Fish microbiota repel ovipositing mosquitoes.

The mere presence of predators causes prey organisms to display predation-avoidance strategies. Predator presence is often communicated through predator-released chemical signals. Ovipositing female mosquitoes of several species are repelled by unknown signals released from larvivorous fish. It was previously suggested that in many cases, a predator's microbiota plays an important role in the release of these signals; however, this mechanism is still poorly understood. In this study, we looked into the effects of the microbiota originating from the larvivorous Gambusia affinis (Baird and Girard) on the oviposition behaviour of gravid female mosquitoes. We used fish with altered microbiota and bacterial isolates in a set of outdoor mesocosm experiments to address this aim. We show that interference with the fish microbiota significantly reduces fish's repellent effect. We further show that the bacterium Pantoea pleuroti, isolated from the skin of the fish, repels oviposition of Culex laticinctus Edwards and Culiseta longiareolata Macquart mosquitoes similarly to the way in which live fish repel them. Our results highlight the importance of bacteria in the interspecies interactions of their hosts. Furthermore, this finding may lead to the development of an ecologically friendly mosquito repellent, that may reduce the use of larvivorous fish for mosquito control.

Cortisol enhances aerobic metabolism and locomotor performance during the transition to land in an amphibious fish.

Amphibious fishes on land encounter higher oxygen (O2) availability and novel energetic demands, which impacts metabolism. Previous work on the amphibious mangrove killifish (Kryptolebias marmoratus) has shown that cortisol becomes elevated in response to air exposure, suggesting a possible role in regulating metabolism as fish move into terrestrial environments. We tested the hypothesis that cortisol is the mechanism by which oxidative processes are upregulated during the transition to land in amphibious fishes. We used two groups of fish, treated fish (+metyrapone, a cortisol synthesis inhibitor) and control (-metyrapone), to determine the impact of cortisol during air exposure (0 and 1 h, 7 days) on O2 consumption, terrestrial locomotion, the phenotype of red skeletal muscle, and muscle lipid concentration. Metyrapone-treated fish had an attenuated elevation in O2 consumption rate during the water to air transition and an immediate reduction in terrestrial exercise performance relative to control fish. In contrast, we found no short- (0 h) or long-term (7 days) differences between treatments in the oxidative phenotype of red muscles, nor in muscle lipid concentrations. Our results suggest that cortisol stimulates the necessary increase in aerobic metabolism needed to fuel the physiological changes that amphibious fishes undergo during the acclimation to air, although further studies are required to determine specific mechanisms of cortisol regulation.

Predictability and conceptual repeatability of the predator-associated burst speed ecophenotype in western mosquitofish (Gambusia affinis).

Predation exerts strong selection pressure on morphological traits and is often studied in freshwater fishes. A common morphological ecophenotype driven by predation from pursuit predators is the predator-associated burst speed (PABS) ecophenotype. This ecophenotype, characterized by a smaller head, smaller body, and larger caudal region, is commonly found in western mosquitofish (Gambusia affinis) residing in environments with sunfish (family Centrarchidae) predators. However, the repeatability and transferability of the PABS ecophenotype across populations have not been tested. The four objectives of this study were to (1) assess the repeatability of methods to confirm the presence of the PABS ecophenotype in independent populations, (2) test whether morphology is correlated with predator abundance, (3) assess the repeatability of utilizing a discriminant function analysis (DFA) to accurately classify fish to predator status, and (4) use a DFA to conduct a cross-validation test of the PABS model using previously studied populations of G. affinis to predict predator status of newly studied populations. There was consistency in the occurrence of the PABS ecophenotype among populations with predators. Permuted MANOVA (PMANOVA) models fit to each dataset revealed a significant effect of collection site nested within predator status (i.e., presence or absence of predator), suggesting location-specific variations of the PABS ecophenotype. In addition, a PMANOVA model fit to body shape as a function of predator abundance (i.e., 0, 0.1-1.0, or >1.0 predators per minute of electrofishing) revealed a significant effect of predation level in the newly studied populations, suggesting morphology differs among populations with varying predator abundances. Cross-study validation of the DFA revealed low between-study accuracy compared to within-study accuracy, but elongation of the caudal region in the presence of predators was consistent across studies. Our findings ultimately suggest that although the PABS ecophenotype at studied locations provides partial predictive capacity at unstudied locations, the nature and magnitude of the PABS ecophenotype depend on environmental settings, encounter histories with predators, level of abundance of predators, or other spatially structured mechanisms.

