Biology subjects

Bell, R. C.

Publications and source records attributed to Bell, R. C..

8 recordsLinked to original sources

bioRxiv2025

Eyes wide open: Frogs with reduction of auditory communication ability show lower eye, but higher corneal, investment

Vocal communication is the primary mechanism of mate attraction in frogs, but some species have lost the auditory structures needed to hear airborne sounds, raising the question of how they communicate in their absence. We hypothesized that frog species with auditory system reductions have compensated through enhanced visual ability, including through enlarged eyes. To test this trade-off hypothesis, we estimated relative eye and corneal investment across 264 species and tested for correlations with auditory reduction. Contrary to our expectations, we found that species with auditory reductions had smaller eye size and lower eye investment. Instead, we found that non-fossorial species with auditory reductions had significantly greater corneal investment, after controlling for habitat-based variation. Enlarged corneas enhance light sensitivity but are expected to have a lower metabolic cost than larger eyes. Relatively larger corneas may, in combination with other changes, allow frogs with auditory reductions to rely more on vision for communication, while at the same time minimizing energetic costs in small animals that are already metabolically constrained. Our findings suggest that the visual system plays an adaptive role in response to the reduction of auditory communication and highlights the condition of sensory systems as an important factor shaping frog evolution.

evolutionary biology↗

bioRxiv2025

Evolutionary and ontogenetic shifts from aquatic to terrestrial light environments in frogs provide new insights into the vertebrate phototransduction cascade

Evolutionary studies of vertebrate vision have mainly focused on the light-sensitive opsins neglecting the downstream phototransduction cascade. Here, we investigate patterns of gene loss and expression across jawed vertebrates, providing clarity on the evolution of phototransduction. We next focus on an ecologically diverse sample of frogs represented by whole-eye transcriptomes and genomes from 113 species. We tested the hypothesis that phototransduction genes were lost in the common ancestor of frogs due to nocturnality and that functional differences in phototransduction are driven by variation in habitat, activity pattern, and life history. Across frogs, we recovered 38 of 39 vertebrate phototransduction genes, with all but one of these (GUCA1B2) consistently expressed in frog eyes. This contrasts the high level of gene loss found in other ancestrally nocturnal tetrapods (e.g., mammals and snakes) and is more similar to ray-finned fishes. More than other ecological traits, we found that loss of the larval aquatic life stage (direct development) and inhabiting aquatic and semiaquatic habitats as adults were most strongly associated with shifts in selective strength, with widespread signals of both positive and relaxed selection across frog phototransduction genes. These findings suggest that phototransduction is under different functional constraints in aquatic versus terrestrial light environments and that shifts in the use of these environments have played a strong role in frog visual evolution. Collectively, these results reinforce that frogs are of particular interest for vertebrate visual evolution because they span both an evolutionary and an ontogenetic transition from vision underwater to vision on land.

evolutionary biology↗

bioRxiv2022

Molecular evolution of non-visual opsin genes across environmental, developmental, and morphological adaptations in frogs

Non-visual opsins are transmembrane proteins expressed in the eyes, skin, and brain of many animals. When paired with a light-sensitive chromophore, non-visual opsins form photopigment systems involved in various non-visual, light-detection functions, including circadian rhythm regulation, light-seeking behavior, and detection of seasonality. Previous research has primarily explored the diversity and function of non-visual opsins in model organisms, with few studies investigating their molecular evolution in non-model species. Here we explored molecular evolution of non-visual opsin genes in anuran amphibians (frogs and toads). With diverse lifestyles ranging from fossorial to aquatic, anurans inhabit a diverse array of light environments, which makes them a compelling system for studying the evolution of light detection mechanisms. Using whole-eye transcriptomes from 79 anuran species, as well as genomes and multi-tissue transcriptomes from an additional 15 species, we 1) identify which non-visual opsin genes are expressed in the eyes of anurans; 2) compare selective constraint ({omega}, or dN/dS) among non-visual opsin genes; and 3) test for potential adaptive evolution by comparing selection between discrete ecological classes in anurans (e.g. diurnal versus non-diurnal). We consistently recovered 14 non-visual opsin genes from anuran eye transcriptomes, compared to 18 genes that we recovered genome wide, and detected positive selection in a subset of these genes. We also found variation in selective constraint between discrete ecological and life-history classes, which may reflect functional adaptation in non-visual opsin genes. Although non-visual opsins remain poorly understood, these findings provide insight into their molecular evolution and set the stage for future research on their potential function across taxa with diverse environmental, developmental, and morphological adaptations.

