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    Genic distribution modelling predicts adaptation of the bank vole to climate change

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    Recapping and mite removal behaviour in Cuba: home to the world’s largest population of Varroa-resistant European honeybees

    We confirm that Cuba is home to the world’s largest European honeybee population that has naturally become Varroa-resistant, with an estimated 220,000 colonies being maintained without any form of chemical treatment for over two decades19 although some drone-trapping occurred during the early years of the transition period This is despite the presence of the K-haplotype of the mite20 and the widespread occurrence of DWV19 throughout Cuba. Hence, the Cuban honeybee population is the first major case of Varroa-resistant European bees occupying an entire country of a large size (109,884 km2). In Europe the proportion of varroa-resistant honeybee populations in each country is highly variable21,22, but they still consist of small, isolated populations within any country. For example, the second largest known area of European Varroa-resistant honeybees is in North Wales, UK where 104 beekeepers have managed around 500 honey bee colonies over an area of 2500 km2 without treatment for over a decade23.It has long been established that sub-Sharan African and Africanised honeybees are Varroa-resistant and both populations cover much larger areas than Cuba, but these honeybee races are not capable of thriving in temperate regions or are rejected by beekeepers in Northern hemispheres. However, previous studies on African/Africanised and European honeybees4,5,6,9 all appear to have evolved with the same resistance mechanism7 and Cuban honeybees follow this pattern showing high recapping behaviour, high mite removal behaviour and low mite reproduction (Figs. 1, 4, Table 1).The strongest evidence that increased recapping behaviour is a direct response to the presence of Varroa, is the very low recapping rates in Varroa-naïve colonies. This is evidenced by the recapping baseline data that has now been collected from four different Varroa-naïve (Varroa free) honeybee populations (Australia, UK [two populations] and Hawaii [this study]) all producing similar results (Fig. 1). Across the four populations, a total of 9542 worker cells from 15 colonies have been studied with an average recapping rate of 2.0% (+ SD 3.2). Interestingly, only two of the colonies had atypical recapping rates of 8.5% and 10.7%, from Australia and Kauai respectively. This may suggest increased sensitivity in these colonies as no obvious causes e.g., wax moth or dead pupa, were detected in either colony. The data summary in Fig. 1 indicates that even in Varroa-treated populations the workers are still able to detect mite infested cells, but the average consistently falls significantly below that found in resistant populations. That is, in non-infested worker cells recapping rates are significantly higher in resistant populations in comparison to susceptible populations (Fig. 1) t4, 5 = − 4.185, p = 0.0023 as well as for infested cells t4, 5 = − 6.905, p = 0.00007.The ability of Cuban honeybees to detect infested cells causes not only high recapping levels but also high removal rates of artificially mite-infested cells. A mean removal rate of 81% is among one of the highest recorded in Apis mellifera7. The average control rate of 45% is driven by three colonies that all removed more than 75% of the controls, while the average of the remaining seven colonies was 28%. During the mite-removal studies in March 2022 natural Varroa infestation was 23%, whereas in December 2021 it was only 13%. This is due to decreasing worker brood rearing, caused by a shortage of nectar during the annual dry season. During this time there is an increase in hygienic behaviour in the colonies24, which could help explain the higher-than-expected removal of control cells.The reproductive ability of Varroa to produce viable i.e., mated, female offspring (r) in infested worker cells in resistant colonies in South Africa4 (r = 0.9), Brazil4 (r = 0.8), Mexico18 (r = 0.73), Europe3 (r = 0.84) is similar to the 0.87 found in Cuba (this study). In Cuba ‘r’ reduces to 0.77 when both single and multiple infested cells are considered. This reduction in mite reproduction, relative to susceptible colonies that have values of r greater than one, is directly linked to the increased ability of resistant workers to both detect and remove, by cannibalisation, the infested pupa. Hence, this ensures the invading mite fails to