Social experience influences thermal sensitivity: lessons from an amphibious mangrove fish.

Understanding the factors affecting the capacity of ectothermic fishes to cope with warming temperature is critical given predicted climate change scenarios. We know that a fish's social environment introduces plasticity in how it responds to high temperature. However, the magnitude of this plasticity and the mechanisms underlying socially modulated thermal responses are unknown. Using the amphibious hermaphroditic mangrove rivulus fish Kryptolebias marmoratus as a model, we tested three hypotheses: (1) social stimulation affects physiological and behavioural thermal responses of isogenic lineages of fish; (2) social experience and acute social stimulation result in distinct physiological and behavioural responses; and (3) a desensitization of thermal receptors is responsible for socially modulated thermal responses. To test the first two hypotheses, we measured the temperature at which fish emerged from the water (i.e. pejus temperature) upon acute warming with socially naive isolated fish and with fish that were raised alone and then given a short social experience prior to exposure to increasing temperature (i.e. socially experienced fish). Our results did not support our first hypothesis as fish socially stimulated by mirrors during warming (i.e. acute social stimulation) emerged at similar temperatures to isolated fish. However, in support of our second hypothesis, a short period of prior social experience resulted in fish emerging at a higher temperature than socially naive fish suggesting an increase in pejus temperature with social experience. To test our third hypothesis, we exposed fish that had been allowed a brief social interaction and naive fish to capsaicin, an agonist of TRPV1 thermal receptors. Socially experienced fish emerged at significantly higher capsaicin concentrations than socially naive fish suggesting a desensitization of their TRPV1 thermal receptors. Collectively, our data indicate that past and present social experiences impact the behavioural response of fish to high temperature. We also provide novel data suggesting that brief periods of social experience affect the capacity of fish to perceive warm temperature.

How maternal age and environmental cues influence embryonic developmental pathways and diapause dynamics in a North American annual killifish.

BACKGROUND: Temporary pools are variable environments with seasonal drought/flood phases. Annual killifish have adapted to life in temporary pools by producing embryos that undergo diapause to traverse the dry phase. To fill existing knowledge gaps about embryo diapause regulation and evolution in annual killifishes, we test the effect of maternal age, incubation temperature, and incubation medium on diapause induction and length in Millerichthys robustus, the only North American fish species that has evolved an annual life history. RESULTS: All embryos at extreme temperatures follow a defined developmental pathway: skipping diapause at 30°C, and entering diapause at 18°C, both regardless of maternal age, and incubation medium. However, maternal age, and incubation medium influenced whether diapause is entered, and time arrested in diapause for embryos incubated at 25°C. At 25°C, five-week-old, and 52-week-old females produced more embryos that entered diapause than 26-week-old females. Also, embryos incubated in aqueous medium skipped diapause more frequently at this intermediate temperature. CONCLUSIONS: Millerichthys developmental dynamics associated with maternal age under intermediate range of temperatures are likely adapted to the particular patterns of flood/drought in North American temporary pools. Millerichthys also exhibits developmental patterns largely comparable with other annual fishes, probably due to common seasonal patterns in temporary pools.

Context-dependent relationships between swimming, terrestrial jumping and body composition in the amphibious fish Kryptolebias marmoratus.

Understanding the mechanisms that create phenotypic variation within and among populations is a major goal of physiological ecology. Variation may be a consequence of functional trade-offs (i.e. improvement in one trait comes at the expense of another trait) or alternatively may reflect the intrinsic quality of an organism (i.e. some individuals are simply better overall performers than others). There is evidence for both ideas in the literature, suggesting that environmental context may mediate whether variation results from trade-offs or differences in individual quality. We tested this overarching 'context dependence' hypothesis by comparing the aquatic and terrestrial athletic performance of the amphibious fish Kryptolebias marmoratus captured from two contrasting habitats, a large pond and small burrows. Overall, pond fish were superior terrestrial athletes but burrow fish were better burst swimmers, suggestive of a performance trade-off at the population level. Within each population, however, there was no evidence of a performance trade-off. In burrow fish, athletic performance was positively correlated with muscle content and body condition, consistent with the individual quality hypothesis. In pond fish, there was only a relationship between glycolytic white muscle and aquatic burst performance. Notably, pond fish were in better body condition, which may mask relationships between condition and athletic performance. Overall, our data highlight that population-level trends are insufficient evidence for the existence of phenotypic trade-offs in the absence of similar within-population patterns. Furthermore, we only found evidence for the individual quality hypothesis in one population, suggesting that patterns of phenotypic covariance are context dependent.