evolutionary biology↗

bioRxiv2021

Diversity and evolution of amphibian pupil shapes

Pupil constriction has important functional consequences for animal vision, yet the evolutionary mechanisms underlying diverse pupil sizes and shapes, often among animals that occupy optically similar environments, are poorly understood. We aimed to quantify the diversity and evolution of pupil shapes among amphibians and test for potential correlations to ecology based on functional hypotheses. Using photographs, we surveyed pupil shape and the orientation of the constricted pupil across adults of 1293 amphibian species, 72 families, and 3 orders, and additionally for larval life stages for all families of frogs and salamanders with a biphasic ontogeny. Pupil shape is exceptionally diverse in amphibians with evolutionary transitions throughout the amphibian tree of life. For amphibians with a biphasic life history, we found that pupils change in many species that occupy distinct habitats before and after metamorphosis. Finally, we found that non-elongated (round or diamond) constricted pupils were correlated with species inhabiting consistently dim light environments (burrowing and aquatic species) and that elongated pupils (vertical and horizontal) were more common in species with larger absolute eye sizes. We propose that amphibians provide a valuable group within which to explore the anatomical, physiological, optical, and ecological mechanisms underlying the evolution of pupil shape.

evolutionary biology↗

bioRxiv2021

Genomic and Spectral Visual Adaptation in Southern Leopard Frogs during the Ontogenetic Transition from Aquatic to Terrestrial Light Environments

Many animals have complex life cycles where larval and adult forms have distinct ecologies and habitats that impose different demands on their sensory systems. While the adaptive decoupling hypothesis predicts reduced genetic correlations between life stages, how sensory systems adapt across life stages at the molecular level is not well understood. Frogs are a compelling system to study this question in because most species rely on vision as both aquatic tadpoles and terrestrial adults, but these habitats present vastly different light environments. Here we used whole eye transcriptome sequencing to investigate differential expression between aquatic tadpoles and terrestrial juveniles of the southern leopard frog (Lithobates sphenocephalus). Because visual physiology changes with light levels, we also tested how constant light or dark exposure affected gene expression. We found 42% of genes were differentially expressed in the eyes of tadpoles versus juveniles, versus 5% for light/dark exposure. Analyses targeting a curated set of visual genes revealed significant differential expression between life stages in genes that control aspects of visual function and development, including spectral sensitivity and lens composition. Light/dark exposure had a significant effect on a smaller set of visual genes. Finally, microspectrophotometry of photoreceptors confirmed shifts in spectral sensitivity predicted by the expression results, consistent with adaptation to distinct light environments. Overall, we identified extensive expression-level differences in the eyes of tadpole and juvenile frogs related to observed morphological and physiological changes through metamorphosis, and corresponding adaptive shifts to optimize vision in aquatic versus terrestrial environments.

genomics↗

bioRxiv2021

Association between breastfeeding during infancy and white matter microstructure in early childhood

