Phenotypic differentiation in a heterogeneous environment: morphological and life‐history responses to ecological gradients in a livebearing fish

Characterizing functional morphology and trophic niches in a neotropical Characiforms (Actinopterygii: Teleostei) assemblage in middle Munim River basin, Maranhão, Brazil

Understanding how functionally similar species segregate resources to minimize competition is vital for predicting evolutionary factors and patterns of coexistence. We conducted a study in Mata de Itamacaoca, in the middle Munim River basin, Maranhão, northeastern Brazil, to characterize the functional morphology and trophic niches of five coexisting Characiform species in this area - including a recently described species, and to investigate whether their functional morphology is a key determinant of their trophic niches. Our analysis of functional morphology and diet, employing linear measurements to predict dietary specializations, showed that these species are predominantly generalist insectivores with a significant morphological overlap. This study underscores the influence of species' natural history on their ecological characteristics, contributing to more effective conservation strategies.

Unstable environment of coastal lagoons drives genetic variation in the amphipod Quadrivisio lutzi

The freshwater/brackish amphipod Quadrivisio lutzi inhabits coastal lagoons, highly unstable environments subject to sudden inflow of marine water. Our aim was to evaluate how the genetic composition varies in these populations. Brazilian populations were compared by 16S rRNA and COI gene sequences. The genetic structure of four Rio de Janeiro amphipod populations was evaluated during the period of 2011-2019 by COI. Rio de Janeiro population was compared with Alagoas and São Paulo populations, which was genetically distinct, at species level (16S, d > 7%; COI, d >14%). The genetic structure in Rio de Janeiro showed the Imboassica subpopulation as the most divergent (Imboassica & Carapebus, F ST = 0.238), followed by Lagamar population (Lagamar & Carapebus, F ST = 0.049). The geographic distance and urbanization around these lagoons explain the degree of genetic isolation of these amphipod subpopulations. Paulista and Carapebus populations were not structured. Temporal variation in haplotype number and frequency were evident in both populations that were evaluated (Carapebus and Imboassica). Changes in salinity and water volume variation at these lagoons may be responsible for the observed changes in genetic composition, which may be the results of genetic drift effects over temporally fluctuating size subpopulations, without loss of genetic diversity.

Compiling forty years of guppy research to investigate the factors contributing to (non)parallel evolution

Examples of parallel evolution have been crucial for our understanding of adaptation via natural selection. However, strong parallelism is not always observed even in seemingly similar environments where natural selection is expected to favour similar phenotypes. Leveraging this variation in parallelism within well-researched study systems can provide insight into the factors that contribute to variation in adaptive responses. Here we analyse the results of 36 studies reporting 446 average trait values in Trinidadian guppies, Poecilia reticulata, from different predation regimes. We examine how the extent of predator-driven phenotypic parallelism is influenced by six factors: sex, trait type, rearing environment, ecological complexity, evolutionary history, and time since colonization. Analyses show that parallel evolution in guppies is highly variable and weak on average, with only 24.7% of the variation among populations being explained by predation regime. Levels of parallelism appeared to be especially weak for colour traits, and parallelism decreased with increasing complexity of evolutionary history (i.e., when estimates of parallelism from populations within a single drainage were compared to estimates of parallelism from populations pooled between two major drainages). Suggestive - but not significant - trends that warrant further research include interactions between the sexes and different trait categories. Quantifying and accounting for these and other sources of variation among evolutionary 'replicates' can be leveraged to better understand the extent to which seemingly similar environments drive parallel and nonparallel aspects of phenotypic divergence.