Increased superfetation precedes the evolution of advanced degrees of placentotrophy in viviparous fishes of the family Poeciliidae.

The causes and consequences of the evolution of placentotrophy (post-fertilization nutrition of developing embryos of viviparous organisms by means of a maternal placenta) in non-mammalian vertebrates are still not fully understood. In particular, in the fish family Poeciliidae there is an evolutionary link between placentotrophy and superfetation (ability of females to simultaneously bear embryos at distinct developmental stages), with no conclusive evidence for which of these two traits facilitates the evolution of more advanced degrees of the other. Using a robust phylogenetic comparative method based on Ornstein-Uhlenbeck models of adaptive evolution and data from 36 poeciliid species, we detected a clear causality pattern. The evolution of extensive placentotrophy has been facilitated by the preceding evolution of more simultaneous broods. Therefore, placentas became increasingly complex as an adaptive response to evolutionary increases in the degree of superfetation. This finding represents a substantial contribution to our knowledge of the factors that have shaped placental evolution in poeciliid fishes.

Effects of crowding stress on the hypothalamo-pituitary-interrenal axis of the self-fertilizing fish, Kryptolebias marmoratus.

We tested whether crowding stress affects the hypothalamo-pituitary-interrenal (HPI) axis of the self-fertilizing fish, Kryptolebias marmoratus, which is known to be aggressive in the laboratory conditions but sometimes found as a group from a single land crab burrow in the wild. The projection of corticotropin-releasing hormone (CRH) neurons to the adrenocorticotropic hormone (ACTH) cells in the pituitary was confirmed by dual-label immunohistochemistry; CRH-immunoreactive (ir) fibers originating from cell bodies located in the lateral tuberal nucleus (NLT) of the hypothalamus were observed to project to ACTH-ir cells in the rostral pars distalis of the pituitary. Then, fish were reared solitary or in pairs for 14 days, and the number of CRH-ir cell bodies in the NLT of the hypothalamus and cortisol levels in the body without head region were compared. The number of CRH-ir cell bodies and cortisol levels were significantly higher in paired fish. These results indicate that crowding stress affects the HPI axis in K. marmoratus which thrive in small burrows with limited water volume.

Starter feed for carnivorous species as a practical replacement of bloodworms for a vertebrate model organism in ageing, the turquoise killifish Nothobranchius furzeri.

The absence of a controlled diet is unfortunate in a promising model organism for ageing, the turquoise killifish (Nothobranchius furzeri Jubb, 1971). Currently captive N. furzeri are fed bloodworms but it is not known whether this is an optimal diet. Replacing bloodworms with a practical dry feed would reduce diet variability. In the present study, we estimated the nutritional value of the diet ingested by wild fish and determined the fish-body amino acid profile as a proxy for their nutritional requirements. We compared the performance of fish fed four commercial feeds containing 46%-64% protein to that achieved with bloodworms and that of wild fish. Wild fish target a high-protein (60%) diet and this is supported by their superior performance on high-protein diets in captivity. In contrast, feeds for omnivores led to slower growth, lower fecundity and unnatural liver size. In comparison to wild fish, a bloodworm diet led to lower body condition, overfeeding and male liver enlargement. Out of the four dry feeds tested, the fish fed Aller matched wild fish in body condition and liver size, and was comparable to bloodworms in terms of growth and fecundity. A starter feed for carnivorous species appears to be a practical replacement for bloodworms for N. furzeri. The use of dry feeds improved performance in comparison to bloodworms and thus may contribute to reducing response variability and improving research reproducibility in N. furzeri research.

Neuronal Phenotype of col4a1 and col25a1: An Intriguing Hypothesis in Vertebrates Brain Aging.

