High-resolution mapping reveals that microniches in the gastric glands control Helicobacter pylori colonization of the stomach.

Lifelong infection of the gastric mucosa by Helicobacter pylori can lead to peptic ulcers and gastric cancer. However, how the bacteria maintain chronic colonization in the face of constant mucus and epithelial cell turnover in the stomach is unclear. Here, we present a new model of how H. pylori establish and persist in stomach, which involves the colonization of a specialized microenvironment, or microniche, deep in the gastric glands. Using quantitative three-dimensional (3D) confocal microscopy and passive CLARITY technique (PACT), which renders tissues optically transparent, we analyzed intact stomachs from mice infected with a mixture of isogenic, fluorescent H. pylori strains with unprecedented spatial resolution. We discovered that a small number of bacterial founders initially establish colonies deep in the gastric glands and then expand to colonize adjacent glands, forming clonal population islands that persist over time. Gland-associated populations do not intermix with free-swimming bacteria in the surface mucus, and they compete for space and prevent newcomers from establishing in the stomach. Furthermore, bacterial mutants deficient in gland colonization are outcompeted by wild-type (WT) bacteria. Finally, we found that host factors such as the age at infection and T-cell responses control bacterial density within the glands. Collectively, our results demonstrate that microniches in the gastric glands house a persistent H. pylori reservoir, which we propose replenishes the more transient bacterial populations in the superficial mucosa.

Laterality is Universal Among Fishes but Increasingly Cryptic Among Derived Groups.

Laterality has been studied in several vertebrates, mainly in terms of brain lateralization and behavioral laterality, but morphological asymmetry has not been extensively investigated. Asymmetry in fishes was first described in scale-eating cichlids from Lake Tanganyika, in the form of bilateral dimorphism in which some individuals, when opening their mouths, twist them to the right and others to the left. This asymmetry has a genetic basis, and is correlated with lateralized attack behaviors. This has subsequently been found in fishes from numerous taxa with various feeding habits. The generality of such morphological laterality should thus be investigated in as wide a range of fishes as possible. Using specific indicators of lateral differences in mandibles and head inclination, we find that representative species from all 60 orders of extant gnathostome fishes (both bony and cartilaginous) possess morphological laterality. Furthermore, we identify the same laterality in agnathans (hagfish and lamprey), suggesting that this trait appeared early in fish evolution and has been maintained across fish lineages. However, a comparison of asymmetry among groups of bony fishes reveals, unexpectedly, that phylogenetically more recent-groups possess less asymmetry in body structures. The universality of laterality in fishes indicates a monophyletic origin, and may have been present in the ancestors of vertebrates. Ecological factors, predator-prey interactions in particular, may be key drivers in the evolution and maintenance of dimorphism, and may also be responsible for the cryptic trend of asymmetry in derived groups. Because lungfish and coelacanths share this trait, it is likely that tetrapods also inherited it. We believe that study of this morphological laterality will provide insights into the behavioral and sensory lateralization of vertebrates.

Geographical Variation in Community Divergence: Insights from Tropical Forest Monodominance by Ectomycorrhizal Trees.

Convergence occurs in both species traits and community structure, but how convergence at the two scales influences each other remains unclear. To address this question, we focus on tropical forest monodominance, in which a single, often ectomycorrhizal (EM) tree species occasionally dominates forest stands within a landscape otherwise characterized by diverse communities of arbuscular mycorrhizal (AM) trees. Such monodominance is a striking potential example of community divergence resulting in alternative stable states. However, it is observed only in some tropical regions. A diverse suite of AM and EM trees locally codominate forest stands elsewhere. We develop a hypothesis to explain this geographical difference using a simulation model of plant community assembly. Simulation results suggest that in a region with a few EM species (e.g., South America), EM trees experience strong selection for convergent traits that match the abiotic conditions of the environment. Consequently, EM species successfully compete against other species to form monodominant stands via positive plant-soil feedbacks. By contrast, in a region with many EM species (e.g., Southeast Asia), species maintain divergent traits because of complex plant-soil feedbacks, with no species having traits that enable monodominance. An analysis of plant trait data from Borneo and Peruvian Amazon was inconclusive. Overall, this work highlights the utility of geographical comparison in understanding the relationship between trait convergence and community convergence.

Fine-Grained Distribution of a Non-Native Resource Can Alter the Population Dynamics of a Native Consumer.

New interactions with non-native species can alter selection pressures on native species. Here, we examined the effect of the spatial distribution of a non-native species, a factor that determines ecological and evolutionary outcomes but that is poorly understood, particularly on a fine scale. Specifically, we explored a native butterfly population and a non-native plant on which the butterfly oviposits despite the plant's toxicity to larvae. We developed an individual-based model to describe movement and oviposition behaviors of each butterfly, which were determined by plant distribution and the butterfly's host preference genotype. We estimated the parameter values of the model from rich field data. We simulated various patterns of plant distributions and compared the rates of butterfly population growth and changes in the allele frequency of oviposition preference. Neither the number nor mean area of patches of non-native species affected the butterfly population, whereas plant abundance, patch shape, and distance to the nearest native and non-native patches altered both the population dynamics and genetics. Furthermore, we found a dramatic decrease in population growth rates when we reduced the distance to the nearest native patch from 147 m to 136 m. Thus changes in the non-native resource distribution that are critical to the fate of the native herbivore could only be detected at a fine-grained scale that matched the scale of a female butterfly's movement. In addition, we found that the native butterfly population was unlikely to be rescued by the exclusion of the allele for acceptance of the non-native plant as a host. This study thus highlights the importance of including both ecological and evolutionary dynamics in analyses of the outcome of species interactions and provides insights into habitat management for non-native species.

