The critical role that gut microbiota play in many aspects of an animal’s life, including pathogen resistance, detoxification, digestion, and nutritional physiology, is becoming more and more apparent (Engel and Moran 2013; Lindsay et al., 2020). Gut microbiota recruitment and maintenance can be largely affected by the surrounding environment (Chandler et al., 2011; Callens et al., 2020). The environment may thus dictate gut microbiota composition and diversity, which in turn can affect organismal responses to stress. Only few studies have, however, taken the gut microbiota into account to estimate life histories in response to multiple stressors in aquatic systems (Macke et al., 2016). 

Houwenhuyse et al., investigate how the microbiome affects life histories in response to ecologically relevant single and multiple biotic stressors (an oomycete-like parasite, and a toxic cyanobacterium) in Daphnia magna (Houwenhuyse et al., 2023). Daphnia is an excellent model, because this aquatic system lends itself extremely well for gut microbiota transplantation and manipulation. This is due to the possibility to sterilize eggs (making them free of bacteria), horizontal transmission of bacteria from the environment, and the relative ease of culturing genetically similar Daphnia clones in large numbers. 

The authors use an elegant experimental design to show that the Daphnia gut microbial community differs when derived from a laboratory versus natural inoculum, the latter being more diverse. The authors subsequently show that key life history traits (survival, fecundity, and body size) depend on the stressors (and combination thereof), the microbiota (structure and diversity), and Daphnia genotype. A key finding is that Daphnia exposed to both biotic stressors show an antagonistic interaction effect on survival (being higher), but only in individuals containing laboratory gut microbiota. The exact mechanism remains to be determined, but the authors propose several interesting hypotheses as to why Daphnia with more diverse gut microbiota do less well. This could be due, for example, to increased inter-microbe competition or an increased chance of contracting opportunistic, parasitic bacteria. For Daphnia with less diverse laboratory gut microbiota, a monopolizing species may be particularly beneficial for stress tolerance. Alongside these interesting findings, the paper also provides extensive information about the gut microbiota composition (available in the supplementary files), which is a very useful resource for other researchers. 

Overall, this study reveals that multiple, interacting factors affect the performance of Daphnia under stressful conditions. Of importance is that laboratory studies may be based on simpler microbiota systems, meaning that stress responses measured in the laboratory may not accurately reflect what is happening in nature. 

REFERENCES

Callens M, De Meester L, Muylaert K, Mukherjee S, Decaestecker E. The bacterioplankton community composition and a host genotype dependent occurrence of taxa shape the Daphnia magna gut bacterial community. FEMS Microbiology Ecology. 2020;96(8):fiaa128. https://doi.org/10.1093/femsec/fiaa128

Chandler JA, Lang JM, Bhatnagar S, Eisen JA, Kopp A. Bacterial communities of diverse Drosophila species: ecological context of a host-microbe model system. PLOS Genetics. 2011;7(9):e1002272. https://doi.org/10.1371/journal.pgen.1002272

Engel P, Moran NA. The gut microbiota of insects - diversity in structure and function. FEMS Microbiology Reviews. 2013;37(5):699-735. https://doi.org/10.1111/1574-6976.12025

Houwenhuyse S, Bulteel L, Vanoverberghe I, Krzynowek A, Goel N et al. Microbiome mediated tolerance to biotic stressors: a case study of the interaction between a toxic cyanobacterium and an oomycete-like infection in Daphnia magna. 2023. OSF, ver. 2 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.31219/osf.io/9n4mg

Lindsay EC, Metcalfe NB, Llewellyn MS. The potential role of the gut microbiota in shaping host energetics and metabolic rate. Journal of Animal Ecology. 2020;89(11):2415-2426. https://doi.org/10.1111/1365-2656.13327

Macke E, Tasiemski A, Massol F, Callens M, Decaestecker E. Life history and eco-evolutionary dynamics in light of the gut microbiota. Oikos. 2017;126(4):508-531. https://doi.org/10.1111/oik.03900