Collagens are the most abundant proteins in vertebrates and constitute the major components of the extracellular matrix. Collagens play an important and multifaceted role in the development and functioning of the nervous system and undergo structural remodeling and quantitative modifications during aging. Here, we investigated the age-dependent regulation of col4a1 and col25a1 in the brain of the short-lived vertebrate Nothobranchius furzeri, a powerful model organism for aging research due to its natural fast-aging process and further characterized typical hallmarks of brain aging in this species. We showed that col4a1 and col25a1 are relatively well conserved during vertebrate evolution, and their expression significantly increases in the brain of N. furzeri upon aging. Noteworthy, we report that both col4a1 and col25a1 are expressed in cells with a neuronal phenotype, unlike what has already been documented in mammalian brain, in which only col25a1 is considered a neuronal marker, whereas col4a1 seems to be expressed only in endothelial cells. Overall, our findings encourage further investigation on the role of col4a1 and col25a1 in the biology of the vertebrate brain as well as the onset of aging and neurodegenerative diseases.

Administration of krill oil extends lifespan of fish Nothobranchius guentheri via enhancement of antioxidant system and suppression of NF-κB pathway.

Krill oil (KO) extracted from Antarctic krill (Euphausia superba) mainly comprises phospholipids and triglycerides. KO has been shown to prolong the median lifespan of the nematode Caenorhabditis elegans, but to shorten the lifespan of long-lived F1 mice; therefore, it remains controversial over the life-extending property of KO. In this study, we clearly demonstrated that dietary intake of KO extended both the mean and maximum lifespans of aged male Nothobranchius guentheri (p < 0.05), reduced the accumulation of lipofuscin (LF) (p < 0.05) in the gills and senescence-associated ß-galactosidase (SA-ß-Gal) (p < 0.05) in the caudal fins, and lowered the levels of protein oxidation (p < 0.05), lipid peroxidation (p < 0.01), and reactive oxygen species (ROS) (p < 0.01) in the muscles and livers, indicating that KO possesses rejuvenation and anti-aging activity. We also showed that KO enhanced the activities of antioxidant enzymes catalase (CAT) (p < 0.05), superoxide dismutase (SOD) (p < 0.05), and glutathione peroxidase (GPX) (p < 0.05) in aged male N. guentheri. In addition, KO administration effectively reversed histological lesions including inflammatory cell infiltration and structural collapse in the muscles and livers of aged N. guentheri and suppressed the nuclear factor kappa-B (NF-κB) signaling pathway (p < 0.05), a master regulator of inflammation. Altogether, our study indicates that KO has anti-aging and rejuvenation property. It also suggests that KO exerts its anti-aging and rejuvenation effects via enhancement of the antioxidant system and suppression of the NF-κB signaling pathway.

Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish.

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus (Kryptolebias marmoratus) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na+-K+-ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.

Cutaneous respiration and osmoregulation in amphibious fishes.

In his early career, August Krogh made fundamental discoveries of the properties of cutaneous respiration in fish, frogs and other vertebrates. Following Krogh's example, the study of amphibious fishes provides an excellent model to understand how the skin morphology and physiological mechanisms evolved to meet the dual challenges of aquatic and terrestrial environments. The skin of air-exposed fishes takes on many of the functions that are typically associated with the gills of fish in water: gas exchange, gas sensing, iono- and osmoregulation, and nitrogen excretion. The skin of amphibious fishes has capillaries close to the surface in the epidermis. Skin ionocytes or mitochondrial-rich cells (MRCs) in the epidermis are thought to be responsible for ion exchange, as well as ammonia excretion in the amphibious mangrove rivulus Kryptolebias marmoratus. Ammonia gas (NH3) moves down the partial pressure gradient from skin capillaries to the surface through ammonia transporters (e.g., Rhcg) and NH3 is volatilized from the mucus film on the skin. Future studies are needed on the skin of amphibious fishes from diverse habitats to understand more broadly the role of the skin as a multifunctional organ.

Histopathology and transcriptome reveals the tissue-specific hepatotoxicity and gills injury in mosquitofish (Gambusia affinis) induced by sublethal concentration of triclosan.

Triclosan (TCS), a ubiquitous antimicrobial agent, has been frequently detected in wild fish, leading to concerns regarding TCS safety in the aquatic environment. The present work aims to investigate the TCS-mediated effects on various tissues (the liver, gills, brain, and testes) of wild-sourced adult mosquitofish based on histological analysis and transcriptome. Severe morphological injuries were only found in the liver and gills. The histopathological alterations in the liver were characterized by cytoplasmic vacuolation and degeneration, eosinophilic cytoplasmic inclusions, and nuclear polymorphism. The gill lesions contained epithelial lifting, intraepithelial edema, fusion and shortening of the secondary lamellae. Consistently, the numbers of differently expressed genes (DEGs) identified by transcriptome were in the order of liver (1627) > gills (182) > brain (9) > testes (4). Trend-aligned histopathological and transcriptomic changes in the 4 tissues, suggesting the tissue-specific response manner of mosquitofish to TCS, and the liver and gills were the target organs. TCS interrupted many biological pathways associated with lipogenesis and lipid metabolism, transmembrane transporters, protein synthesis, and carbohydrate metabolism in the liver, and it induced nonspecific immune response in the gills. TCS-triggered hepatotoxicity and gills damnification may lead to inflammation, apoptosis, diseases, and even death in mosquitofish. TCS showed moderate acute toxicity and bioaccumulative property on mosquitofish, suggesting that prolonged or massive use of TCS may pose an ecological risk.

