Adaptive value of a predatory mouth-form in a dimorphic nematode

Polyphenisms can be adaptations to environments that are heterogeneous in space and time, but to persist they require conditional-specific advantages. The nematode Pristionchus pacificus is a facultative predator that displays an evolutionarily conserved polyphenism of its mouthparts. During development, P. pacificus irreversibly executes either a eurystomatous (Eu) or stenostomatous (St) mouth-form, which differ in the shape and number of movable teeth. The Eu form, which has an additional tooth, is more complex than the St form and is thus more highly derived relative to species lacking teeth. Here, we investigate a putative fitness trade-off for the alternative feeding-structures of P. pacificus. We show that the complex Eu form confers a greater ability to kill prey. When adults were provided with a prey diet, Eu nematodes exhibited greater fitness than St nematodes by several measures, including longevity, offspring survival and fecundity when followed by bacterial feeding. However, the two mouth-forms had similar fecundity when fed ad libitum on bacteria, a condition that would confer benefit on the more rapidly developing St form. Thus, the two forms show conditional fitness advantages in different environments. This study provides, to our knowledge, the first functional context for dimorphism in a model for the genetics of plasticity.     

MATE PREFERENCE FOR A PHENOTYPICALLY PLASTIC TRAIT IS LEARNED,AND MAY FACILITATE PREFERENCE‐PHENOTYPE MATCHING

Fixed, genetically determined, mate preferences for species whose adult phenotype varies with rearing environment may be maladaptive, as the phenotype that is most fit in the parental environment may be absent in the offspring environment. Mate preference in species with polyphenisms (environmentally dependent alternative phenotypes) should therefore either not focus on polyphenic traits, be polyphenic themselves, or learned each generation. Here, we test these alternative hypotheses by first describing a female‐limited seasonal polyphenism in a sexually dimorphic trait in the butterfly Bicyclus anynana, dorsal hindwing spot number (DHSN), and then testing whether male and female mate preferences for this trait exist, and whether they are seasonally polyphenic, or learned. Neither naïve males nor naïve females in either seasonal form exhibited mating preferences for DHSN. However, males, but not females, noticed DHSN variation and learned mate preferences for DHSN. These results suggest that individuals may accommodate environmentally dependent variation in morphological traits via learned mate preferences in each generation, and that learned mate preference plasticity can be sexually dimorphic.     

Resource polyphenism increases species richness: a test of the hypothesis

A major goal of evolutionary biology is to identify the causes of diversification and to ascertain why some evolutionary lineages are especially diverse. Evolutionary biologists have long speculated that polyphenism—where a single genome produces alternative phenotypes in response to different environmental stimuli—facilitates speciation, especially when these alternative phenotypes differ in resource or habitat use, i.e. resource polyphenism. Here, we present a series of replicated sister-group comparisons showing that fishes and amphibian clades in which resource polyphenism has evolved are more species rich, and have broader geographical ranges, than closely related clades lacking resource polyphenism. Resource polyphenism may promote diversification by facilitating each of the different stages of the speciation process (isolation, divergence, reproductive isolation) and/or by reducing a lineage''s risk of extinction. Generally, resource polyphenism may play a key role in fostering diversity, and species in which resource polyphenism has evolved may be predisposed to diversify.     

Translating environmental gradients into discontinuous reaction norms via hormone signalling in a polyphenic butterfly

Polyphenisms—the expression of discrete phenotypic morphs in response to environmental variation—are examples of phenotypic plasticity that may potentially be adaptive in the face of predictable environmental heterogeneity. In the butterfly Bicyclus anynana, we examine the hormonal regulation of phenotypic plasticity that involves divergent developmental trajectories into distinct adult morphs for a suite of traits as an adaptation to contrasting seasonal environments. This polyphenism is induced by temperature during development and mediated by ecdysteroid hormones. We reared larvae at separate temperatures spanning the natural range of seasonal environments and measured reaction norms for ecdysteroids, juvenile hormones (JHs) and adult fitness traits. Timing of peak ecdysteroid, but not JH titres, showed a binary response to the linear temperature gradient. Several adult traits (e.g. relative abdomen mass) responded in a similar, dimorphic manner, while others (e.g. wing pattern) showed a linear response. This study demonstrates that hormone dynamics can translate a linear environmental gradient into a discrete signal and, thus, that polyphenic differences between adult morphs can already be programmed at the stage of hormone signalling during development. The range of phenotypic responses observed within the suite of traits indicates both shared regulation and independent, trait-specific sensitivity to the hormone signal.     

