Resurrection genomics provides molecular and phenotypic evidence of rapid adaptation to salinization in a keystone aquatic species.

Ecologists and evolutionary biologists are increasingly cognizant of rapid adaptation in wild populations. Rapid adaptation to anthropogenic environmental change is critical for maintaining biodiversity and ecosystems services into the future. Anthropogenic salinization of freshwater ecosystems is quickly emerging as a primary threat, which is well documented in the northern temperate ecoregion. Specifically, many northern temperate lakes have undergone extensive salinization because of urbanization and the associated increase in impervious surfaces causing runoff, and the extensive use of road deicing salts (e.g., NaCl). It remains unclear whether increasing salinization will lead to extirpation of species from these systems. Using a "resurrection genomics" approach, we investigated whether the keystone aquatic herbivore, Daphnia pulicaria, has evolved increased salinity tolerance in a severely salinized lake located in Minnesota, USA. Whole-genome resequencing of 54 Daphnia clones from the lake and hatched from resting eggs that represent a 25-y temporal contrast demonstrates that many regions of the genome containing genes related to osmoregulation are under selection in the study population. Tolerance assays of clones revealed that the most recent clones are more tolerant to salinity than older clones; this pattern is concomitant with the temporal pattern of stabilizing salinity in this lake. Together, our results demonstrate that keystone species such as Daphnia can rapidly adapt to increasing freshwater salinization. Further, our results indicate that rapid adaptation to salinity may allow lake Daphnia populations to persist in the face of anthropogenic salinization maintaining the food webs and ecosystem services they support despite global environmental change.

The roles of recombination and selection in shaping genomic divergence in an incipient ecological species complex.

Speciation genomic studies have revealed that genomes of diverging lineages are shaped jointly by the actions of gene flow and selection. These evolutionary forces acting in concert with processes such as recombination and genome features such as gene density shape a mosaic landscape of divergence. We investigated the roles of recombination and gene density in shaping the patterns of differentiation and divergence between the cyclically parthenogenetic ecological sister-taxa, Daphnia pulicaria and Daphnia pulex. First, we assembled a phased chromosome-scale genome assembly using trio-binning for D. pulicaria and constructed a genetic map using an F2-intercross panel to understand sex-specific recombination rate heterogeneity. Finally, we used a ddRADseq data set with broad geographic sampling of D. pulicaria, D. pulex, and their hybrids to understand the patterns of genome-scale divergence and demographic parameters. Our study provides the first sex-specific estimates of recombination rates for a cyclical parthenogen, and unlike other eukaryotic species, we observed male-biased heterochiasmy in D. pulicaria, which may be related to this somewhat unique breeding mode. Additionally, regions of high gene density and recombination are generally more divergent than regions of suppressed recombination. Outlier analysis indicated that divergent genomic regions are probably driven by selection on D. pulicaria, the derived lineage colonizing a novel lake habitat. Together, our study supports a scenario of selection acting on genes related to local adaptation shaping genome-wide patterns of differentiation despite high local recombination rates in this species complex. Finally, we discuss the limitations of our data in light of demographic uncertainty.

The effects of different cold-temperature regimes on development, growth, and susceptibility to an abiotic and biotic stressor.

Global climate change is expected to both increase average temperatures as well as temperature variability.Increased average temperatures have led to earlier breeding in many spring-breeding organisms. However, individuals breeding earlier will also face increased temperature fluctuations, including exposure to potentially harmful cold-temperature regimes during early developmental stages.Using a model spring-breeding amphibian, we investigated how embryonic exposure to different cold-temperature regimes (control, cold-pulse, and cold-press) affected (a) compensatory larval development and growth, (b) larval susceptibility to a common contaminant, and (c) larval susceptibility to parasites.We found: (a) no evidence of compensatory development or growth, (b) larvae exposed to the cold-press treatment were more susceptible to NaCl at 4-days post-hatching but recovered by 17-days post-hatching, and (c) larvae exposed to both cold treatments were less susceptible to parasites.These results demonstrate that variation in cold-temperature regimes can lead to unique direct and indirect effects on larval growth, development, and response to stressors. This underscores the importance of considering cold-temperature variability and not just increased average temperatures when examining the impacts of climate disruption.

Direct and indirect effects of a common cyanobacterial toxin on amphibian-trematode dynamics.

Wildlife diseases are emerging at unprecedented rates. While there are likely several factors at play, human-mediated environmental alterations may play a significant role. Of growing interest is the effect that microcystin-LR (MC-LR), a cyanotoxin, may have on disease outcomes. In this study, using an amphibian-trematode model we examined (1) the lethal effects of MC-LR on cercariae of trematodes; (2) the sublethal effects of MC-LR exposure on the ability for trematodes to infect green frog tadpoles; and (3) the sublethal effects of MC-LR on green frog tadpole susceptibility to trematodes. We found that environmentally-relevant concentrations of MC-LR at 50, 100, and 500 µg L-1 increased cercariae rate of mortality (LC50-14h = 134.24 µg L-1). However, sublethal exposure of trematodes to 2 and 10 µg L-1 MC-LR did not alter their infectivity. Conversely, sublethal exposure of tadpoles to 2 µg L-1 increased their susceptibility to trematodes by 147%. However, 10 µg L-1 of MC-LR did not affect tadpole susceptibility to trematodes, indicating a non-linear response to sublethal MC-LR exposure. Overall, our findings suggest that high concentrations of MC-LR (≥50 µg L-1) have the potential to limit trematode transmission to amphibian hosts through MC-LR-induced mortality. However, at lower concentrations (<10 µg L-1) MC-LR's effect on tadpole-cercariae disease outcome is likely driven by its effect on the tadpole host. Collectively, this work highlights the need to consider how toxicants influence both host and parasite at multiple concentrations to better understand the impacts of cyanotoxins on disease dynamics.