Endocrine disrupting effects of binary mixtures of 17ß-estradiol and testosterone in adult female western mosquitofish (Gambusia affinis).

Androgens and estrogens often co-exist in aquatic environments and pose potential risks to fish populations. However, little is known about the endocrine disrupting effects of the mixture of androgens and estrogens in fish. In this study, transcriptional level of target genes related to the hypothalamic-pituitary-gonadal-liver (HPGL) axis, sex hormone level, VTG protein concentration, histology and secondary sex characteristic were assessed in the ovaries and livers of adult female western mosquitofish (Gambusia affinis) exposed to 17ß-estradiol (E2), testosterone (T), and mixtures of E2 and T for 91 days. The results showed that the transcriptional expression of cytochrome P450, family 19, subfamily A, polypeptide 1a (Cyp19a1a) was suppressed in the 200 ng/L T treatment and the 50 ng/L E2 + 200 ng/L T treatment in the ovaries. Steroidogenic acute regulatory protein (Star) and Cyp11a1 showed a similar expression pattern in the T treatment to its corresponding T + E2 mixtures. In the ovaries, the concentrations of 17ß-estradiol and testosterone were decreased in most treatments compared with the solvent control. VTG protein was induced in all steroid treatment. However, exposure to T or E2 + T mixture did not cause the abnormal cells of the ovaries and livers and an extension of the anal fins in female G. affinis. This study demonstrates that chronic exposure to E2, T and their mixtures affects the transcripts of genes in the HPGL axis, steroid hormone level and VTG protein concentration in the ovaries and livers, but fails to cause the histopathological effect of the ovaries and livers and alter the morphology of the anal fins in G. affinis.

Reproductive strategies and chromosomal aberrations affect survival in the Rivuliid fish Hypsolebias sertanejo.

Rivulidae comprises a family of fish largely distributed in Brazil that includes 201 species, of which 125 are considered endangered. This fact emphasizes the need for development of conservation strategies including studies on genetics and reproduction. In this paper, we describe aspects of biology and reproduction of the rivuliid species Hypsolebias sertanejo. We outline the reproductive behaviour of this species under laboratory conditions, analyze ploidy status by flow cytometry, describe reproductive behaviour and performance and test dry and wet incubation of eggs. Although H. sertanejo showed well known patterns of reproductive behaviour, we verified many peculiarities inherent to its reproductive biology. As expected, most individuals were diploid (87.71%), however 14.29% were considered mosaics. Although no sterility was observed within mosaics, infertility of these fish was not fully evaluated. Hatching rate of the eggs collected was very low following both dry and wet incubation (5.04 and 3.79%, respectively). These results provide interesting information regarding the reproductive success of this species, and suggest that chromosomal abnormalities described may reduce the survival of H. sertanejo under natural conditions, limiting the perpetuation of this species, and emphasizing the need for more preservation efforts, including artificial propagation and gene banking.

Shifts in Reproductive Investment in Response to Competitors Lower Male Reproductive Success.

AbstractIn many species, males exhibit phenotypic plasticity in sexually selected traits when exposed to social cues about the intensity of sexual competition. To date, however, few studies have tested how this plasticity affects male reproductive success. We initially tested whether male mosquitofish, Gambusia holbrooki (Poeciliidae), change their investment in traits under pre- and postcopulatory sexual selection depending on the social environment. For a full spermatogenesis cycle, focal males were exposed to visual and chemical cues of rivals that were either present (competitive treatment) or absent (control). Males from the competitive treatment had significantly slower-swimming sperm but did not differ in sperm count from control males. When two males competed for a female, competitive treatment males also made significantly fewer copulation attempts and courtship displays than control males. Further, paternity analysis of 708 offspring from 148 potential sires, testing whether these changes in reproductive traits affected male reproductive success, showed that males previously exposed to cues about the presence of rivals sired significantly fewer offspring when competing with a control male. We discuss several possible explanations for these unusual findings.