Population genetics and independently replicated evolution of predator-associated burst speed ecophenotypy in mosquitofish

Many species show replicated ecophenotypy due to recurring patterns of natural selection. Based on the presence or absence of pursuit predators, at least 17 species of fish repeatedly differentiated in body shape in a manner that increases burst swimming speed and the likelihood of predator escape. The predator-associated burst speed (PABS) ecophenotype is characterized by a small head and trunk and enlarged caudal region. Mechanisms promoting replicated phenotype-environment association include selection (without evolution), a single instance of adaptive evolution followed by biased habitat occupation, repeated instances of local adaptation, or adaptive phenotypic plasticity. Common garden rearing of mosquitofish, Gambusia affinis, demonstrated a likely heritable basis for PABS phenotypy, but it is unknown whether populations are otherwise genetically distinct or whether replicated ecophenotypy represents a single or replicated instances of adaptation. To genetically characterize the populations and test hypotheses of single or multiple adaptations, we characterized variation in 12 polymorphic DNA microsatellites in the previously studied G. affinis populations. Populations were genetically distinct by multilocus analysis, exhibited high allelic diversity, and were heterozygote deficient, which effects were attributed to G. affinis's shoaling nature and habitat patchiness. Genetic and phenotypic distances among populations were correlated for non-PABS but not PABS morphology. Multilocus analysis demonstrated ecophenotype polyphyly and scattered multivariate genetic structure which support only the replicated-adaptation model. As all of the diverse tests performed demonstrated lack of congruence between patterns of molecular genetic and PABS differentiation, it is likely that divergent natural selection drove multiple instances of adaptive evolution.

Maternal food restriction during pregnancy affects offspring development and swimming performance in a placental live-bearing fish

How pregnant mothers allocate limited resources to different biological functions such as maintenance, somatic growth, and reproduction can have profound implications for early life development and survival of offspring. Here, we examined the effects of maternal food restriction during pregnancy on offspring in the matrotrophic (i.e. mother-nourishment throughout gestation) live-bearing fish species Phalloptychus januarius (Poeciliidae). We fed pregnant females with either low or high food levels for 6 weeks and quantified the consequences for offspring size and body fat at birth and 1 week after birth. We further measured fast-start escape performance of offspring at birth, as well as swimming kinematics during prey capture at 0, 2 and 7 days after birth. We found that the length of maternal food restriction during pregnancy negatively affected offspring dry mass and lean dry mass at birth, as well as body fat gain during the first week after birth. Moreover, it impacted the locomotor performance of offspring during prey capture at birth and during the first week after birth. We did not observe an effect of food restriction on fast-start escape performance of offspring. Our study suggests that matrotrophic poeciliid fish are maladapted to unpredictably fluctuating resource environments, because sudden reductions in maternal food availability during pregnancy result in smaller offspring with slower postnatal body fat gain and an inhibition of postnatal improving swimming skills during feeding, potentially leading to lower competitive abilities after birth.

Highlighted Article: Maternal food restriction during pregnancy results in smaller offspring, slower postnatal body fat gain and an inhibition of postnatal improvement of swimming skills during feeding, possibly leading to lower competitive abilities after birth.

Phenotypic responses to oil pollution in a poeciliid fish

Pollution damages ecosystems around the globe and some forms of pollution, like oil pollution, can be either man-made or derived from natural sources. Despite the pervasiveness of oil pollution, certain organisms are able to colonise polluted or toxic environments, yet we only have a limited understanding of how they are affected by it. Here, we analysed phenotypic responses to oil pollution in guppies (Poecilia reticulata) living in oil-polluted habitats across southern Trinidad. We analysed body-shape and life-history traits for 352 individuals from 11 independent populations, six living in oil-polluted environments (including the naturally oil-polluted Pitch Lake), and five stemming from non-polluted habitats. Based on theory of, and previous studies on, responses to environmental stressors, we predicted guppies from oil-polluted waters to have larger heads and shallower bodies, to be smaller, to invest more into reproduction, and to produce more but smaller offspring compared to guppies from non-polluted habitats. Contrary to most of our predictions, we uncovered strong population-specific variation regardless of the presence of oil pollution. Moreover, guppies from oil-polluted habitats were characterised by increased body size; rounder, deeper bodies with increased head size; and increased offspring size, when compared to their counterparts from non-polluted sites. This suggests that guppies in oil-polluted environments are not only subject to the direct negative effects of oil pollution, but might gain some (indirect) benefits from other concomitant environmental factors, such as reduced predation and reduced parasite load. Our results extend our knowledge of organismal responses to oil pollution and highlight the importance of anthropogenic pollution as a source of environmental variation. They also emphasise the understudied ecological heterogeneity of extreme environments.

