Joint Evolution of Kin Recognition and Cooperation in Spatially Structured Rhizobium Populations

In the face of costs, cooperative interactions maintained over evolutionary time present a central question in biology. What forces maintain this cooperation? Two potential ways to explain this problem are spatially structured environments (kin selection) and kin-recognition (directed benefits). In a two-locus population genetic model, we investigated the relative roles of spatial structure and kin recognition in the maintenance of cooperation among rhizobia within the rhizobia-legume mutualism. In the case where the cooperative and kin recognition loci are independently inherited, spatial structure alone maintains cooperation, while kin recognition decreases the equilibrium frequency of cooperators. In the case of coinheritance, spatial structure remains a stronger force, but kin recognition can transiently increase the frequency of cooperators. Our results suggest that spatial structure can be a dominant force in maintaining cooperation in rhizobium populations, providing a mechanism for maintaining the mutualistic nodulation trait. Further, our model generates unique and testable predictions that could be evaluated empirically within the legume-rhizobium mutualism

On the X-ray Emission from Massive Star Clusters and their Evolving Superbubbles

The X-ray emission properties from the hot thermalized plasma that results from the collisions of individual stellar winds and supernovae ejecta within rich and compact star clusters are discussed. We propose a simple analytical way of estimating the X-ray emission generated by super star clusters and derive an expression that indicates how this X-ray emission depends on the main cluster parameters. Our model predicts that the X-ray luminosity from the star cluster region is highly dependent on the star cluster wind terminal speed, a quantity related to the temperature of the thermalized ejecta.We have also compared the X-ray luminosity from the SSC plasma with the luminosity of the interstellar bubbles generated from the mechanical interaction of the high velocity star cluster winds with the ISM.We found that the hard (2.0 keV - 8.0 keV) X-ray emission is usually dominated by the hotter SSC plasma whereas the soft (0.3 keV - 2.0 keV) component is dominated by the bubble plasma. This implies that compact and massive star clusters should be detected as point-like hard X-ray sources embedded into extended regions of soft diffuse X-ray emission. We also compared our results with predictions from the population synthesis models that take into consideration binary systems and found that in the case of young,massive and compact super star clusters the X-ray emission from the thermalized star cluster plasma may be comparable or even larger than that expected from the HMXB population.Comment: 24 pages, 8 figures, Accepted for publication in The Astrophysical Journa

The Extent of Seasonally Suitable Habitats May Limit Forage Fish Production in a Temperate Estuary

The sustained production of sufficient forage is critical to advancing ecosystem-based management, yet factors that affect local abundances and habitat conditions necessary to support aggregate forage production remain largely unexplored. We quantified suitable habitat in the Chesapeake Bay and its tidal tributaries for four key forage fishes: juvenile spotted hake Urophycis regia, juvenile spot Leiostomus xanthurus, juvenile weakfish Cynoscion regalis, and bay anchovy Anchoa mitchilli. We used information from monthly fisheries surveys from 2000 to 2016 coupled with hindcasts from a spatially interpolated model of dissolved oxygen and a 3-D hydrodynamic model of the Chesapeake Bay to identify influential covariates and construct habitat suitability models for each species. Suitable habitat conditions resulted from a complex interplay between water quality and geophysical properties of the environment and varied among species. Habitat suitability indices ranging between 0 (poor) and 1 (superior) were used to estimate seasonal and annual extents of suitable habitats. Seasonal variations in suitable habitat extents in Chesapeake Bay, which were more pronounced than annual variations during 2000–2016, reflected the phenology of estuarine use by these species. Areas near shorelines served as suitable habitats in spring for juvenile spot and in summer for juvenile weakfish, indicating the importance of these shallow areas for production. Tributaries were more suitable for bay anchovy in spring than during other seasons. The relative baywide abundances of juvenile spot and bay anchovy were significantly related to the extent of suitable habitats in summer and winter, respectively, indicating that Chesapeake Bay habitats may be limiting for these species. In contrast, the relative baywide abundances of juvenile weakfish and juvenile spotted hake varied independently of the spatial extent of suitable habitats. In an ecosystem-based approach, areas that persistently provide suitable conditions for forage species such as shoreline and tributary habitats may be targeted for protection or restoration, thereby promoting sufficient production of forage for predators. Further, quantitative habitat targets or spatial thresholds may be developed for habitat-limited species using estimates of the minimum habitat area required to produce a desired abundance or biomass; such targets or thresholds may serve as spatial reference points for management

Extent of Suitable Habitats for Juvenile Striped Bass: Dynamics and Implications for Recruitment in Chesapeake Bay

