Environment not dispersal limitation drives clonal composition of Arctic Daphnia in a recently deglaciated area

The role of neutral and adaptive evolutionary processes on patterns of genetic diversity across small cave-dwelling populations of Icelandic Arctic charr (Salvelinus alpinus)

Understanding the adaptability of small populations in the face of environmental change is a central problem in evolutionary biology. Solving this problem is challenging because neutral evolutionary processes that operate on historical and contemporary timescales can override the effects of selection in small populations. We assessed the effects of isolation by colonization (IBC), isolation by dispersal limitation (IBDL) as reflected by a pattern of isolation by distance (IBD), and isolation by adaptation (IBA) and the roles of genetic drift and gene flow on patterns of genetic differentiation among 19 cave-dwelling populations of Icelandic Arctic charr (Salvelinus alpinus). We detected evidence of IBC based on the genetic affinity of nearby cave populations and the genetic relationships between the cave populations and the presumed ancestral population in the lake. A pattern of IBD was evident regardless of whether high-level genetic structuring (IBC) was taken into account. Genetic signatures of bottlenecks and lower genetic diversity in smaller populations indicate the effect of drift. Estimates of gene flow and fish movement suggest that gene flow is limited to nearby populations. In contrast, we found little evidence of IBA as patterns of local ecological and phenotypic variation showed little association with genetic differentiation among populations. Thus, patterns of genetic variation in these small populations likely reflect localized gene flow and genetic drift superimposed onto a larger-scale structure that is largely a result of colonization history. Our simultaneous assessment of the effects of neutral and adaptive processes in a tractable and replicated system has yielded novel insights into the evolution of small populations on both historical and contemporary timescales and over a smaller spatial scale than is typically studied.

Limnological factors that affect waterbird assemblages in semi-arid reservoirs of Tigray National Regional State, northern Ethiopia

Published accounts of the conservation of biodiversity indicate that understanding patterns of species distribution and richness is crucial. However, what drives patterns of species composition in a landscape remains debatable. I examined the relationship between limnological characteristics of reservoirs, morpho-edaphic variables, biological variables, and patterns of bird species richness and distribution. Six limnological, three morpho-edaphic variables, and biological variables were recorded for 35 reservoirs and analyzed by multivariate statistical techniques. To investigate the most important explanatory factors influencing avian species richness and their distribution, redundancy analysis (RDA) was used. A total of 85 bird species from 54 genera, with a mean species richness 14.23 ± 6.72 (mean ± standard deviation) per reservoir, were recorded. The RDA analysis identified two significant RDA axes, and 34.4% of the variation in species richness is explained by environmental variation (R²_adj = 0.34375; P < 0.001). Bird species richness was positively correlated with the surface area of reservoirs. I showed here that reservoir size and environmental heterogeneity were the important features that affect bird species richness, thus providing an important insight into the ecological relationship between waterbird species richness and the limnological characteristics of reservoirs. The strong positive correlation between species richness and both size and environmental variables underscores the importance of these reservoir features in the management of wildlife conservation. Large, environmentally heterogeneous reservoirs can support more species than small, environmentally homogeneous reservoirs because large, environmentally heterogeneous limnetic ecosystems can provide different resources for nesting, foraging, and roosting habitats for a diversified bird species. The result here also plays a role towards strengthening our knowledge of aquatic bird ecology and the natural history of African-Eurasian Migratory waterbirds.

Speciation in Daphnia

The microcrustacean Daphnia is arguably one of the most studied zooplankton species, having a well understood ecology, life history, and a relatively well studied evolutionary history. Despite this wealth of knowledge, species boundaries within closely related species in this genus often remain elusive and the major evolutionary forces driving the diversity of daphniids remain controversial. This genus contains more than 80 species with multiple cryptic species complexes, with many closely related species able to hybridize. Here, we review speciation research in Daphnia within the framework of current speciation theory. We evaluate the role of geography, ecology, and biology in restricting gene flow and promoting diversification. Of the 253 speciation studies on Daphnia, the majority of studies examine geographic barriers (55%). While evidence shows that geographic barriers play a role in species divergence, ecological barriers are also probably prominent in Daphnia speciation. We assess the contribution of ecological and nonecological reproductive isolating barriers between closely related species of Daphnia and found that none of the reproductive isolating barriers are restricting gene flow completely. Research on reproductive isolating barriers has disproportionally focused on two species complexes, the Daphnia pulex and Daphnia longispina species complexes. Finally, we identify areas of research that remain relatively unexplored and discuss future research directions that build our understanding of speciation in daphniids.