Redistribution of domesticated and non domesticated species by humans profoundly affected earth biogeography and in return human activities. This process accelerated exponentially since human expansion out of Africa, leading to the modern global, highly connected and homogenized, agriculture and trade system (Mack et al. 2000, Jaksic and Castro 2021), that threatens biological diversity and genetic resources. To accompany quarantine and control effort, the reconstruction of invasion routes provides valuable information that help identifying critical nodes and edges in the global networks (Estoup and Guillemaud 2010, Cristescu 2015). Historical records and genetic markers are the two major sources of information of this corpus of knowledge on Anthropocene historical phylogeography. With the advances of molecular genetics tools, the genealogy of these introductions events could be revisited and empowered. Due to their idiosyncrasy and intimate association with the contingency of human trades and activities, understanding the invasion and domestication routes require particular statistical tools (Fraimout et al. 2017).

Because it encompasses all these theoretical, ecological and economical implications, I am pleased to recommend the readers of PCI Zoology this article by Mallez et al. (2021) on pine wood nematode invasion route inference from genetic markers using Approximate Bayesian Computation (ABC) methods.

Economically and ecologically, this pest, is responsible for killing millions of pines worldwide each year. The results show these damages and the global genetic patterns are due to few events of successful introductions. The authors consider that this low probability of introductions success reinforces the idea that quarantine measures are efficient. This is illustrated in Europe where the pine-worm has been quarantined successfully in the Iberian Peninsula since 1999. Another relevant conclusion is that hybridization between invasive populations have not been observed and implied in the invasion process. Finally the present study reinforced the role of Asiatic bridgehead populations in invasion process including in Europe.

Methodologically, for the first time, ABC was applied to this species. A total of 310 individual sequences were added to the Mallez et al. (2015) microsatellite dataset. Fraimoult et al. (2017) showed the interest to apply random forest to improve scenario selection in ABC framework. This method, implemented in the DiYABC software (Collin et al. 2020) for invasion route scenario selection allows to handle more complex scenario alternatives and was used in this study. In this article by Mallez et al. (2021), you will also find a clear illustration of the step-by-step approach to select scenario using ABC techniques (Lombaert et al. 2014). The rationale is to reduce number of scenario to be tested by assuming that most recent invasions cannot be the source of the most ancient invasions and to use posterior results on most ancient routes as prior hypothesis to distinguish following invasions. The other simplification is to perform classical population genetic analysis to characterize genetic units and representative populations prior to invasion routes scenarios selection by ABC.

Yet, even when using the most advanced Bayesian inference methods, it is recognized by the authors that the method can be pushed to its statistical power limits. The method is appropriate when population show strong inter-population genetic structure. But the high number of differentiated populations in native area can be problematic since it is generally associated to incomplete sampling scheme. The hypothesis of ghost populations source allowed to bypass this difficulty, but the authors consider simulation studies are needed to assess the joint effect of genetic diversity and number of genetic markers on the inference results in such situation. Also the need to use a stepwise approach to reduce the number of scenario to test has to be considered with caution. Scenarios that are not selected but have non negligible posterior, cannot be ruled out in the constitution of next step scenarios hypotheses.

Due to its interest to understand this major facet of Anthropocene, reconstruction of invasion routes should be more considered as a guide to damper biological homogenization process.

References

Collin, F.-D., Durif, G., Raynal, L., Lombaert, E., Gautier, M., Vitalis, R., Marin, J.-M. and Estoup, A. (2020) Extending Approximate Bayesian Computation with Supervised Machine Learning to infer demographic history from genetic polymorphisms using DIYABC Random Forest. Authorea. doi: https://doi.org/10.22541/au.159480722.26357192

Cristescu, M.E. (2015) Genetic reconstructions of invasion history. Molecular Ecology, 24, 2212–2225. doi: https://doi.org/10.1111/mec.13117

Estoup, A. and Guillemaud, T., (2010) Reconstructing routes of invasion using genetic data: Why, how and so what? Molecular Ecology, 9, 4113-4130. doi: https://doi.org/10.1111/j.1365-294X.2010.04773.x

