A field-study of inducible molecular defenses,ultraviolet radiation,and melanomagenesis in natural <Emphasis Type="Italic">Xiphophorus</Emphasis> hybrids

Ultraviolet radiation—the primary natural pollutant affecting melanomagenesis—may represent a widespread ecological stressor for many fishes, and yet the relationship between UV-exposure and stress has not been investigated in natural fish populations. Recent lab-based studies have sought to characterize the relationship between tumorigenesis and the induction of molecular defenses, such as heat shock proteins. Here we show that ultraviolet radiation and heat shock protein gene expression explain a significant amount of the variation in hyper-melanization—the phenotypic precursor to melanoma—in wild hybrids of Xiphophorus, laboratory models in cancer research. Our results suggest exposure to UV radiation causes stress which induces molecular defense mechanisms, which in turn may facilitate tumorigenesis in natural fish populations. Studies of laboratory-based model organisms in natural settings, like this one, may provide important insights into ecological and evolutionary relationships obscured in controlled laboratory environments. We hope that ours is only the first of many studies to investigate the such relationships between environmental stress, stress-induced molecular defenses, and cancer in fishes.     

Sequence and structural characterization of great salt lake bacteriophage CW02, a member of the T7-like supergroup

Halophage CW02 infects a Salinivibrio costicola-like bacterium, SA50, isolated from the Great Salt Lake. Following isolation, cultivation, and purification, CW02 was characterized by DNA sequencing, mass spectrometry, and electron microscopy. A conserved module of structural genes places CW02 in the T7 supergroup, members of which are found in diverse aquatic environments, including marine and freshwater ecosystems. CW02 has morphological similarities to viruses of the Podoviridae family. The structure of CW02, solved by cryogenic electron microscopy and three-dimensional reconstruction, enabled the fitting of a portion of the bacteriophage HK97 capsid protein into CW02 capsid density, thereby providing additional evidence that capsid proteins of tailed double-stranded DNA phages have a conserved fold. The CW02 capsid consists of bacteriophage lambda gpD-like densities that likely contribute to particle stability. Turret-like densities were found on icosahedral vertices and may represent a unique adaptation similar to what has been seen in other extremophilic viruses that infect archaea, such as Sulfolobus turreted icosahedral virus and halophage SH1.     

Physiological adaptation along environmental gradients and replicated hybrid zone structure in swordtails (Teleostei: Xiphophorus)

Local adaptation is often invoked to explain hybrid zone structure, but empirical evidence of this is generally rare. Hybrid zones between two poeciliid fishes, Xiphophorus birchmanni and X. malinche, occur in multiple tributaries with independent replication of upstream‐to‐downstream gradients in morphology and allele frequencies. Ecological niche modelling revealed that temperature is a central predictive factor in the spatial distribution of pure parental species and their hybrids and explains spatial and temporal variation in the frequency of neutral genetic markers in hybrid populations. Among populations of parentals and hybrids, both thermal tolerance and heat‐shock protein expression vary strongly, indicating that spatial and temporal structure is likely driven by adaptation to local thermal environments. Therefore, hybrid zone structure is strongly influenced by interspecific differences in physiological mechanisms for coping with the thermal environment.     

Resilience to extreme temperature events: acclimation capacity and body condition of a polymorphic fish in response to thermal stress

Considerable attention has been given to the potential impacts of global climate change on biodiversity. In the present study, we combine understudied themes by examining the ability of a freshwater fish (polymorphic for heat‐sensitivity) to respond to short‐term thermal stress mimicking an extreme temperature event. We simultaneously measured the effect of thermal stress on the body condition of heat‐sensitive and heat‐tolerant forms to evaluate an existing hypothesis regarding the underlying mechanism by which temperature affects the maintenance of genetic variation in this species. Surprisingly, the heat‐sensitive allelic variant increased in body condition equally as much as a heat‐tolerant variant under acute heat stress. More importantly, the heat‐sensitive variant exhibited a significant response to thermal stress, with an upward shift of greater than 2 °C in critical thermal maximum. Our findings suggest a complexity to the relationship between thermal stress and male body condition that may depend on an interaction with other factors such as resource level. Although the evolutionary fate of species with respect to climate change is typically evaluated in terms long‐term adaptive response, short‐term selection events could drastically reduce fitness and reduce evolutionary potential. Our results suggest that heat‐sensitive species may have considerably greater resilience to the short‐term, extreme perturbations to the environment that are expected under climate change. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 504–510.     