reproduce7 or reduces mite fertility due to the recapping process4. Although, in this study no significant difference was found in the reproduction of Varroa in recapped or non-recapped cells, supporting the findings of two previous studies5,9. Therefore, recapping may be playing a minor role in resistance. However, recapping remains the best indicator or ‘proxy’ of resistance within the vast majority of honeybee populations since it’s easier, quicker, and it requires less skill to measure recapping rates than mite removal rates. However, recapping is a highly variable trait7, hence both many cells (200–300) per colony and many colonies ( > 10) per population ideally need to be studied to help reduce the variablity, also in temperate countries measuring recapping when mite-infestation rates peak in autumn maximises detecting infested cells since the recapping of cells is spatially associated with infested cells11.Despite the current focus on what is happening in worker cells, studies focusing on the role of recapping in drone brood are still in their infancy with. Currently, data is only available from South Africa9 (Fig. 1) and now Cuba (this study). Interestingly, both studies indicate no significant difference in recapping rates between infested and non-infested brood. This is caused by some colonies performing no recapping of drone brood, while some colonies do recap cells but in a non-targeted manner. Whereas there is a significant increase in the size of the recapped area between infested (3.1 mm) and non-infested (2.3 mm) worker cells (Fig. 3), this does not occur in drone brood, as it appears that the holes are entirely exploratory. However, the lack of removal of infested drone brood may be playing an important role in mite-resistance (see below).The mite infestation of worker cells currently varies between 23 and 13% in Cuba (this study), roughly 25 years after it was first detected (1996). Whereas, in Mexico and Brazil, infestation rates of worker brood have fallen from around 20% in 1996/1999 down to 4% in 2018/197. Although, Varroa was first detected in Brazil much earlier, in 197225 and the Africanised honeybees adapted to the mite and spread northward replacing the susceptible European colonies. Therefore, we predict that the worker infestation rate in Cuba will continue to fall over the next 20 years, especially if high mite-removal rates persist. Correspondingly, we would expect to see the infestation rates of the drone brood (currently at 40%) to remain high as mites potentially avoid reproduction in worker cells. This potentially is a key, but currently overlooked part, of the resistance mechanism. Since an empirical model26 indicated that negative mite population growth occurs in (resistant) Africanised honeybee colonies only when the initial drone cells are present. This is thought to arise because mites also show a tenfold preference to reproduce in drone cells (which comprises only 1–5% of all the honeybee brood) and they soon become overcrowded as the mite population increases. This leads to inter-mite competition for the limited food and space, causing an increase in mite mortality27, resulting in negative reproductive success for mites entering these overcrowded drone cells. Thus, mite population growth in drone brood cells is limited by a density-dependent mechanism. In Cuba it has been observed that strong colonies typically with drone brood do not weaken during the drought season, whereas colonies without drone brood are weak and often die during the drought (APP personal comm).Although Cuban beekeepers have been aware of their mite-resistant honeybees for 15 to 20 years’, Cuba’s situation has only recently come to light16,18. The main reason for Varroa-resistance in Cuba is due to the centralised decision to allow natural resistance to evolve, as also was done successfully in South Africa3, rather than becoming locked into using miticides, as has happened throughout the Northern hemisphere. The CIAPI and Veterinarian Services central decision to ‘not treat’ was greatly assisted by all Cuban beekeepers being professional, registered and embedded within a strong locally based beekeeping community where colony movement and exchange of queens is within each province.There is also a large feral population and due to Cuba’s sub-tropical climate, queens are replaced annually in managed colonies because of almost continuous egg-laying, similar to honeybees in Hawaii. This rapid queen turnover speeds up natural selection relative to honeybee populations in more temperate climates. Finally, Cuba’s 60-year ban on honeybee importation has helped isolate the country from been invaded by Africanised bees which has occurred in many nearby regions (eg. Mexico, Southern USA, Puerto Rico, neighbouring Dominican Republic13 and Haiti (D. Macdonald, Apiary Inspector, Min. of Agi BC, Canada, pers. Comm.). Cuba has many managed European colonies coupled with many queen rearing stations. These colonies are productive and mild mannered. Thus, Cuba is an excellent example of the power of natural selection in honeybees when they are allowed to adapt naturally to Varroa with minimal human interference. More