Specialization of a polyphenism switch gene following serial duplications in Pristionchus nematodes

Polyphenism is an extreme manifestation of developmental plasticity, requiring distinct developmental programs and the addition of a switch mechanism. Because the genetic basis of polyphenism switches has only begun to be understood, how their mechanisms arise is unclear. In the nematode Pristionchus pacificus, which has a mouthpart polyphenism specialized for alternative diets, a gene (eud‐1) executing the polyphenism switch was recently identified as the product of lineage‐specific duplications. Here, we infer the role of gene duplications in producing a switch gene. Using reverse genetics and population genetic analyses, we examine evidence for competing scenarios of degeneration and complementation, neutral evolution, and functional specialization. Of the daughter genes, eud‐1 alone has assumed switch‐like regulation of the mouth polyphenism. Measurements of life‐history traits in single, double, and triple sulfatase mutants did not, given a benign environment, identify alternative or complementary roles for eud‐1 paralogs. Although possible roles are still unknown, selection analyses of the sister species and 104 natural isolates of P. pacificus detected purifying selection on the genes, suggesting their functionality by their fixation and evolutionary maintenance. Our approach shows the tractability of reverse genetics in a nontraditional model system to study evolution by gene duplication.     

Environmental Variables Explain Genetic Structure in a Beetle-Associated Nematode

The distribution of a species is a complex expression of its ecological and evolutionary history and integrating population genetic, environmental, and ecological data can provide new insights into the effects of the environment on the population structure of species. Previous work demonstrated strong patterns of genetic differentiation in natural populations of the hermaphroditic nematode Pristionchus pacificus in its La Réunion Island habitat, but gave no clear understanding of the role of the environment in structuring this variation. Here, we present what is to our knowledge the first study to statistically evaluate the role of the environment in shaping the structure and distribution of nematode populations. We test the hypothesis that genetic structure in P. pacificus is influenced by environmental variables, by combining population genetic analyses of microsatellite data from 18 populations and 370 strains, with multivariate statistics on environmental data, and species distribution modelling. We assess and quantify the relative importance of environmental factors (geographic distance, altitude, temperature, precipitation, and beetle host) on genetic variation among populations. Despite the fact that geographic populations of P. pacificus comprise vast genetic diversity sourced from multiple ancestral lineages, we find strong evidence for local associations between environment and genetic variation. Further, we show that significantly more genetic variation in P. pacificus populations is explained by environmental variation than by geographic distances. This supports a strong role for environmental heterogeneity vs. genetic drift in the divergence of populations, which we suggest may be influenced by adaptive forces.     

Assaying Environmental Nickel Toxicity Using Model Nematodes

Although nickel exposure results in allergic reactions, respiratory conditions, and cancer in humans and rodents, the ramifications of excess nickel in the environment for animal and human health remain largely undescribed. Nickel and other cationic metals travel through waterways and bind to soils and sediments. To evaluate the potential toxic effects of nickel at environmental contaminant levels (8.9-7,600 µg Ni/g dry weight of sediment and 50-800 µg NiCl₂/L of water), we conducted assays using two cosmopolitan nematodes, Caenorhabditis elegans and Pristionchus pacificus. We assayed the effects of both sediment-bound and aqueous nickel upon animal growth, developmental survival, lifespan, and fecundity. Uncontaminated sediments were collected from sites in the Midwestern United States and spiked with a range of nickel concentrations. We found that nickel-spiked sediment substantially impairs both survival from larval to adult stages and adult longevity in a concentration-dependent manner. Further, while aqueous nickel showed no adverse effects on either survivorship or longevity, we observed a significant decrease in fecundity, indicating that aqueous nickel could have a negative impact on nematode physiology. Intriguingly, C. elegans and P. pacificus exhibit similar, but not identical, responses to nickel exposure. Moreover, P. pacificus could be tested successfully in sediments inhospitable to C. elegans. Our results add to a growing body of literature documenting the impact of nickel on animal physiology, and suggest that environmental toxicological studies could gain an advantage by widening their repertoire of nematode species.     