Spatiotemporal environmental heterogeneity and the maintenance of the tailspot polymorphism in the variable platyfish (Xiphophorus variatus)

Genetic variation is critical for adaptive evolution. Despite its importance, there is still limited evidence in support of some prominent theoretical models explaining the maintenance of genetic polymorphism within populations. We examined 84 populations of Xiphophorus variatus, a livebearing fish with a genetic polymorphism associated with physiological performance, to test: (1) whether niche differentiation explains broad‐scale maintenance of polymorphism, (2) whether polymorphism is maintained among populations by local adaptation and migration, or (3) whether heterogeneity in explicit environmental variables could be linked to levels of polymorphism within populations. We found no evidence of climatic niche differentiation that could generate or maintain broad geographic variation in polymorphism. Subsequently, hierarchical partitioning of genetic richness and partial mantel tests revealed that 76% of the observed genetic richness was partitioned within populations with no effect of geographic distance on polymorphism. These results strongly suggest a lack of migration‐selection balance in the maintenance of polymorphism, and model selection confirmed a significant relationship between environmental heterogeneity and genetic richness within populations. Few studies have demonstrated such effects at this scale, and additional studies in other taxa should examine the generality of gene‐by‐environment interactions across populations to better understand the dynamics and scale of balancing selection.     

Impacts of yeast metabolic network structure on enzyme evolution

A comment on D Vitkup, P Kharchenko and A Wagner: Influence of metabolic network structure and function on enzyme evolution. Genome Biol 2006, 7:R39.     

Genetic structure of Eurasian and North American Leymus (Triticeae) wildryes assessed by chloroplast DNA sequences and AFLP profiles

Leymus is a genomically defined allopolyploid of genus Triticeae with two distinct subgenomes. Chloroplast DNA sequences of Eurasian and North American species are distinct and polyphyletic. However, phylogenies derived from chloroplast and nuclear DNA sequences are confounded by polyploidy and lack of polymorphism among many taxa. The AFLP technique can resolve phylogenetic relationships between closely related species, with a curvilinear relationship expected between the proportion of shared bands and nucleotide substitution rate (D), up to about 0.100 D. The objective of this study was to compare D and phylogenetic relationships among 16 Leymus taxa, based on chloroplast DNA sequences and multi-locus AFLP genotypes. Estimates of chloroplast D between taxa were 0.002 and 0.013 within and among continental regions, respectively. Estimates of AFLP D between taxa were 0.076 and 0.093 compared within and between continental regions, respectively, versus 0.024 within taxa. Bayesian and neighbor-joining cluster analyses effectively separated all AFLP genotypes by species, but showed that North American L. ambiguus is a hybrid species with nearly equal contributions from sympatric L. cinereus and L. salinus taxa. Two hierarchical AFLP clades, containing six North American taxa and four Eurasian taxa, had more than 98% bootstrap confidence with 0.071 and 0.055 D among taxa. Three other Eurasian taxa clustered with 79% and 89% confidence, with up to 0.79 D between taxa. These estimates provide benchmarks for phylogenetic comparisons of AFLP profiles, but three taxa could not be reliably grouped, which may reflect concurrent radiation of multiple lineages or lack of homologous AFLP characters caused by a high D.     

Variation in behavioral traits across a broad latitudinal gradient in a livebearing fish

Evolutionary Ecology - Variation in animal behavior can have important consequences for ecological processes like mate choice, resource competition and invasive potential. Characterizing behavioral...