Fraimout, A., Debat, V., Fellous, S., Hufbauer, R.A., Foucaud, J., Pudlo, P., Marin, J.M., Price, D.K., Cattel, J., Chen, X., Deprá, M., Duyck, P.F., Guedot, C., Kenis, M., Kimura, M.T., Loeb, G., Loiseau, A., Martinez-Sañudo, I., Pascual, M., Richmond, M.P., Shearer, P., Singh, N., Tamura, K., Xuéreb, A., Zhang, J., Estoup, A. and Nielsen, R. (2017) Deciphering the routes of invasion of Drosophila suzukii by Means of ABC Random Forest. Molecular Biology and Evolution, 34, 980-996. doi: https://doi.org/10.1093/molbev/msx050

Jaksic, F.M. and Castro, S.A. (2021). Biological Invasions in the Anthropocene, in: Jaksic, F.M., Castro, S.A. (Eds.), Biological Invasions in the South American Anthropocene: Global Causes and Local Impacts. Springer International Publishing, Cham, pp. 19-47. doi: https://doi.org/10.1007/978-3-030-56379-0_2

Lombaert, E., Guillemaud, T., Lundgren, J., Koch, R., Facon, B., Grez, A., Loomans, A., Malausa, T., Nedved, O., Rhule, E., Staverlokk, A., Steenberg, T. and Estoup, A. (2014) Complementarity of statistical treatments to reconstruct worldwide routes of invasion: The case of the Asian ladybird Harmonia axyridis. Molecular Ecology, 23, 5979-5997. doi: https://doi.org/10.1111/mec.12989

Mack, R.N., Simberloff, D., Lonsdale, M.W., Evans, H., Clout, M., Bazzaz, F.A. (2000) Biotic Invasions : Causes , Epidemiology , Global Consequences , and Control. Ecological Applications, 10, 689-710. doi: https://doi.org/10.1890/1051-0761(2000)010[0689:BICEGC]2.0.CO;2

Mallez, S., Castagnone, C., Lombaert, E., Castagnone-Sereno, P. and Guillemaud, T. (2021) Inference of the worldwide invasion routes of the pinewood nematode Bursaphelenchus xylophilus using approximate Bayesian computation analysis. bioRxiv, 452326, ver. 6 peer-reviewed and recommended by Peer community in Zoology. doi: https://doi.org/10.1101/452326

Wildlife is increasingly threatened by drops in number of individuals and populations, and eventually by extinction. Besides loss of habitat, persecution, pet trade,… a decrease in individual health status is an important factor to consider. In this article, Ballouard et al (2021)  perform a thorough analysis on the prevalence of two pathogens (herpes virus and mycoplasma) in (mainly) Western Hermann’s tortoises in south-east France. This endangered species was suspected to suffer from infections obtained through released/escaped pet tortoises. By incorporating samples of captive as well as wild tortoises, they convincingly confirm this and identify some possible ‘pet’ vectors. 

In February this year, a review paper on health assessments in wildlife was published (Kophamel et al 2021). Amongst others, it shows reptilia/chelonia are relatively well-represented among publications. It also contains a useful conceptual framework, in order to improve the quality of the assessments to better facilitate conservation planning. The recommended manuscript (Ballouard et al 2021) adheres to many aspects of this framework (e.g. minimum sample size, risk status, …) while others might need more (future) attention. For example, climate/environmental changes are likely to increase stress levels, which could lead to more disease symptoms. So, follow-up studies should consider conducting endocrinological investigations to estimate/monitor stress levels. Kophamel et al (2021) also stress the importance of strategic international collaboration, which may allow more testing of Eastern Hermann’s Tortoise, as these were shown to be infected by mycoplasma.

The genetic health of individuals/populations shouldn’t be forgotten in health/stress assessments. As noted by Ballouard et al (2021), threatened species often have low genetic diversity which makes them more vulnerable to diseases. So, it would be interesting to link the infection data with (individual) genetic characteristics. In future research, the samples collected for this paper could fit that purpose.

