Comparison of methods for mapping rhizosphere processes in the context of their surrounding root and soil environments

The rhizosphere embodies a complex biogeochemical zone with enhanced rates of nutrient exchange between plants, soil, and microbial communities. Understanding controls on rhizosphere dynamics is critical to support emerging concepts including rhizosphere engineering and reduced dependence on chemical fertilizers which have direct application to food production, increased biofuel generation, and habitat restoration efforts. Yet, its fine spatial scale and complex interactions between geochemical and microbial processes within complex spatiotemporal gradients make the rhizosphere notoriously difficult to study. Emerging instrumentation and methodologies, however, are providing improved resolution to rhizosphere measurements and helping to address critical knowledge gaps in rhizosphere function, ecology, and establishment. Here, we examine recent advances in analysis techniques and the resulting potential for improved understanding of rhizosphere function.

Residential Segregation under Jim Crow: Whites, Blacks, and Mulattoes in Southern Cities, 1880–1920

We study the residential patterns of blacks and mulattoes in 10 Southern cities in 1880 and 1920. Researchers have documented the salience of social differences among African Americans in this period, partly related to mulattoes’ higher occupational status. Did these differences result in clustering of these two groups in different neighborhoods, and were mulattoes less separated from whites? If so, did the differences diminish in these decades after Reconstruction due a Jim Crow system that did not distinguish between blacks and mulattoes? We use geocoded census microdata for 1880 and 1920 to address these questions. Segregation between whites and both blacks was already high in 1880, especially at a fine spatial scale, and it increased sharply by 1920. In this respect, whites did not distinguish between these two groups. However, blacks and mulattoes were quite segregated from one another in 1880, and even more so by 1920. This pattern did not result from mulattoes’ moderately higher-class position. Hence, as the color line between whites and all non-whites was becoming harder, blacks and mulattoes were separating further from each other. Understanding what led to this pattern remains a key question about racial identities and racialization in the early twentieth century.

Opportunistic gut sampling indicates differential diet and plastic ingestion risk in Indian dugongs

Dugongs, exclusively seagrass foragers, are globally threatened marine mammals. Knowledge on their feeding biology has been derived from few direct observations and mostly by analysis of stomach contents. Given limitations in data from Indian populations, dugong strandings serve as an opportunity to understand their dietary composition through gut sampling. In this paper, we utilize the gut contents collected from stranded dugongs to detect differences in the seagrass foraging between two isolated pockets of dugong distribution (Tamil Nadu and Gujarat) and supplement existing knowledge on dugong feeding biology in Indian waters. We extracted, enumerated and identified seagrass species from dugong gut contents. The proportion of seagrass leaf fragments were found higher (>40%) than other fragments in all the gut samples analysed. We recorded two seagrass genera (Halophila spp. and Halodule spp.) from Gujarat and five seagrass genera (Halophila spp., Halodule spp., Cymodocea spp., Enhalus spp., Syringodium spp.) from Tamil Nadu dugong individuals. We also obtained anthropogenic debris such as plastic, fishing net and wood fragments from the gut samples. We suggest enhanced monitoring of seagrass habitats and fine spatial scale threat mapping in entire dugong distribution range in India.

The influence of roads on the fine-scale population genetic structure of the dengue vector Aedes aegypti (Linnaeus)

Dengue is endemic in tropical and subtropical countries and is transmitted mainly by Aedes aegypti. Mosquito movement can be affected by human-made structures such as roads that can act as a barrier. Roads can influence the population genetic structure of Ae. aegypti. We investigated the genetic structure and gene flow of Ae. aegypti as influenced by a primary road, España Boulevard (EB) with 2000-meter-long stretch and 24-meters-wide in a very fine spatial scale. We hypothesized that Ae. aegypti populations separated by EB will be different due to the limited gene flow as caused by the barrier effect of the road. A total of 359 adults and 17 larvae Ae. aegypti were collected from June to September 2017 in 13 sites across EB. North (N1-N8) and South (S1-S5) comprised of 211 and 165 individuals, respectively. All mosquitoes were genotyped at 11 microsatellite loci. AMOVA FST indicated significant genetic differentiation across the road. The constructed UPGMA dendrogram found 3 genetic groups revealing the clear separation between North and South sites across the road. On the other hand, Bayesian cluster analysis showed four genetic clusters (K = 4) wherein each individual samples have no distinct genetic cluster thus genetic admixture. Our results suggest that human-made landscape features such as primary roads are potential barriers to mosquito movement thereby limiting its gene flow across the road. This information is valuable in designing an effective mosquito control program in a very fine spatial scale.