Predation and Resource Availability Interact to Drive Life-History Evolution in an Adaptive Radiation of Livebearing Fish

Predation risk and resource availability are two primary factors predicted by theory to drive the evolution of life histories. Yet, disentangling their roles in life-history evolution in the wild is challenging because (1) the two factors often co-vary across environments, and (2) environmental effects on phenotypes can mask patterns of genotypic evolution. Here, we use the model system of the post-Pleistocene radiation of Bahamas mosquitofish (Gambusia hubbsi) inhabiting blue holes to provide a strong test of the roles of predation and resources in life-history evolution, as the two factors do not co-vary in this system and we attempted to minimize environmental effects by raising eight populations under common laboratory conditions. We tested a priori predictions of predation- and resource-driven evolution in five life-history traits. We found that life-history evolution in Bahamas mosquitofish largely reflected complex interactions in the effects of predation and resource availability. High predation risk has driven the evolution of higher fecundity, smaller offspring size, more frequent reproduction, and slower growth rate—but this predation-driven divergence primarily occurred in environments with relatively high resource availability, and the effects of resources on life-history evolution was generally greater within environments having high predation risk. This implies that resource-driven selection on life histories overrides selection from predators when resources are particularly scarce. While several results matched a priori predictions, with the added nuance of interdependence among selective agents, some did not. For instance, only resource levels, not predation risk, explained evolutionary change in male age at maturity, with more rapid sexual maturation in higher-resource environments. We also found faster (not slower) juvenile growth rates within low-resource and low-predation environments, probably caused by selection in these high-competition scenarios favoring greater growth efficiency. Our approach, using common-garden experiments with a natural system of low- and high-predation populations that span a continuum of resource availability, provides a powerful way to deepen our understanding of life-history evolution. Overall, it appears that life-history evolution in this adaptive radiation has resulted from a complex interplay between predation and resources, underscoring the need for increased attention on more sophisticated interactions among selective agents in driving phenotypic diversification.

A century later: Adaptive plasticity and rapid evolution contribute to geographic variation in invasive mosquitofish

One century after their introduction to Europe, eastern mosquitofish (Gambusia holbrooki) represent a natural experiment to determine the relative contributions of adaptive plasticity and rapid evolutionary change in creating large-scale geographic variation in phenotypes. We evaluated the population-genetic structure and invasion history based on allele length polymorphisms of 15 nuclear microsatellites, which we quantified for N = 660 individuals from 23 populations sampled in 2013 across the invasive range of G. holbrooki in Europe. We analysed body-shape and life-history variation in N = 1331 individuals from 36 populations, sampled in 2013 and 2017, and tested heritability of phenotypic differences in a subset of four populations using a common-garden experiment. The genetic structure of wild-caught individuals suggested a single introduction for all European mosquitofish, which were genetically impoverished compared to their native counterparts. We found some convergent patterns of phenotypic divergence across native and invasive climatic gradients (e.g., increased body size in colder/more northern populations); however, several phenotypic responses were not consistent between sampling years, pointing towards plastic phenotypes. Our analysis of common-garden reared individuals uncovered moderate heritability estimates only for two measures of male body size (intraclass correlation coefficient, ICC = 0.628 and 0.556) and offspring fat content (ICC = 0.734), while suggesting high levels of plasticity in most other phenotypic traits (ICC ≤ 0.407). Our results highlight the importance of phenotypic plasticity in invasive species during range expansions and demonstrate that strong selective pressures-in this case towards increased body size in colder environments-simultaneously promote rapid evolutionary divergence.