• European Marine Science
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Although sexual dimorphism is widespread in nature, its evolutionary causes often remain elusive. Here we report a case where a sex-specific conflicting functional demand related to parental care but not to sexual selection explains sexual dimorphism in a primarily trophic structure, the gill rakers of cichlid fishes. More specifically, we examined gill raker length in a representative set of cichlid fish species from Lake Tanganyika featuring three different parental care strategies: (i) uni-parental mouthbrooding, whereby only one parental sex incubates the eggs in the buccal cavity; (ii) bi-parental mouthbrooding, whereby both parents participate in mouthbrooding; and (iii) nest guarding without any mouthbrooding involved. As predicted from these different parental care strategies, we find sexual dimorphism in gill raker length to be present only in uni-parental mouthbrooders, but neither in bi-parental mouthbrooders nor in nest guarders. Moreover, variation in the extent of sexual dimorphism among uni-parental mouthbrooders appears to be related to trophic ecology. Overall, we present a so far unrecognised scenario for the evolution of sexual dimorphism that is not related to sexual selection or initial niche divergence between sexes. Instead, sexual dimorphism in gill raker length in uni-parental mouthbrooding cichlid fish appears to be the consequence of a sex-specific functional trade-off between a trophic function present in both sexes, and a reproductive function present only in the brooding sex. Gill raker length measurementsRonco_et_al_Supplementary_data1_gill_raker.txtStable isotope dataRonco_et_al_Supplementary_data2_stable_isotope.txt

Adaptive phenotypic plasticity and fixed genotypic differences have long been considered opposing strategies in adaptation. More recently, these mechanisms have been proposed to act complementarily and under certain conditions jointly facilitate evolution, speciation and even adaptive radiations. Here we investigate the relative contributions of adaptive phenotypic plasticity vs. local adaptation to fitness, using an emerging model system for studying early phases of adaptive divergence, the generalist cichlid fish species Astatotilapia burtoni. We tested direct fitness consequences of morphological divergence between lake and river populations in nature by performing two transplant experiments in Lake Tanganyika. In the first experiment we used wild-caught juvenile lake and river individuals, while in the second experiment we used F1 crosses between lake and river fish bred in a common garden setup. By tracking the survival and growth of translocated individuals in enclosures in the lake over several weeks, we revealed local adaptation evidenced by faster growth of the wild-caught resident population in the first experiment. On the other hand, we did not find difference in growth between different types of F1 crosses in the second experiment, suggesting a substantial contribution of adaptive phenotypic plasticity to increased immigrant fitness. Our findings highlight the value of formally comparing fitness of wild-caught and common-garden-reared individuals and emphasize the necessity of considering adaptive phenotypic plasticity in the study of adaptive divergence. Individual measurement data from transplant experiement 1 using wild-caught Astatotilapia burtoni transplanted to the lake habitatSL - standard length, TL - total length, W -weight, m- male, f - female, ? - unknown, KaL - Kalambo lake, Ka2 - Kalambo river upstreamexperiement1.txtIndividual measurement data from transplant experiment 2 using F1 crosses between lake and stream Astatotilapia burtoni transplanted to the lake habitatSL- standard length, TL - total length, W - weight, m- male, f - female, ? - unknown, KaL x KaL - pure lake cross, Ka2 x KaL - hybrid, Ka2 x Ka2 - pure river crossexperiment2.txt

Understanding how reproductive barriers evolve and which barriers contribute to speciation requires the examination of organismal lineages that are still in the process of diversification and the study of the full range of reproductive barriers acting at different life stages. Lake and river ecotypes of the East African cichlid fish Astatotilapia burtoni show habitat specific adaptations, despite different levels of genetic differentiation, and thus represent an ideal model to study the evolution of reproductive barriers. To evaluate the degree of reproductive isolation between genetically divergent lake and river populations, we performed a mesocosm mating experiment in a semi-natural setting at Lake Tanganyika. We assessed reproductive isolation in the presence of male–male competition by analysing survival and growth rates of introduced adults and their reproductive success from genetic parentage of surviving offspring. The genetically divergent river population showed reduced fitness in terms of survival, growth rate, and mating success in a lake-like environment. Hybrid offspring between different populations showed intermediate survival consistent with extrinsic postzygotic reproductive barriers. Our results suggest that both prezygotic (immigrant inviability) and postzygotic reproductive barriers contribute to divergence, and highlight the value of assessing multiple reproductive barriers acting at different stages and in natural contexts to understand speciation mechanisms. Body shape dataTps file created using TPSDIG2 v.2.26 containing x and y coordinates of 17 homologous landmarks that were placed on the image of each fish. The ID contains the year, generation (pb - parents before the experiment, pa - parents after the experiment, F1- offspring), population or type of cross (KaL - Kalambo lake, NdL - Ndole lake, LfR - Lufubu river), individual ID and sex.Body_shape_morphometrics.tpsMeasurements, survival and number of matings for the introduced adultsStandard length (SL), total length (TL) and weight (W) of all the introduced (SL0, TL0, W0) and all the recovered (SL1, TL1, W1) adults .parents.txtMating data per female-male pairNumber of mating events and number of surviving free-swimming offspring per each female-male pairmatings.txt