Molecular phylogeny of beetle associated diplogastrid nematodes suggests host switching rather than nematode-beetle coevolution

Background  

Nematodes are putatively the most species-rich animal phylum. They have various life styles and occur in a variety of habitats, ranging from free-living nematodes in aquatic or terrestrial environments to parasites of animals and plants. The rhabditid nematode Caenorhabditis elegans is one of the most important model organisms in modern biology. Pristionchus pacificus of the family of the Diplogastridae has been developed as a satellite model for comparison to C. elegans. The Diplogastridae, a monophyletic clade within the rhabditid nematodes, are frequently associated with beetles. How this beetle-association evolved and whether beetle-nematode coevolution occurred is still elusive. As a prerequisite to answering this question a robust phylogeny of beetle-associated Diplogastridae is needed.     

Life-History Traits of the Model Organism Pristionchus pacificus Recorded Using the Hanging Drop Method: Comparison with Caenorhabditis elegans

The nematode Pristionchus pacificus is of growing interest as a model organism in evolutionary biology. However, despite multiple studies of its genetics, developmental cues, and ecology, the basic life-history traits (LHTs) of P. pacificus remain unknown. In this study, we used the hanging drop method to follow P. pacificus at the individual level and thereby quantify its LHTs. This approach allowed direct comparisons with the LHTs of Caenorhabditis elegans recently determined using this method. When provided with 5×10⁹ Escherichia coli cells ml^–1 at 20°C, the intrinsic rate of natural increase of P. pacificus was 1.125 (individually, per day); mean net production was 115 juveniles produced during the life-time of each individual, and each nematode laid an average of 270 eggs (both fertile and unfertile). The mean age of P. pacificus individuals at first reproduction was 65 h, and the average life span was 22 days. The life cycle of P. pacificus is therefore slightly longer than that of C. elegans, with a longer average life span and hatching time and the production of fewer progeny.     

Leptojacobus dorci n. gen., n. sp. (Nematoda: Diplogastridae), an Associate of Dorcus Stag Beetles (Coleoptera: Lucanidae)

A new species of diplogastrid nematode, Leptojacobus dorci n. gen., n. sp., was isolated from adults of the stag beetle Dorcus ritsemae (Coleoptera: Lucanidae) that were purchased from a pet shop in Japan. Leptojacobus n. gen. is circumscribed by a very thin, delicate body and by a small stoma with minute armature. A combination of other stomatal characters, namely the division of the cheilostom into adradial plates, the symmetry of the subventral stegostomatal sectors, and the presence of a thin, conical dorsal tooth, further distinguishes Leptojacobus n. gen. from other genera of Diplogastridae. Phylogenetic analysis of nearly full-length SSU rRNA sequences support the new species, together with an isolate identified previously as Koerneria luziae, to be excluded from a clade including all other molecularly characterized diplogastrids with teeth and stomatal dimorphism. Therefore, the new species will be of importance for reconstruction of ancestral character histories in Diplogastridae, a family circumscribed by a suite of feeding-related novelties.     

Stress-induced changes in gene interactions in human cells

Cells respond to variable environments by changing gene expression and gene interactions. To study how human cells response to stress, we analyzed the expression of >5000 genes in cultured B cells from nearly 100 normal individuals following endoplasmic reticulum stress and exposure to ionizing radiation. We identified thousands of genes that are induced or repressed. Then, we constructed coexpression networks and inferred interactions among genes. We used coexpression and machine learning analyses to study how genes interact with each other in response to stress. The results showed that for most genes, their interactions with each other are the same at baseline and in response to different stresses; however, a small set of genes acquired new interacting partners to engage in stress-specific responses. These genes with altered interacting partners are associated with diseases in which endoplasmic reticulum stress response or sensitivity to radiation has been implicated. Thus, our findings showed that to understand disease-specific pathways, it is important to identify not only genes that change expression levels but also those that alter interactions with other genes.     

Comparative Genomic Analysis of Buffalo (Bubalus bubalis) NOD1 and NOD2 Receptors and Their Functional Role in In-Vitro Cellular Immune Response

Nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs) are innate immune receptors that recognize bacterial cell wall components and initiate host immune response. Structure and function of NLRs have been well studied in human and mice, but little information exists on genetic composition and role of these receptors in innate immune system of water buffalo—a species known for its exceptional disease resistance. Here, a comparative study on the functional domains of NOD1 and NOD2 was performed across different species. The NOD mediated in-vitro cellular responses were studied in buffalo peripheral blood mononuclear cells, resident macrophages, mammary epithelial, and fibroblast cells. Buffalo NOD1 (buNOD1) and buNOD2 showed conserved domain architectures as found in other mammals. The domains of buNOD1 and buNOD2 showed analogy in secondary and tertiary conformations. Constitutive expressions of NODs were ubiquitous in different tissues. Following treatment with NOD agonists, peripheral lymphocytes showed an IFN-γ response along-with production of pro-inflammatory cytokines. Alveolar macrophages and mammary epithelial cells showed NOD mediated in-vitro immune response through NF-κB dependent pathway. Fibroblasts showed pro-inflammatory cytokine response following agonist treatment. Our study demonstrates that both immune and non-immune cells could generate NOD-mediated responses to pathogens though the type and magnitude of response depend on the cell types. The structural basis of ligand recognition by buffalo NODs and knowledge of immune response by different cell types could be useful for development of non-infective innate immune modulators and next generation anti-inflammatory compounds.     

Two TOBAMOVIRUS MULTIPLICATION 2A homologs in tobacco control asymptomatic response to tobacco mosaic virus

The most common response of a host to pathogens is arguably the asymptomatic response. However, the genetic and molecular mechanisms responsible for asymptomatic responses to pathogens are poorly understood. Here we report on the genetic cloning of two genes controlling the asymptomatic response to tobacco mosaic virus (TMV) in cultivated tobacco (Nicotiana tabacum). These two genes are homologous to tobamovirus multiplication 2A (TOM2A) from Arabidopsis, which was shown to be critical for the accumulation of TMV. Expression analysis indicates that the TOM2A genes might play fundamental roles in plant development or in responses to stresses. Consistent with this hypothesis, a null allele of the TOM2A ortholog in tomato (Solanum lycopersicum) led to the development of bent branches and a high tolerance to both TMV and tomato mosaic virus (ToMV). However, the TOM2A ortholog in Nicotiana glauca did not account for the asymptomatic response to TMV in N. glauca. We showed that TOM2A family is plant-specific and originated from Chlorophyte, and the biological functions of TOM2A orthologs to promote TMV accumulation are highly conserved in the plant kingdom—in both TMV host and nonhost species. In addition, we showed that the interaction between tobacco TOM1 and TOM2A orthologs in plant species is conserved, suggesting a conserved nature of TOM1–TOM2A module in promoting TMV multiplication in plants. The tradeoff between host development, the resistance of hosts to pathogens, and their influence on gene evolution are discussed. Our results shed light on mechanisms that contribute to asymptomatic responses to viruses in plants and provide approaches for developing TMV/ToMV-resistant crops.

Tobacco TOBAMOVIRUS MULTIPLICATION 2A homologs control the asymptomatic response to tobacco mosaic virus and have highly conserved biological functions related to virus multiplication.     

Life cycle polyphenism as a factor affecting ecological divergence within <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis>

Polyphenism, which allows a single genotype to express multiple discrete phenotypes in response to environmental cues, is an adaptive trait in heterogeneous environments. Pond hydroperiod is an important ecological parameter affecting amphibian life history, and variation in local pond hydrology has been hypothesized to play a role in species divergence via changes in polyphenism. The eastern newt (Notophthalmus viridescens) expresses life cycle polyphenism. Larvae develop along three possible pathways: metamorphosis to aquatic lunged adult via a terrestrial juvenile (eft) stage, metamorphosis directly to an aquatic lunged adult, or maturation directly to an aquatic gilled adult without metamorphosis (i.e., paedomorphosis). Subspecies of N. viridescens vary in their polyphenic patterns, suggesting possible adaptation to different environments. However, no studies have experimentally tested how genetic and environmental components contribute to the observed differences among subspecies and whether such differences may facilitate divergence. We tested whether adaptation to local pond hydrology via polyphenic changes existed among subspecies by rearing larvae of three subspecies (N. v. dorsalis, N. v. louisianensis, and N. v. viridescens) along three hydroperiod regimes (short, long, and constant) in outdoor artificial ponds. We found that larval N. v. viridescens obligately metamorphosed to efts under all hydroperiods, whereas N. v. dorsalis and N. v. louisianensis exhibited plasticity: larvae metamorphosed to efts under drying conditions but metamorphosed directly to aquatic adults or became paedomorphic in constant water. Also, N. v. viridescens metamorphosed to efts faster and at a smaller body size than the other two subspecies. These data suggest that subspecies of N. viridescens are adapted to different pond hydroperiods, supporting the potential for polyphenism to facilitate divergence. Canalizing selection for certain alternative phenotypes within a single species in which other populations remain plastic may play an important role in the initiation of ecological divergence.