Finally, it is expected that this paper will contribute to the conservation management strategy of the Hermann’s tortoises. As such,  it will be interesting to see how the results of the current paper will be implemented in the ‘field’. As the infections are likely caused by releases/escaped pets and as treating the wild animals is difficult, preventing them from getting infected through pets seems a priority.  Awareness building among pet holders and monitoring/treating pets should be highly effective.

References

Ballouard J-M, Bonnet X, Jourdan J, Martinez-Silvestre A, Gagno S, Fertard B, Caron S (2021) First detection of herpesvirus and mycoplasma in free-ranging Hermann’s tortoises (Testudo hermanni), and in potential pet vectors. bioRxiv, 2021.01.22.427726, ver. 4 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.1101/2021.01.22.427726

Kophamel S, Illing B, Ariel E, Difalco M, Skerratt LF, Hamann M, Ward LC, Méndez D, Munns SL (2021), Importance of health assessments for conservation in noncaptive wildlife. Conservation Biology. https://doi.org/10.1111/cobi.13724

Polyphenisms offer an opportunity to study the links between phenotype, development, and environment in a controlled genomic context (Simpson, Sword, & Lo, 2011). In organisms with short generation times, individuals living and developing in different seasons encounter different environmental conditions. Adaptive plasticity allows them to express different phenotypes in response to seasonal cues, such as temperature or photoperiod. Such phenotypes can be morphological variants, for instance displaying different wing patterns as seen in butterflies (Brakefield & Larsen, 1984; Nijhout, 1991; Windig, 1999), or physiological variants, characterized for instance by direct development vs winter diapause in temperate insects (Dalin & Nylin, 2012; Lindestad, Wheat, Nylin, & Gotthard, 2019; Shearer et al., 2016). 

Many aphids display cyclical parthenogenesis, a remarkable seasonal polyphenism for reproductive mode (Tagu, Sabater-Muñoz, & Simon, 2005), also sometimes coupled with wing polyphenism (Braendle, Friebe, Caillaud, & Stern, 2005), which allows them to switch between parthenogenesis during spring and summer to sexual reproduction and the production of diapausing eggs before winter. In the pea aphid Acyrthosiphon pisum, photoperiod shortening results in the production, by parthenogenetic females, of embryos developing into the parthenogenetic mothers of sexual individuals. The link between parthenogenetic reproduction and sexual reproduction, therefore, occurs over two generations, changing from a parthenogenetic form producing parthenogenetic females (virginoparae), to a parthenogenetic form producing sexual offspring (sexuparae), and finally sexual forms producing overwintering eggs (Le Trionnaire et al., 2022).  

The molecular basis for the transduction of the environmental signal into reproductive changes is still unknown, but the dopamine pathway is an interesting candidate. Form-specific expression of certain genes in the dopamine pathway occurs downstream of the perception of the seasonal cue, notably with a marked decrease in the heads of embryos reared under short-day conditions and destined to become sexuparae. Dopamine has multiple roles during development, with one mode of action in cuticle melanization and sclerotization, and a neurological role as a synaptic neurotransmitter. Both modes of action might be envisioned to contribute functionally to the perception and transduction of environmental signals. 

In this study, Le Trionnaire and colleagues aim at clarifying this role in the pea aphid (Le Trionnaire et al., 2022). Using quantitative RT-PCR, RNA-seq, and in situ hybridization of RNA probes, they surveyed the timing and spatial patterns of expression of dopamine pathway genes during the development of different stages of embryo to larvae reared under long and short-day conditions, and destined to become virginoparae or sexuparae females, respectively. The genes involved in the synaptic release of dopamine generally did not show differences in expression between photoperiodic treatments. By contrast, pale and ddc, two genes acting upstream of dopamine production, generally tended to show a downregulation in sexuparare embryo, as well as genes involved in cuticle development and interacting with the dopamine pathway. The downregulation of dopamine pathway genes observed in the heads of sexuparare juveniles is already detectable at the embryonic stage, suggesting embryos might be sensing environmental cues leading them to differentiate into sexuparae females.