IntroductionAssociations between breastfeeding and brain development, in the context of child, perinatal, and sociodemographic variables, remain unclear. This study investigates whether exclusive breastfeeding for the first 6 months and total duration of any breastfeeding are associated with brain white matter microstructure in young children. MethodsThis study included a sample of 83 mothers and 85 typically developing children (42 males). Children underwent their first diffusion tensor imaging scan between ages 2.34-6.97 years; some children returned multiple times, providing a total of 331 datasets. Feeding information was collected from the mothers at 3, 6, and 12 months postpartum and at their childs scan to calculate breastfeeding status at 6 months (exclusive or not) as well as total duration of any breastfeeding. Linear regression was used to investigate associations between breastfeeding exclusivity/duration and fractional anisotropy (FA, a measure sensitive to myelination/axonal packing/fibre coherence) for the whole brain and 10 individual white matter tracts. ResultsBreastfeeding exclusivity and duration were associated with global and regional white matter microstructure, even after controlling for perinatal and sociodemographic factors. Greater exclusivity was associated with higher FA in females and lower FA in males. ConclusionsThese findings suggest white matter differences associated with breastfeeding that differ by sex. These may stem from different trajectories in white matter development between males and females in early childhood and suggest possible long-term white matter differences associated with breastfeeding.

neuroscience↗

bioRxiv2020

Evolutionary drivers of polymorphic sexual signals in slender anoles

Phenotypic variation among populations, as seen in the signaling traits of many species, provides an opportunity to test whether similar factors generate shared phenotypic patterns in different parts of a species range. We investigate whether genetic divergence, abiotic gradients, and sympatry with closely related species explain variation in the dewlap colors of slender anoles, Anolis fuscoauratus. To this aim, we characterized dewlap diversity in the field, inferred population genetic structure and evolutionary relationships, assessed whether dewlap morphs are associated with climate and landscape variables, and tested for non-random associations in the distribution of A. fuscoauratus morphs and sympatric Anolis species. We found that dewlap colors vary among but not within sites in A. fuscoauratus. Regional genetic clusters included multiple morphs, while populations with similar dewlaps were often distantly related. Morphs did not segregate in environmental space, suggesting that dewlaps are not locally adapted to abiotic factors. Instead, we found a negative association between certain morphs and Anolis species with similar relative dewlap brightness, suggesting that interactions with closely related species promoted dewlap divergence among A. fuscoauratus populations. Slender anoles emerge as a promising system to address questions about parallel trait evolution and the contribution of signaling traits to speciation.

evolutionary biology↗

bioRxiv2018

Sexual Dichromatism Drives Diversification Within a Major Radiation of African Amphibians

Theory predicts that sexually dimorphic traits under strong sexual selection, particularly those involved with intersexual signaling, can accelerate speciation and produce bursts of diversification. Sexual dichromatism (sexual dimorphism in color) is widely used as a proxy for sexual selection and is associated with rapid diversification in several animal groups, yet studies using phylogenetic comparative methods to explicitly test for an association between sexual dichromatism and diversification have produced conflicting results. Sexual dichromatism is rare in frogs, but it is both striking and prevalent in African reed frogs, a major component of the diverse frog radiation termed Afrobatrachia. In contrast to most other vertebrates, reed frogs display female-biased dichromatism in which females undergo color transformation, often resulting in more ornate coloration in females than in males. We produce a robust phylogeny of Afrobrachia to investigate the evolutionary origins of sexual dichromatism in this radiation and examine whether the presence of dichromatism is associated with increased rates of net diversification. We find that sexual dichromatism evolved once within hyperoliids and was followed by numerous independent reversals to monochromatism. We detect significant diversification rate heterogeneity in Afrobatrachia and find that sexually dichromatic lineages have double the average net diversification rate of monochromatic lineages. By conducting trait simulations on our empirical phylogeny, we demonstrate our inference of trait-dependent diversification is robust. Although sexual dichromatism in hyperoliid frogs is linked to their rapid diversification and supports macroevolutionary predictions of speciation by sexual selection, the function of dichromatism in reed frogs remains unclear. We propose that reed frogs are a compelling system for studying the roles of natural and sexual selection on the evolution of sexual dichromatism across both micro- and macroevolutionary timescales.

evolutionary biology↗