Beyond Pairwise Interactions: Multispecies Character Displacement in Mexican Freshwater Fish Communities.

Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased, as it has focused almost exclusively on pairs of interacting species while ignoring multispecies interactions. Communities are seldom so simple that only pairs of species interact, and it is not clear whether inferences from pairwise interactions are sufficient to explain patterns of phenotypes in nature. Here, we test for character displacement in a natural system of freshwater fishes in western Mexico that contains up to four congeneric species of the genus Poeciliopsis. We analyzed body shape differences between populations with different numbers of competitors while accounting for confounding environmental variables. Surprisingly, we found evidence for convergent character displacement in populations of P. prolifica, P. viriosa, and P. latidens. We also found that the convergence in body shape was not consistently in the same direction, meaning that when three or more competitors co-occurred, we did not find more extreme body shapes compared with when there were only two competitors. Instead, when three or more competitors co-occurred, body shape was intermediate between the shape found with a pair of species and the shape found with no competitor present. This intermediate shape suggests that evolution in multispecies communities likely occurs in response to several competitors rather than to simple pairwise interactions. Overall, our results suggest that competition among multiple species is more complex than simple pairwise competitive interactions.

Extreme genomic volatility characterizes the evolution of the immunoglobulin heavy chain locus in cyprinodontiform fishes.

The evolution of the adaptive immune system has provided vertebrates with a uniquely sophisticated immune toolkit, enabling them to mount precise immune responses against a staggeringly diverse range of antigens. Like other vertebrates, teleost fishes possess a complex and functional adaptive immune system; however, our knowledge of the complex antigen-receptor genes underlying its functionality has been restricted to a small number of experimental and agricultural species, preventing systematic investigation into how these crucial gene loci evolve. Here, we analyse the genomic structure of the immunoglobulin heavy chain (IGH) gene loci in the cyprinodontiforms, a diverse and important group of teleosts present in many different habitats across the world. We reconstruct the complete IGH loci of the turquoise killifish (Nothobranchius furzeri) and the southern platyfish (Xiphophorus maculatus) and analyse their in vivo gene expression, revealing the presence of species-specific splice isoforms of transmembrane IGHM. We further characterize the IGH constant regions of 10 additional cyprinodontiform species, including guppy, Amazon molly, mummichog and mangrove killifish. Phylogenetic analysis of these constant regions suggests multiple independent rounds of duplication and deletion of the teleost-specific antibody class IGHZ in the cyprinodontiform lineage, demonstrating the extreme volatility of IGH evolution. Focusing on the cyprinodontiforms as a model taxon for comparative evolutionary immunology, this work provides novel genomic resources for studying adaptive immunity and sheds light on the evolutionary history of the adaptive immune system.

Reversals in complex traits uncovered as reticulation events: Lessons from the evolution of parity-mode, chromosome morphology, and maternal resource transfer.

Complex traits include, among many others, the evolution of eyes, wings, body forms, reproductive modes, human intelligence, social behavior, diseases, and chromosome morphology. Dollo's law states that the evolution of complex traits is irreversible. However, potential exceptions have been proposed. Here, we investigated whether reticulation, a simple and elegant means by which complex characters may be reacquired, could account for suggested reversals in the evolution of complex characters using two datasets with sufficient genetic coverage and a total of five potential reversals. Our analyses uncovered a potential reversal in the evolution of parity mode and a potential reversal in the evolution of placentotrophy of fish (Cyprinodontiformes) as reticulation events. Moreover, in a reptile that exhibits a potential reversal from viviparity to oviparity (Zootoca vivipara), reticulation provided the most parsimonious explanation for sex chromosome evolution. Therefore, three of the five studied potential reversals were unraveled as reticulation events. This constitutes the first evidence that accounting for reticulation can fundamentally influence the interpretation of the evolution of complex traits, that testing for reticulation is crucial for obtaining robust phylogenies, and that complex ancestral characters may be reacquired through hybridization with a lineage that still exhibits the trait. Hybridization, rather than reappearance of ancestral traits by means of small evolutionary steps, may thus account for suggested exceptions to Dollo's law. Consequently, ruling out reticulation is required to claim the evolutionary reversal of complex characters and potential exceptions to Dollo's rule.