The way pale and ddc expression differences could influence environmental sensitivity is still unclear. The results suggest that a cuticle phenotype specifically in the heads of larvae could be explored, perhaps in the form of a reduction in cuticle sclerotization and melanization which might allow photoperiod shortening to be perceived and act on development. Although its causality might be either way, such a link would be exciting to investigate, yet the existence of cuticle differences between the two reproductive types is still a hypothesis to be tested. The lack of differences in the expression of synaptic release genes for dopamine might seem to indicate that the plastic response to photoperiod is not mediated via neurological roles. Yet, this does not rule out the role of decreasing levels of dopamine in mediating this response in the central nervous system of embryos, even if the genes regulating synaptic release are equally expressed. 

To test for a direct role of ddc in regulating the reproductive fate of embryos, the authors have generated CrispR-Cas9 knockout mutants. Those mutants displayed egg cuticle melanization, but with lethal effects, precluding testing the effect of ddc at later stages in development. Gene manipulation becomes feasible in the pea aphid, opening up certain avenues for understanding the roles of other genes during development.

This study adds nicely to our understanding of the intricate changes in gene expression involved in polyphenism. But it also shows the complexity of deciphering the links between environmental perception and changes in physiology, which mobilise multiple interacting gene networks. In the era of manipulative genetics, this study also stresses the importance of understanding the traits and phenotypes affected by individual genes, which now seems essential to piece the puzzle together.

References

Braendle C, Friebe I, Caillaud MC, Stern DL (2005) Genetic variation for an aphid wing polyphenism is genetically linked to a naturally occurring wing polymorphism. Proceedings of the Royal Society B: Biological Sciences, 272, 657–664. https://doi.org/10.1098/rspb.2004.2995

Brakefield PM, Larsen TB (1984) The evolutionary significance of dry and wet season forms in some tropical butterflies. Biological Journal of the Linnean Society, 22, 1–12. https://doi.org/10.1111/j.1095-8312.1984.tb00795.x

Dalin P, Nylin S (2012) Host-plant quality adaptively affects the diapause threshold: evidence from leaf beetles in willow plantations. Ecological Entomology, 37, 490–499. https://doi.org/10.1111/j.1365-2311.2012.01387.x

Le Trionnaire G, Hudaverdian S, Richard G, Tanguy S, Gleonnec F, Prunier-Leterme N, Gauthier J-P, Tagu D (2022) Dopamine pathway characterization during the reproductive mode switch in the pea aphid. bioRxiv, 2020.03.10.984989, ver. 4 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.1101/2020.03.10.984989

Lindestad O, Wheat CW, Nylin S, Gotthard K (2019) Local adaptation of photoperiodic plasticity maintains life cycle variation within latitudes in a butterfly. Ecology, 100, e02550. https://doi.org/10.1002/ecy.2550

Nijhout HF (1991). The development and evolution of butterfly wing patterns. Washington, DC: Smithsonian Institution Press.

Shearer PW, West JD, Walton VM, Brown PH, Svetec N, Chiu JC (2016) Seasonal cues induce phenotypic plasticity of Drosophila suzukii to enhance winter survival. BMC Ecology, 16, 11. https://doi.org/10.1186/s12898-016-0070-3

Simpson SJ, Sword GA, Lo N (2011) Polyphenism in Insects. Current Biology, 21, R738–R749. https://doi.org/10.1016/j.cub.2011.06.006

Tagu D, Sabater-Muñoz B, Simon J-C (2005) Deciphering reproductive polyphenism in aphids. Invertebrate Reproduction & Development, 48, 71–80. https://doi.org/10.1080/07924259.2005.9652172

Windig JJ (1999) Trade-offs between melanization, development time and adult size in Inachis io and Araschnia levana (Lepidoptera: Nymphalidae)? Heredity, 82, 57–68. https://doi.org/10.1038/sj.hdy.6884510

In pollinator insects, especially bees, foraging is almost exclusively performed by females due to the close linkage with brood care. They collect pollen as a protein- and lipid-rich food to feed developing larvae in solitary and social species. Bees take carbohydrate-rich nectar in small quantities to fuel their flight and carry the pollen load. To optimise the foraging flight, they tend to be flower constant, reducing the flower handling time and time among individual inflorescences (Goulson, 1999). Males of pollinator species might be found on flowers as well. As they do not collect any pollen for brood care, their foraging flights and visits to flowers might not be shaped by the selective forces that optimise the foraging flights of females. They might stay longer in individual flowers, take up nectar if needed, but might unintentionally carry pollen on their body surface (Wolf & Moritz, 2014).
 