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    Role of saltmarsh systems in estuarine trapping of microplastics

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    Iron-dependent mutualism between Chlorella sorokiniana and Ralstonia pickettii forms the basis for a sustainable bioremediation system

    Iron and carbon dependent mutualism between Chlorella sorokiniana and Ralstonia pickettii forms a synthetic phototrophic communityThe synthetic microalgal-bacterial community based on the active exchange of iron and carbon was developed by screening multiple siderophore producer bacteria and dye decolorizer algae (Fig. 1; refer to Supplementary Data S1 for detailed results). Out of seven bacterial isolates obtained from untreated textile wastewater, five showed relatively high siderophore production in CAS agar plates and broth (Fig. S1). In broth, Serratia plymuthica PW1, Serratia liquefaciens PW71, and Ralstonia pickettii PW2 produced siderophores in decreasing order of concentration, i.e., 15.26 ± 1.3  > 13.28 ± 0.9  > 10.85 ± 0.7 µMmL−1 (Table 1). Arnow’s assay confirmed that S. plymuthica PW1 (81.10 ± 9.8 µMmL−1), R. pickettii PW2 (97.43 ± 16.8 µMmL−1), and S. liquefaciens PW71 (103.1 ± 8.3 µMmL−1) produced catecholate-type siderophores. On the other hand, Csaky’s assay confirmed that Stenotrophomonas maltophilia PW5 (37.86 ± 0.4 µMmL−1) and Stenotrophomonas maltophilia PW6 (17.73 ± 0.2 µMmL−1) produced hydroxamate-type of siderophores. Out of the five algal species, only freshwater microalgae Chlorella sorokiniana and Scenedesmus sp. showed the highest dye degradation potential; therefore, they were selected for further experiments (Data S1).Fig. 1: The study design explains different stages of experiments to develop a phototrophic community of previously non-associated algae and bacteria.The stages include (A) isolation of bacterial strains from textile wastewater collected from Panipat Industrial area, Haryana (India); B cultivation of freshwater and marine algal strains; C assessment of siderophore production in bacterial strains using Schwyn and Neilands’s universal Chrome Azurol S (CAS) assay; D assessment of dye degradation potential of algae strains using Acid Black 1 (AB1) dye; E interaction study between siderophore producing bacteria and dye degrader microalgae to identify bacterial strains that could sustain on algae-derived DOM secreted in algal exudates; F algal-bacterial co-culturability assessment to study different types of microbial interactions viz. antagonism, mutualism, or no interaction between the two organisms, and G identification of algal-bacterial model phototrophic community based on the active exchange of iron and DOM (refer to Data S1 for detailed results).Full size imageTable 1 Characterization of siderophore production in bacterial strains isolated from textile wastewater.Full size tableAfter that, the sterile exudates from C. sorokiniana and Scenedesmus sp. were used as the sole source of dissolved organic matter for bacterial growth and selection of appropriate microalgal-bacterial partners comprising the phototrophic community (Fig. 1E; Data S2). All five bacterial isolates grew well on the exudate of C. sorokiniana as a sole source of carbon. On the contrary, on exudates of Scenedesmus sp., S. plymuthica PW1 showed moderate growth in 20 h, while the growth of R. pickettii PW2 and S. liquefaciens PW71 remained insignificant. S. maltophilia PW5 and PW6 failed to grow in the exudate of Scenedesmus sp. (Fig. S2B).Finally, the compatibility between the phototrophic community of selected microalgae (C. sorokiniana/ Scenedesmus sp.) and siderophore-producer bacteria (S. plymuthica PW1/ R. pickettii PW2/ S. liquefaciens PW71) was tested by co-culturing them in iron limiting BBM media (BBM-Fe; without EDTA) (Fig. 1F). In the absence of EDTA, Fe precipitates rapidly as iron oxyhydroxides and becomes unavailable to microbes. Microalgal growth curves in co-culture assays were used to measure and compare population characteristics such as carrying capacity ‘k’, growth rate ‘r’, etc., in axenic and consortium setups. Algal growth parameters in co-culture with a bacterial partner were used to categorize their interaction as putative mutualistic, antagonistic, and neutral (Data S1, Tables S1 and S2) [42]. Under iron-limiting conditions, axenic C. sorokiniana experienced iron stress as the cell growth was 4.2 ± 0.4 × 106 cells mL−1 after 200 h incubation. On the other hand, axenic Scenedesmus sp. showed a significantly higher growth (11.3 ± 1.2 × 106 cells mL−1) than C. sorokiniana suggesting an effective iron uptake mechanism under iron-limiting conditions (k; t-test, p = 0.001) (Table S1). In contrast to the axenic microalgal culture, C. sorokiniana in co-culture with R. pickettii PW2 showed a significant increase in cell count at 200 h (6.2 ± 0.85 × 106 cells mL−1) (auc; p = 0.000). However, S. plymuthica PW1 exerted a negative effect on C. sorokiniana (Fig. 2A), as indicated by its significant increase in doubling time (p = 0.009) and reduction in auc (p = 0.001) (Fig. 3A). While S. liquefaciens PW71 remained neutral to C. sorokiniana (auc; p = 0.430) (Fig. 2A, Table 2). On the other hand, the interaction of Scenedesmus sp. with both R. pickettii PW2 and S. liquefaciens PW71 was neutral, while S. plymuthica PW1 showed a negative effect (Figs. 2A and 3A).Fig. 2: Assessment of algal and bacterial growth in co-culture experiments.A The growth curves represent the difference in the growth of C. sorokiniana when grown axenically or in co-culture with S. plymuthica PW1, R. pickettii PW2, and S. liquefaciens PW71 under iron limiting conditions. Whereas, the effect of bacteria on the growth of Scenedesmus sp. was less prominent. The difference in the CFUs of bacterial strains in axenic culture and co-culture suggests the growth-promoting effect of C. sorokiniana on S. plymuthica PW1 and R. pickettii PW2. B Anion-exchange chromatography suggests a difference in the glycosyl composition in the EPS of C. sorokiniana and Scenedesmus sp. C The area under curve (auc) of S. plymuthica PW1 and R. pickettii PW2 obtained after growth curves in different sugars. Here, ‘a’, ‘b’, etc., represent grouping after Tukey’s post hoc test.Full size imageFig. 3: Assessment of algal growth parameters in the algal-bacterial phototrophic community under iron-limiting conditions.A The confidence interval plots represent the significant difference in the growth parameters i.e., growth rate ‘r’, carrying capacity ‘k’, doubling time ‘Dt’, and area under curve ‘auc’, of C. sorokiniana (left panel) and Scenedesmus sp. (right panel) in algal-bacterial co-cultures w.r.t. to axenic culture (horizontal blue dashed line). The symbols ‘*’ and ‘**’ represent p values with statistical significance of ‘p  More

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    Maladaptive evolution or how a beneficial mutation may get lost due to nepotism

    Our model results indicate that in species with a strict social dominance hierarchy where social rank is determined by nepotism, a beneficial mutation occurring in a low-ranking female is not very likely to get established. This outcome emerged despite the immense advantage of the modeled mutation, which doubled its carrier’s survival probability. Moreover, the reproductive skew in our model (see Supplementary Fig. 1) was less radical than the skew reported for the spotted hyena females21, which means that in the model, low-ranking females had a relatively higher reproductive success potential than in reality. In other words, our model may be underestimating the severity of the negative selection a low rank induces.It is reasonable to assume that a low-ranking mutant female in a female dominant society would produce very few surviving offspring due to her low rank and ensuing lack of access to resources. Thus, this female would have only a slight chance to transmit the mutation to the next generation. If this female does reproduce successfully and produces a female which also inherits the mutation, chances of this daughter to pass on the mutation are also slim, as her rank would be even lower than that of her mother. However, if the young produced is a male and has inherited the mutation, chances of transmitting the mutation may increase depending on the male’s reproduction odds. As demonstrated by the four scenarios, the reduction in mutation establishment with decreasing mutant female’s rank became more and more prominent with increasing restrictions on male reproduction. In all four scenarios, the mutation establishment rate median was zero for the lowest ranking mutants, and in all cases but Scenario I, it was 41. Although female