape in eastern Australia. Flying beetles were caught in traps at grain storages and in fields at least 1 km from the nearest stored grain at regular intervals for an entire year. Significantly more beetles were trapped at storages than in fields, and almost no beetles were caught in native vegetation reserves many kilometres from the nearest stored grain. Genetic differentiation between beetles caught at storages and in fields was low, indicating that although T. castaneum is predominantly aggregated around grain storages, active dispersal takes place to the extent that significant gene flow occurs between them, mitigating founder effects and genetic drift. By combining ecological and molecular techniques, we reveal much higher levels of active dispersal through adult flight in T. castaneum than previously thought. We conclude that the implications of adult flight to previous and future studies on this model organism warrant consideration.

Strain-specific priming of resistance in the red flour beetle, Tribolium castaneum

As invertebrates lack the molecular machinery employed by the vertebrate adaptive immune system, it was thought that they consequently lack the ability to produce lasting and specific immunity. However, in recent years, it has been demonstrated that the immune defence of invertebrates is by far more complicated and specific than previously envisioned. Lasting immunity following an initial exposure that proves protection on a secondary exposure has been shown in several species of invertebrates. This phenomenon has become known as immune priming. In the cases where it is explicitly tested, this priming can also be highly specific. In this study, we used survival assays to test for specific priming of resistance in the red flour beetle, Tribolium castaneum, using bacteria of different degrees of relatedness. Our results suggest an unexpected degree of specificity that even allows for differentiation between different strains of the same bacterium. However, our findings also demonstrate that specific priming of resistance in insects may not be ubiquitous across all bacteria.