Background: Parental care, while increasing parental fitness through offspring survival also bears cost to the care-giving parent. Consequentially, trade offs between parental care and other vitally important traits, such as the immune system seem evident. In co-occurring phases of parental care and immunological challenges negative consequences through a resource allocation trade off on both the parental and the offspring conditions can be predicted. While the immune system is reflecting parental stress conditions, parental immunological investments also boost offspring survival via the transfer of immunological substances (trans-generational immune priming).We investigated this relationship adult and juvenile mouth brooding East African cichlid Astotatilapia burtoni. Prior to mating, females were exposed to an immunological activation, while others remained immunologically naive. Correspondingly, immunological status of females was either examined directly after reproduction or after mouth brooding had ceased. Offspring from both groups were exposed to immunological challenges to assess the extent of trans-generational immune priming. As proxy for immune status, cellular immunological activity and gene expression were determined.Results: Both reproducing and mouthbrooding females allocate their resources towards reproduction. While upon reproduction the innate immune system was impeded, mouthbrooding females showed an attenuation of inflammatory components and an elevated stress levels. Juveniles from immune challenged mouthbrooding females showed downregulation of immune and life history candidate genes, implying a limitation of trans-generational plasticity when parents experience stress during the costly reproductive phase.Conclusion: Our results provide evidence that parental investment via mouthbrooding is beneficial for the offspring. However, both parental investment and the rise of the immunological activity upon an immune challenge are costly traits. If applied simultaneously, not only mothers seem to be impacted in their performance, but also offspring are impeded in their ability to react upon a potentially virulent pathogen exposure. Supplement to: Keller, Isabel Salome; Salzburger, Walter; Roth, Olivia (2017): Parental investment matters for maternal and offspring immune defense in the mouthbrooding cichlid Astatotilapia burtoni. BMC Evolutionary Biology, 17(1)

Abstract Background Parental care, while increasing parental fitness through offspring survival, also bears cost to the care-giving parent. Consequentially, trade offs between parental care and other vitally important traits, such as the immune system seem evident. In co-occurring phases of parental care and immunological challenges negative consequences through a resource allocation trade off on both the parental and the offspring conditions can be predicted. While the immune system reflects parental stress conditions, parental immunological investments also boost offspring survival via the transfer of immunological substances (trans-generational immune priming). We investigated this relationship in the mouthbrooding East African cichlid Astotatilapia burtoni. Prior to mating, females were exposed to an immunological activation, while others remained immunologically naĂŻve. Correspondingly, the immunological status of females was either examined directly after reproduction or after mouthbrooding had ceased. Offspring from both groups were exposed to immunological challenges to assess the extent of trans-generational immune priming. As proxy for immune status, cellular immunological activity and gene expression were determined. Results Both reproducing and mouthbrooding females allocate their resources towards reproduction. While upon reproduction the innate immune system was impeded, mouthbrooding females showed an attenuation of inflammatory components. Juveniles from immune challenged mouthbrooding females showed downregulation of immune and life history candidate genes, implying a limitation of trans-generational plasticity when parents experience stress during the costly reproductive phase. Conclusion Our results provide evidence that both parental investment via mouthbrooding and the rise of the immunological activity upon an immune challenge are costly traits. If applied simultaneously, not only mothers seem to be impacted in their performance, but also offspring are impeded in their ability to react upon a potentially virulent pathogen exposure.