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Abstract Sterility among hybrids is one of the most prevalent forms of reproductive isolation delineating species boundaries and is expressed disproportionately in heterogametic XY males. While hybrid male sterility (HMS) due to the “large X effect” is a well-recognized mechanism of reproductive isolation, it is less clear how HMS manifests in species that lack heteromorphic sex chromosomes. We evaluated differences in allele frequencies at approximately 460,000 SNPs between fertile and sterile F2 interpopulation male hybrids to characterize the genomic architecture of HMS in a species without sex chromosomes (Tigriopus californicus). We tested associations between HMS and mitochondrial-nuclear and/or nuclear-nuclear signatures of incompatibility. Genomic regions associated with HMS were concentrated on a single chromosome with the same primary 2-Mbp regions identified in one pair of reciprocal crosses. Gene Ontology analysis revealed that annotations associated with spermatogenesis were the most overrepresented within the implicated region, with nine protein-coding genes connected with this process found in the quantitative trait locus of chromosome 2. Our results indicate that a narrow genomic region was associated with the sterility of male hybrids in T. californicus and suggest that incompatibilities among select nuclear loci may replace the large X effect when sex chromosomes are absent. 

Abstract Invasive plants formed via hybridization, especially those that modify the structure and function of their ecosystems, are of particular concern given the potential for hybrid vigor. In the U.S. Pacific Northwest, two invasive, dune‐building beachgrasses,Ammophila arenaria(European beachgrass) andA. breviligulata(American beachgrass), have hybridized and formed a new beachgrass taxa (Ammophila arenaria × A. breviligulata), but little is known about its distribution, spread, and ecological consequences. Here, we report on surveys of the hybrid beachgrass conducted across a 250‐km range from Moclips, Washington to Pacific City, Oregon, in 2021 and 2022. We detected nearly 300 hybrid individuals, or an average of 8–14 hybrid individuals per km of surveyed foredune. The hybrid was more common at sites within southern Washington and northern Oregon whereA. breviligulatais abundant (75%–90% cover) andA. arenariais sparse and patchy. The hybrid displayed morphological traits such as shoot density and height that typically exceeded its parent species suggesting hybrid vigor. We measured an average growth rate of 30% over one year, with individuals growing faster at the leading edge of the foredune, nearest to the beach. We also found a positive relationship between hybrid abundance andA. arenariaabundance, suggesting thatA. arenariadensity may be a controlling factor for hybridization rate. The hybrid showed similar sand deposition and associated plant species richness patterns compared with its parent species, although longer term studies are needed. Finally, we found hybrid individuals within and near conservation habitat of two Endangered Species Act‐listed, threatened bird species, the western snowy plover (Charadrius alexandrinus nivosus) and the streaked horned lark (Eremophila alpestris strigata), a concern for conservation management. Documenting this emerging hybrid beachgrass provides insights into how hybridization affects the spread of novel species and the consequences for communities in which they invade. 

Metazoans respond to hypoxic stress via the Hypoxia Inducible Factor (HIF) pathway, a mechanism thought to be extremely conserved due to its importance in monitoring cellular oxygen levels and regulating responses to hypoxia. However, recent work revealed that key members of the HIF pathway have been lost in specific lineages (a tardigrade and a copepod), suggesting this pathway is not as widespread in animals as previously assumed. Using genomic and transcriptomic data from 70 different species across 12 major crustacean groups, we assessed the degree to which the gene HIFα, the master regulator of the HIF pathway, was conserved. Mining of protein domains, followed by phylogenetic analyses of gene families, uncovered group-level losses of HIFα, including one across three orders within Cirripedia, and in three orders within Copepoda. For these groups, additional assessment showed losses of HIF repression machinery (EGLN, VHL). These results suggest the existence of alternative mechanisms for cellular response to low oxygen, and highlight these taxa as models useful for probing these evolutionary outcomes.