dominance hierarchy exists in a few of these species (e.g., Peruvian squirrel monkey41, ring-tailed lemur (Lemur catta)39,42, Verreaux’s sifaka (Propithecus verreauxi))13,25, we did not find any studies indicating female reproductive skew in any of them. Holekamp and Engh25, who reviewed the more classical female dominant species, also reported no evidence for female reproductive skew.This seemingly lack of female reproductive skew among most female dominant species is quite surprising in light of the rather common correlation between social rank and female reproductive success in male dominant species. To mention a few, considerable female reproductive skew is found in baboons (Papio spp), macaques (macaca spp.), feral horses (Equus caballus) and plains zebras (Equus burchelli)8,15,19.Holekamp and Smale28 state that “reproductive skew among female spotted hyenas appears to be greater than that documented among females of male-dominated species characterized by plural breeding”. They suggest that the key determinant of reproductive success among females in this species is rank-related priority of access to food resources. This high priority is reinforced by female dominance over males and is particularly important as this species resides in an environment in which prey availability is seasonal and scarce at times21. Our study suggests that this extreme difference in reproductive success, which, unlike in male-dominated species, is determined by nepotism rather than by physical characters, may induce a handicap on the entire population preventing the establishment of beneficial mutations. This may also hinder adaptation to a changing environment. However, our study results indicate that male equal access to females may, at least partially, counter the inhibition effect on a beneficial mutation establishment. More research is necessary in order to investigate female reproductive skew in species with a social structure similar to that of the spotted hyena, which is characterized by female dominance over males, plural breeding, and a strict female linear social hierarchy determined by nepotism.One intriguing possibility for testing this model’s validity would be an empirical study, provided that the value of some adaptive trait can be measured. In the case of the spotted hyena such a trait may refer to hunting success or physical capabilities. It is well established that adult female spotted hyenas are larger and more aggressive than adult males21, but little attention has been allocated to the study of individual physical differences among females of different ranks. Smith et al.43 studied within clan aggression in the context of the fission-fusion behavior characterizing the spotted hyena clans. Their results indicate more frequent aggression and resulting fissions occurring during times of food shortage. Rank was found to be the major correlate of an aggressive incident result. If it is possible to identify low-ranking females with some beneficial trait (independent of rank), it would be interesting to follow such females’ inclusive reproductive success along time, and even more so, the reproductive success of their sons.Another possible path around the conflict this model suggests would be through the selection of male admission into new clans. Male admission into clans is often constrained by severe aggression of resident immigrant males which may prevent or delay male admission21,26. Such behavior may in fact promote mutant male chances, at least in the case of a mutation that improves physical capabilities.One last, though not very likely possible detour around this difficulty is the occurrence of dominance rank exchanges. Such rank improvements are not very common among female dominated societies, except for in the case of aging females who may clear the way for their daughters44. However, Straus and Holekamp44 found that individuals who repeatedly form coalitions with their top allies are likely to improve their position, and, according to Strauss and Holekamp44, “facilitate revolutionary social change”. It should be kept in mind that not only are such incidents rather rare, but they are unlikely to turn a very low-ranking female into a high-ranking one, especially not when group size is large.More empirical and theoretical research should shed more light on this intriguing question of possible maladaptive evolution. Our model, in line with a few other models such as that of Holman31, suggests that evolution may not always lead to the best solution. As in every process, a local optimum may get evolution trapped and prevent further advance to better optima. More