The production of striped bass Morone saxatilis in Chesapeake Bay supports recreational and commercial fisheries along the Atlantic coast of the United States, but factors that contribute to high abundances of juvenile life stages are not fully understood. In this study, we characterized and quantified suitable and optimal habitat conditions in the Chesapeake Bay for two age groups of juvenile striped bass in discrete portions of the Bay: young-of-the-year (age-0) fish in shoreline and nearshore habitats, and resident sub-adults (age-1 to -4) in the mainstem and Bay-wide. We coupled information from 24 years of monthly fisheries surveys with hindcasts from a 3-D hydrodynamic model of the Bay and a numerical model of dissolved oxygen (DO) conditions. These models provided estimates of habitat conditions for 1996 to 2019 for 33 metrics of temperature, salinity, current speed, depth, DO, and physical features of habitats. Boosted regression trees were used to identify influential habitat covariates for each group, and those covariates were used to develop nonparametric habitat suitability models based on environmental conditions at the time and location of sampling. Habitat suitability indices (HSI), ranging from 0 (poor habitat) to 1 (high-quality habitat), were assigned to each grid in the 3-D model for each season in 1996 to 2019. We quantified suitable (HSI \u3e 0.5) and optimal (HSI \u3e 0.7) on a seasonal and annual basis, and across a range of environmental conditions (wet vs. dry years; warm vs. cool years). We also estimated the persistence of suitable habitats through time as the percent of years during which conditions were suitable at a given site; persistence allowed us to identify areas of the Bay and tidal tributaries that consistently supported suitable conditions for juvenile striped bass. Specific habitat conditions that defined suitable and optimal habitats for age-0 and age 1-4 striped bass varied across seasons and among years, reflecting changes in water quality conditions in Chesapeake Bay and changes in habitat use by striped bass during their first few years of life. Metrics of water quality, especially dissolved oxygen, were consistently identified as important covariates for juvenile striped bass; these conditions are of greater importance in determining habitat suitability than specific physical features especially for a highly mobile species and may be used to inform existing decision-support tools. In our study, we found no evidence that habitat use by striped bass in Chesapeake Bay was moderated by a strict threshold for any given covariate, and average to above-average abundances of striped bass were encountered in sub-suitable conditions; thus, habitat use resulted from a combination of abiotic, and likely biotic, conditions. Neither age group exhibited a statistically significant relationship between relative abundance and the extent of suitable habitats, however, for nearly all ages and seasons, relative abundance increased with greater extent of suitable habitats suggesting that detection of this relationship requires additional annual observations. A significant decrease in the extent of suitable habitat through time (1996 to present) was observed in spring and early summer, reflecting a change in suitable environmental conditions; with additional study years, declines in the relative abundance of age-0 and age 1-4 fish may be observed as suitability of habitats continues to decline. Given the high degree of interannual variability in abundance that is characteristic of estuarine-dependent species like striped bass, the availability and quantity of suitable and high-quality habitats at the scale of individual tributaries and Bay-wide may play an important role in production of this species

Seasonal and Annual Variation in the Extent of Suitable Habitats for Forage Fishes in Chesapeake Bay, 2000-2016

The sustained production of sufficient forage is critical to advancing ecosystem-based management in Chesapeake Bay. Yet factors that affect local abundances and habitat conditions necessary to support forage production remain largely unexplored. Here, we quantified suitable habitat in the Chesapeake Bay region for four key forage fishes: bay anchovy Anchoa mitchilli, juvenile spot Leiostomus xanthurus, juvenile weakfish Cynoscion regalis, and juvenile spotted hake Urophycis regia. We coupled information from 17 years of monthly fisheries surveys with hindcasts from a numerical model of dissolved oxygen (DO) conditions and a 3-D hydrodynamic model of the Bay that provided estimates of habitat conditions across 18 covariates of salinity, temperature, DO, depth, and current speed for the period 2000 to 2016. Sediment composition and distance to shore metrics were also considered. The hindcast covariates were subsampled at the times and locations of the fisheries surveys to provide dynamic habitat metrics that are not generally observed at the time of fish sampling (e.g., current velocity, salinity stratification). Hindcast covariates were also used to describe habitat conditions in areas of Chesapeake Bay that are not sampled routinely by fisheries-independent surveys such as the Potomac River and Mobjack Bay. Boosted regression trees were used to identify influential habitat covariates for each species, and these influential covariates were then used to construct habitat suitability models. Habitat suitability indices, which ranged between 0 (poor habitat) and 1 (superior habitat), were assigned to each location in the 3-D model grid for each season in 2000-2016. Based on the estimated habitat suitability index and using a GIS approach, we quantified suitable habitat (defined as habitats with a habitat suitability index \u3e 0.5) throughout the Chesapeake Bay and its tidal tributaries. Furthermore, we validated the modeling approach using out-of-sample observations from Mobjack Bay in 2010-2012. Suitable seasonal habitat extents for forage species exhibited strong seasonal and annual signals reflecting temporal heterogeneity in habitat conditions in Chesapeake Bay. Current speed, water depth, and either temperature or dissolved oxygen were identified as important covariates for the four forage species we examined, and distance to shore was important for three of the four species; thus, suitable habitat conditions resulted from a complex interplay between water quality and the physical properties of the habitat. In our study, two species exhibited a relationship between relative abundance and extent of suitable habitats – juvenile spot in summer and bay anchovy in winter; as such, estimates of the minimum habitat area required to produce a desired abundance (or biomass) of forage fish can be used to establish quantitative habitat targets or spatial thresholds that may serve as spatial reference points for management. In an ecosystem-based approach, important habitats may be targeted for protection (e.g., by limiting fishing activities that may incidentally capture or injure forage fishes) or restoration (e.g., by improving water quality conditions), thereby ensuring production of sufficient forage for predators. In addition, the consequences of aquatic habitat alterations, whether due to climate change or physical disturbances can be investigated using projections of environmental conditions and habitat suitability in the region, though these projections will introduce additional uncertainty