Bumblebees are excellent pollinators (Goulson, 2010), and a few species are exploited commercially for their delivery of pollination services (Velthuis & van Doorn, 2006). However, a monophyletic group of socially parasitic species – cuckoo bumblebees – has evolved amongst the bumblebees, lacking a worker caste. Cuckoo bee gynes usurp nests of free-living bumblebees, kill the resident queen, and forces the host workers to rear their offspring consisting of gynes and males (Lhomme & Hines, 2019). The level of affected colonies in an area can be up to 42% (Erler & Lattorff, 2010).
 
The behaviour of the cuckoo bumblebees, especially that of the males, has been rarely studied. The present study by Fisogni et al. (2021) has targeted the flower-visiting behaviour of workers and males of free-living bumblebees and males of the cuckoo species. They used behavioural observations of flower-visiting insects on Gentiana lutea, a plant from south-eastern Europe with yellow flowers arranged in whorls. While all three groups of bees visited the same number of plants, males of both types visited more flowers within a whorl, but cuckoo males spent more time on flowers within a whorl and the whole plant than the free-living bumblebees.
 
The flower visits of bumblebee workers are optimised, aiming at collecting as much pollen as possible within a short time frame. This, in turn, has consequences for the pollination process by enhancing cross-pollination between different plants. By contrast, males and especially cuckoo bumblebee males, are not selected for an optimised foraging pattern. Instead, they spend more time on flowers, eventually resulting in higher levels of pollen transfer within a plant (geitonogamy), which might lead to reduced plant fitness. This is the first study to relate the foraging behaviour of cuckoo bumblebees to pollination and plant fitness.
 
References
 
Erler, S., & Lattorff, H. M. G. (2010). The degree of parasitism of the bumblebee (Bombus terrestris) by cuckoo bumblebees (Bombus (Psithyrus) vestalis). Insectes sociaux, 57(4), 371-377. https://doi.org/10.1007/s00040-010-0093-2
 
Fisogni, A., Bogo, G., Massol, F., Bortolotti, L., Galloni, M. (2021). Cuckoo male bumblebees perform slower and longer flower visits than free-living male and worker bumblebees. Zenodo, 10.5281/zenodo.4489066, ver. 1.2 peer-reviewed and recommended by PCI Zoology. https://doi.org/10.5281/zenodo.4489066
 
Goulson, D. (1999). Foraging strategies of insects for gathering nectar and pollen, and implications for plant ecology and evolution. Perspectives in plant ecology, evolution and systematics, 2(2), 185-209. https://doi.org/10.1078/1433-8319-00070
 
Goulson, D. (2010). Bumblebees. Behaviour, Ecology, and Conservation, 2nd edn. Oxford University Press, Oxford.
 
Lhomme, P., Hines, H. M. (2019). Ecology and evolution of cuckoo bumble bees. Annals of the Entomological Society of America, 112, 122-140. https://doi.org/10.1093/aesa/say031
 
Velthuis, H. H. W., van Doorn, A. (2006). A century of advances in bumblebee domestication and the economic and environmental aspects of its commercialization for pollination. Apidologie, 37, 421-451. https://doi.org/10.1051/apido:2006019
 
Wolf, S., Moritz, R. F. A. (2014). The pollination potential of free-foraging bumblebee (Bombus spp.) males (Hymenoptera. Apidae). Apidologie, 45, 440-450. https://doi.org/10.1007/s13592-013-0259-9