Ecosystem size shapes antipredator trait evolution in estuarine threespine stickleback

Adaptability of Bony Armor Elements of the Threespine Stickleback Gasterosteus aculeatus (Teleostei: Gasterosteidae): Ecological and Evolutionary Insights from Symmetry Analyses

Differentiation in the defensive armor of the threespine stickleback, Gasterosteus aculeatus, is caused by predator-driven divergent selection. Most studies considered armor traits related to swimming behavior, hence combining pre- and post-capture responses to gape-limited predators. Here, we focus exclusively on the defensive complex (DC), the post-capture predator defense. This complex consists of a series of bony elements surrounding the anterior part of the abdomen. Relaxation from predation pressure not only drives reduction of bony elements but is also expected to increase asymmetry in the DC. To test this hypothesis, we used four Austrian freshwater populations that differed distinctly in the formation of the DC. We found significant left–right asymmetries in the DC in the population with a distinctly reduced DC and, surprisingly, also in the population with a significantly enhanced DC. These populations occur in vastly different habitats (stream and lake) characterized by distinct regimes of gape-limited predators (none vs. many). Apparently, both a shift to very low and very high pressure by gape-limited predators can boost asymmetry. We conclude that greater asymmetries in the two populations at the opposite ends of the predatory gradient result from an ongoing process of adaptation to decreased or increased environmental stress.

Ecological Drivers of Community Cohesion

From protocellular to societal, networks of living systems are complex and multiscale. Discerning the factors that facilitate assembly of these intricate interdependencies using pairwise interactions can be nearly impossible. To facilitate a greater understanding, we developed a mathematical and computational model based on a synthetic four-strain Saccharomyces cerevisiae interdependent system. Specifically, we aimed to provide a greater understanding of how ecological factors influence community dynamics. By leveraging transiently structured ecologies, we were able to drive community cohesion. We show how ecological interventions could reverse or slow the extinction rate of a cohesive community. An interconnected system first needs to persist long enough to be a subject of natural selection. Our emulation of Darwin's "warm little ponds" with an ecology governed by transient compartmentalization provided the necessary persistence. Our results reveal utility across scales of organization, stressing the importance of cyclic processes in major evolutionary transitions, engineering of synthetic microbial consortia, and conservation biology. IMPORTANCE We are facing unprecedented disruption and collapse of ecosystems across the globe. To have any hope of mitigating this phenomenon, a much greater understanding of ecosystem dynamics is required. However, ecosystems are typically composed of highly dynamic networks of individual species. These interactions are further modulated by abiotic and biotic factors that vary temporally and spatially. Thus, ecological dynamics are obfuscated by this complexity. Here, we developed a theoretical model, informed by a synthetic experimental system, of Darwin's "warm little ponds." This cycling four-species system seeks to elucidate the ecological factors that drive or inhibit interaction. We show that these factors could provide an essential tool for avoiding the accelerating ecological collapse. Our study also provides a starting point to develop a more encompassing model to inform conservation efforts.

Genetic structure and historic demography of endangered unarmoured threespine stickleback at southern latitudes signals a potential new management approach

Habitat loss, flood control infrastructure, and drought have left most of southern California and northern Baja California's native freshwater fish near extinction, including the endangered unarmoured threespine stickleback (Gasterosteus aculeatus williamsoni). This subspecies, an unusual morph lacking the typical lateral bony plates of the G. aculeatus complex, occurs at arid southern latitudes in the eastern Pacific Ocean and survives in only three inland locations. Managers have lacked molecular data to answer basic questions about the ancestry and genetic distinctiveness of unarmoured populations. These data could be used to prioritize conservation efforts. We sampled G. aculeatus from 36 localities and used microsatellites and whole genome data to place unarmoured populations within the broader evolutionary context of G. aculeatus across southern California/northern Baja California. We identified three genetic groups with none consisting solely of unarmoured populations. Unlike G. aculeatus at northern latitudes, where Pleistocene glaciation has produced similar historical demographic profiles across populations, we found markedly different demographics depending on sampling location, with inland unarmoured populations showing steeper population declines and lower heterozygosity compared to low armoured populations in coastal lagoons. One exception involved the only high elevation population in the region, where the demography and alleles of unarmoured fish were similar to low armoured populations near the coast, exposing one of several cases of artificial translocation. Our results suggest that the current "management-by-phenotype" approach, based on lateral plates, is incidentally protecting the most imperilled populations; however, redirecting efforts toward evolutionary units, regardless of phenotype, may more effectively preserve adaptive potential.