Complementarity and discriminatory power of genotype and otolith shape in describing the fine-scale population structure of an exploited fish, the common sole of the Eastern English Channel

Marine organisms show population structure at a relatively fine spatial scale, even in open habitats. The tools commonly used to assess subtle patterns of connectivity have diverse levels of resolution and can complement each other to inform on population structure. We assessed and compared the discriminatory power of genetic markers and otolith shape to reveal the population structure on evolutionary and ecological time scales of the common sole (Solea solea), living in the Eastern English Channel (EEC) stock off France and the UK. First, we genotyped fish with Single Nucleotide Polymorphisms to assess population structure at an evolutionary scale. Then, we tested for spatial segregation of the subunits using otolith shape as an integrative tracer of life history. Finally, a supervised machine learning framework was applied to genotypes and otolith phenotypes to probabilistically assign adults to subunits and assess the discriminatory power of each approach. Low but significant genetic differentiation was found among subunits. Moreover, otolith shape appeared to vary spatially, suggesting spatial population structure at fine spatial scale. However, results of the supervised discriminant analyses failed to discriminate among subunits, especially for otolith shape. We suggest that the degree of population segregation may not be strong enough to allow for robust fish assignments. Finally, this study revealed a weak yet existing metapopulation structure of common sole at the fine spatial scale of the EEC based on genotypes and otolith shape, with one subunit being more isolated. Our study argues for the use of complementary tracers to investigate marine population structure.

The influence of roads on the fine-scale population genetic structure of the dengue vector Aedes aegypti (Linnaeus)

Dengue is endemic in tropical and subtropical countries and is transmitted mainly by Aedes aegypti. Mosquito movement can be affected by human-made structures such as roads that can act as a barrier. Roads can influence the population genetic structure of Ae. aegypti. We investigated the genetic structure and gene flow of Ae. aegypti as influenced by a primary road, España Boulevard (EB) with 2000-meter-long stretch and 24-meters-wide in a very fine spatial scale. We hypothesized that Ae. aegypti populations separated by EB will be different due to the limited gene flow as caused by the barrier effect of the road. A total of 376 adults and larval Ae. aegypti were collected from June to September 2017 in 21 sites across EB. North (N1-N11) and South (S1-S10) comprised of 211 and 165 individuals, respectively. All mosquitoes were genotyped at 11 microsatellite loci. AMOVA FST indicated significant genetic differentiation across the road. The constructed UPGMA dendrogram found 4 genetic groups revealing the clear separation between North and South sites across the road. On the other hand, Bayesian cluster analysis showed four genetic clusters (K = 4) wherein each individual samples have no distinct genetic cluster thus genetic admixture. Moreover, significant positive spatial autocorrelation was observed at 100 - 200 m distance class, suggesting dispersal potential of the adult mosquitoes within a 200 m spatial scale. Our results suggest that human-made landscape features such as primary roads are potential barriers to mosquito movement thereby limiting its gene flow across the road. This information is valuable in designing an effective mosquito control program in a very fine spatial scale.Author SummaryDengue, a mosquito-borne viral infection is a serious health problem in tropical and subtropical countries such as Philippines. Most dengue prevention programs aim to eradicate its mosquito vector, Aedes aegypti. A successful population control program is reliant in understanding the mosquito behavior and ecology including how human-made structures such as roads influence its expansion and movement. Previous studies have discovered the barrier effect of roads in the movement of mosquitoes. In this study, we examined the influence of roads in the population genetic structure of Ae. aegypti in a fine spatial scale using 11 microsatellite markers. We found significant genetic differentiation of mosquito populations across the road. Our results suggest limited gene flow across the road and supports our hypothesis that roads are potential barriers to mosquito dispersal. This information can be used in designing an effective mosquito population control zones in perceived barrier to mosquito dispersal such as roads.