Population connectivity, dispersal, and swimming behavior in Daphnia

The water flea Daphnia has the capacity to respond rapidly to environmental stressors, to disperse over large geographical scales, and to preserve its genetic material by forming egg banks in the sediment. Spatial and temporal distributions of D. magna have been extensively studied over the last decades using behavioral or genetic tools, although the correlation between the two has rarely been the focus. In the present study, we therefore investigated the population genetic structure and behavioral response to a lethal threat, ultraviolet radiation (UVR), among individuals from two different water bodies. Our results show two genetic populations with moderate gene flow, highly correlated with geographical location and with inheritable traits through generations. However, despite the strong genetic differences between populations, we show homogeneous refuge demand between populations when exposed to the lethal threat solar UVR.

Centennial clonal stability of asexual Daphnia in Greenland lakes despite climate variability

Climate and environmental condition drive biodiversity at many levels of biological organization, from populations to ecosystems. Combined with paleoecological reconstructions, palaeogenetic information on resident populations provides novel insights into evolutionary trajectories and genetic diversity driven by environmental variability. While temporal observations of changing genetic structure are often made of sexual populations, little is known about how environmental change affects the long-term fate of asexual lineages. Here, we provide information on obligately asexual, triploid Daphnia populations from three Arctic lakes in West Greenland through the past 200-300 years to test the impact of environmental change on the temporal and spatial population genetic structure. The contrasting ecological state of the lakes, specifically regarding salinity and habitat structure may explain the observed lake-specific clonal composition over time. Palaeolimnological reconstructions show considerable regional environmental fluctuations since 1,700 (the end of the Little Ice Age), but the population genetic structure in two lakes was almost unchanged with at most two clones per time period. Their local populations were strongly dominated by a single clone that has persisted for 250-300 years. We discuss possible explanations for the apparent population genetic stability: (a) persistent clones are general-purpose genotypes that thrive under broad environmental conditions, (b) clonal lineages evolved subtle genotypic differences unresolved by microsatellite markers, or (c) epigenetic modifications allow for clonal adaptation to changing environmental conditions. Our results motivate research into the mechanisms of adaptation in these populations, as well as their evolutionary fate in the light of accelerating climate change in the polar regions.

Changes in bacterioplankton community structure during early lake ontogeny resulting from the retreat of the Greenland Ice Sheet

Retreating glaciers and ice sheets are among the clearest signs of global climate change. One consequence of glacier retreat is the formation of new meltwater-lakes in previously ice-covered terrain. These lakes provide unique opportunities to understand patterns in community organization during early lake ontogeny. Here, we analyzed the bacterial community structure and diversity in six lakes recently formed by the retreat of the Greenland Ice Sheet (GrIS). The lakes represented a turbidity gradient depending on their past and present connectivity to the GrIS meltwaters. Bulk (16S rRNA genes) and putatively active (16S rRNA) fractions of the bacterioplankton communities were structured by changes in environmental conditions associated to the turbidity gradient. Differences in community structure among lakes were attributed to both, rare and abundant community members. Further, positive co-occurrence relationships among phylogenetically closely related community members dominate in these lakes. Our results show that environmental conditions along the turbidity gradient structure bacterial community composition, which shifts during lake ontogeny. Rare taxa contribute to these shifts, suggesting that the rare biosphere has an important ecological role during early lakes ontogeny. Members of the rare biosphere may be adapted to the transient niches in these nutrient poor lakes. The directionality and phylogenetic structure of co-occurrence relationships indicate that competitive interactions among closely related taxa may be important in the most turbid lakes.