The influence of predator community composition on photoprotective traits of copepods

Trait expression of natural populations often jointly depends on prevailing abiotic environmental conditions and predation risk. Copepods, for example, can vary their expression of compounds that confer protection against ultraviolet radiation (UVR), such as astaxanthin and mycosporine‐like amino acids (MAAs), in relation to predation risk. Despite ample evidence that copepods accumulate less astaxanthin in the presence of predators, little is known about how the community composition of planktivorous fish can affect the overall expression of photoprotective compounds. Here, we investigate how the (co‐)occurrence of Arctic charr (Salvelinus alpinus) and threespine stickleback (Gasterosteus aculeatus) affects the photoprotective phenotype of the copepod Leptodiaptomus minutus in lake ecosystems in southern Greenland. We found that average astaxanthin and MAA contents were lowest in lakes with stickleback, but we found no evidence that these photoprotective compounds were affected by the presence of charr. Furthermore, variance in astaxanthin among individual copepods was greatest in the presence of stickleback and the astaxanthin content of copepods was negatively correlated with increasing stickleback density. Overall, we show that the presence and density of stickleback jointly affect the content of photoprotective compounds by copepods, illustrating how the community composition of predators in an ecosystem can determine the expression of prey traits that are also influenced by abiotic stressors.

The legacy of predator threat shapes prey foraging behaviour

Predators exert strong selection on prey foraging behaviour such that prey responses may reflect a combination of ancestral effects of predators (genetic and nongenetic transgenerational effects), past individual experience with predators (phenotypic plasticity), and current exposure to predators (behavioural response). However, the importance of these factors in shaping prey foraging behaviour is not well understood. To test the relative effects of ancestry, prior experience, and current exposure, we measured foraging rates and food size preference of different ancestry and exposure groups of Western mosquitofish in the presence and absence of immediate threat from predatory largemouth bass. Our results confirm that mosquitofish had lower foraging rate in the immediate presence of predator threat. Mosquitofish also foraged at a lower rate if they had ancestry with predators, regardless of immediate threat. In contrast, individual prior experience with predators only caused reduced foraging rates in the immediate presence of a predator. This suggests that phenotypic plasticity could carry a lower risk of maladaptive antipredator responses-i.e., reduced food intake-in the complete absence of a predator. Finally, in the presence of a predator, mosquitofish with both ancestry and experience with predators consumed larger, presumably more energetically valuable, food items. Overall, our results show that non-consumptive effects of predators on prey behaviour can persist within and across generations, such that the legacy of past predator exposure-or "the ghost of predation past"-may continue to shape prey behaviour even when predators are no longer around.

Using seasonal genomic changes to understand historical adaptation to new environments: Parallel selection on stickleback in highly-variable estuaries

Parallel evolution is considered strong evidence for natural selection. However, few studies have investigated the process of parallel selection as it plays out in real time. The common approach is to study historical signatures of selection in populations already well adapted to different environments. Here, to document selection under natural conditions, we study six populations of threespine stickleback (Gasterosteus aculeatus) inhabiting bar-built estuaries that undergo seasonal cycles of environmental changes. Estuaries are periodically isolated from the ocean due to sandbar formation during dry summer months, with concurrent environmental shifts that resemble the long-term changes associated with postglacial colonization of freshwater habitats by marine populations. We used pooled whole-genome sequencing to track seasonal allele frequency changes in six of these populations and search for signatures of natural selection. We found consistent changes in allele frequency across estuaries, suggesting a potential role for parallel selection. Functional enrichment among candidate genes included transmembrane ion transport and calcium binding, which are important for osmoregulation and ion balance. The genomic changes that occur in threespine stickleback from bar-built estuaries could provide a glimpse into the early stages of adaptation that have occurred in many historical marine to freshwater transitions.