Fitness costs of butterfly oviposition on a lethal non-native plant in a mixed native and non-native plant community.

Non-native plants may be unpalatable or toxic, but have oviposition cues similar to native plants used by insects. The herbivore will then oviposit on the plant, but the offspring will be unable to develop. While such instances have been described previously, the fitness costs at the population level in the wild due to the presence of the lethal host have not been quantified, for this or other related systems. We quantified the fitness cost in the field for the native butterfly Pieris macdunnoughii in the presence of the non-native crucifer Thlaspi arvense, based on the spatial distributions of host plants, female butterflies and eggs in the habitat and the survival of the larvae in the wild. We found that 2.9% of eggs were laid on T. arvense on average, with a survival probability of 0, yielding a calculated fitness cost of 3.0% (95% confidence interval 1.7-3.6%) due to the presence of the non-native in the plant community. Survival probability to the pre-pupal stage for eggs laid on two native crucifers averaged 1.6% over 2 years. The magnitude of the fitness cost will vary temporally and spatially as a function of the relative abundance of the non-native plant. We propose that the fine-scale spatial structure of the plant community relative to the butterflies' dispersal ability, combined with the females' broad habitat use, contributes to the fitness costs associated with the non-native plant and the resulting evolutionary trap.

Community assembly: alternative stable states or alternative transient states?

The concept of alternative stable states has long been a dominant framework for studying the influence of historical contingency in community assembly. This concept focuses on stable states, yet many real communities are kept in a transient state by disturbance, and the utility of predictions for stable states in explaining transient states remains unclear. Using a simple model of plant community assembly, we show that the conditions under which historical contingency affects community assembly can differ greatly for stable versus transient states. Differences arise because the contribution of such factors as mortality rate, environmental heterogeneity and plant-soil feedback to historical contingency changes as community assembly proceeds. We also show that transient states can last for a long time relative to immigration rate and generation time. These results argue for a conceptual shift of focus from alternative stable states to alternative transient states for understanding historical contingency in community assembly.

Does competition between resources change the competition between their consumers to mutualism? Variations on two themes by Vandermeer.

How does competition between resources affect the interaction between consumer species that share those resources? Existing theory suggests that high resource competition can lead to mutualism. However, this is based on an analysis that need only apply near equilibrium, and experimental demonstrations of such mutualism are rare. Two alternative approaches to measuring food web mutualism are examined here. These are based on the population-level effects of adding or removing a consumer species or on the amount of additional mortality that can be applied to one consumer without excluding it. Both measures suggest that mutualism is likely to be confined to two situations: when overlap in resource use by the consumers is very low and when the consumers are inefficient users of their resources. Competition between resources is also likely to increase the occurrence and magnitude of "hypercompetition" between consumers, where the reduction in population size caused by the introduced consumer is greater than that caused by a consumer that is identical to the resident species. Competition between resources can also increase the negative interaction between consumers by destabilizing the dynamics of the system. Such destabilization can cause negative indirect interactions between specialist consumers having no overlap in resource use.

Righty fish are hooked on the right side of their mouths - observations from an angling experiment with largemouth bass, Micropterus salmoides.

The development of muscles and bones in fish is laterally asymmetric (laterality). A "lefty" individual has a "C"-shaped body, with its left-side muscles more developed and the left side of its head facing forward. The body of a "righty" is the mirror-image. This laterality causes asymmetric interactions between individuals of different fish species, in that a righty or lefty fish consumes more lefty or righty fish, respectively. To investigate the coupling mechanisms between body asymmetry and predatory behavior, we conducted angling experiments with largemouth bass (Micropterus salmoides). We used the position of the fishhook set in the mouth to indicate the movement direction of the fish when it took the bait. Righty fish had more hooks set on the right side, whereas lefty fish had more on the left side, indicating that righty fish moved more to the left, and lefty fish moved more to the right, in successful catches. The relationship between the hooked position and movement direction was confirmed by video-image analysis of the angling.

Persistence and fluctuation of lateral dimorphism in fishes.

Two morphological types ("righty" and "lefty") have been discovered in several fish species and are referred to as a typical example of antisymmetry. It has been suggested, first, that this dimorphism (called laterality) is inheritable; second, that the frequencies of laterality in each species fluctuate around 0.5; and third, that predators mainly exploit prey of the opposite laterality; that is, lefty and righty predators prey on righties and lefties, respectively. The latter is defined as "cross predation"; the antonym "parallel predation" means predation within the same laterality. We hypothesized that cross predation drives alternation of the survival and reproductive advantages between two morphological types, leading to frequency-dependent selection that maintains the dimorphism. To investigate this, we constructed mathematical models of population dynamics of one prey/one predator systems and three-trophic-level systems with omnivory. Mathematical analysis and computer simulations explained the behavior of the laterality frequency in nature well, insofar as cross predation dominated over parallel predation. Furthermore, the simulations showed that when only one of the morphological types exists in a species, the other type can invade. This suggests that dimorphism is maintained in all interacting species.