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  1. Abstract

    Hybrid zones are important windows into the evolutionary dynamics of populations, revealing how processes like introgression and adaptation structure population genomic variation. Importantly, they are useful for understanding speciation and how species respond to their environments. Here, we investigate two closely related sea star species,Asterias rubensandA. forbesi, distributed along rocky European and North American coastlines of the North Atlantic, and use genome‐wide molecular markers to infer the distribution of genomic variation within and between species in this group. Using genomic data and environmental niche modelling, we document hybridization occurring between northern New England and the southern Canadian Maritimes. We investigate the factors that maintain this hybrid zone, as well as the environmental variables that putatively drive selection within and between species. We find that the two species differ in their environmental niche breadth;Asterias forbesidisplays a relatively narrow environmental niche while conversely,A. rubenshas a wider niche breadth. Species distribution models accurately predict hybrids to occur within environmental niche overlap, thereby suggesting environmental selection plays an important role in the maintenance of the hybrid zone. Our results imply that the distribution of genomic variation in North Atlantic sea stars is influenced by the environment, which will be crucial to consider as the climate changes.

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  2. Abstract

    A genotypic polymorphism in the sea starPisaster ochraceushas been associated with possible overdominant maintenance of diversity, and subsequent studies of this polymorphism suggested that intermittent disease outbreaks could be a driving factor in this system. However, comparative transcriptomic studies of individuals carrying distinct genotypes at the elongation factor 1-alpha (EF1A) region indicated that the marker was not accurately describing the constitutive differences among individuals. Here we more thoroughly assess this EF1A intron region to better understand how polymorphic diversity could be associated with differential disease outcomes and physiological responses, and find that the underlying genetic model is incorrect. In fact, rather than an instance of homozygous lethality, it is clear that previous genotyping efforts were misled by a PCR artefact. We reanalyze results from two previous studies to show that the effects are not as clear as believed.

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  3. Mass mortality events provide valuable insight into biological extremes and also ecological interactions more generally. The sea star wasting epidemic that began in 2013 catalyzed study of the microbiome, genetics, population dynamics, and community ecology of several high-profile species inhabiting the northeastern Pacific but exposed a dearth of information on the diversity, distributions, and impacts of sea star wasting for many lesser-known sea stars and a need for integration across scales. Here, we combine datasets from single-site to coast-wide studies, across time lines from weeks to decades, for65 species. We evaluated the impacts of abiotic characteristics hypothetically associated with sea star wasting (sea surface temperature, pelagic primary productivity, upwelling wind forcing, wave exposure, freshwater runoff) and species characteristics (depth distribution, developmental mode, diet, habitat, reproductive period). We find that the 2010s sea star wasting out-break clearly affected a little over a dozen species, primarily intertidal and shallow subtidal taxa, causing instantaneous wast-ing prevalence rates of 5%–80%. Despite the collapse of some populations within weeks, environmental and species variation protracted the outbreak, which lasted 2–3 years from onset until declining to chronic background rates of 2% sea star wasting prevalence. Recruitment began immediately in many species, and in general, sea star assemblages trended toward recovery; however, recovery was heterogeneous, and a marine heatwave in 2019 raised concerns of a second decline. The abiotic stressors most associated with the 2010s sea star wasting outbreak were elevated sea surface temperature and low wave exposure, as well as freshwater discharge in the north. However, detailed data speaking directly to the biological, ecological, and environmental cause(s) and consequences of the sea star wasting outbreak remain limited in scope, unavoidably retrospective, andperhaps always indeterminate. Redressing this shortfall for the future will require a broad spectrum of monitoring studies not less than the taxonomically broad cross-scale framework we have modeled in this synthesis. 
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    Free, publicly-accessible full text available December 1, 2024
  4. The global biodiversity crisis unfurling around us often re-quires that we have more complete information to predict how emerging threats will affect ecosystems. We must be able to derive mechanisms from events that we were not prepared to study before they happened. Biologists have learned from undesirable outcomes many times before; the tremendous impacts of species translocations to new localities through human activities are unfortunate—but informative—“experiments” from which we could gain new insights into changing organismal interactions and distributions (Sax et al., 2005. Species Invasions: Insights into Ecology, Evolution, and Biogeography). Similarly, major disruptions to ecosystems have been a source of new understanding when experiments of similar magnitude are not possible, such as new models for community assembly following the massive volcanic eruption that wiped the Krakatau Islands clean of life (MacArthur and Wilson,1963. Evolution 17: 373–387). Although our scientific community can gather more precise and more comprehensive data, collectively glean more insights (e.g., Hewson et al., 2018.Front. Mar. Sci. 5: 77; Wares and Duffin, 2019. bioRxiv:10.1101/584235v1), and continue to reevaluate what we have seen and will see in the years to come, what will such effort achieve? We already have enough evidence that—whether sea stars died as a result of heat, dysoxia, and/or pathogen(s)or some additional combination—this event was the most extreme on record and an illustration of a decline in resilience (e.g., Menge et al., 2021. Proc. Natl. Acad. Sci. U.S.A.119: e2114257119). To avoid another decade of death, it is time to focus on pathways toward recovery of threatened species (Hamilton et al., 2021. Proc. R. Soc. B 288: 20211195)and ecosystem feedback loops (Aquino et al., 2021. Front.Microbiol. 11: 3278) that can rebalance how and where sea stars can thrive, remembering that these animals are typi-cally important consumers that drive diversity in marine ecosystems (Fig. 1F). One way or another, this massive SSW “experiment” is a component of a global problem thatwe must urgently resolve how to address. 
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