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Abstract Science communication (scicomm) shapes our world by helping people use science to make societal and personal decisions. Supporting and doing ethical scicomm requires valuing diverse perspectives and the people who do scicomm. Unfortunately, institutional hurdles ingrained in academia impede and undermine ethical scicomm. The injustices impeding scicomm stem from the prestige paradigm of academia (articulated in the present article), which reinforces hierarchical relationships in an exclusionary and exploitative system. To move academia forward, we name and review these injustices through the lens of five realms of scicomm (scientific communication, teaching scicomm, academics engaging in scicomm, scicomm research, and scicomm careers beyond academia). We then provide a novel framework, helping readers identify axes of influence and how they can leverage their intersectional, academic capital to take concrete action to remove the hurdles impeding ethical scicomm in academia. 

Abstract Hybrid incompatibilities are a critical component of species barriers and may arise due to negative interactions between divergent regulatory elements in parental species. We used a comparative approach to identify common themes in the regulatory phenotypes associated with hybrid male sterility in two divergent rodent crosses, dwarf hamsters and house mice. We investigated three potential characteristic gene expression phenotypes in hybrids including the propensity of transgressive differentially expressed genes toward over or underexpression, the influence of developmental stage on patterns of misexpression, and the role of the sex chromosomes on misexpression phenotypes. In contrast to near pervasive overexpression in hybrid house mice, we found that misexpression in hybrid dwarf hamsters was dependent on developmental stage. In both house mouse and dwarf hamster hybrids, however, misexpression increased with the progression of spermatogenesis, although to varying extents and with potentially different consequences. In both systems, we detected sex chromosome-specific overexpression in stages of spermatogenesis where inactivated X chromosome expression was expected, but the hybrid overexpression phenotypes were fundamentally different. Importantly, misexpression phenotypes support the presence of multiple developmental blocks to spermatogenesis in dwarf hamster hybrids, including a potential role of meiotic stalling or breakdown early in spermatogenesis. Collectively, we demonstrate that while there are some similarities in hybrid expression phenotypes of house mice and dwarf hamsters, there are also clear differences that point toward unique mechanisms underlying hybrid male sterility. Our results highlight the potential of comparative approaches in helping to understand the causes and consequences of disrupted gene expression in speciation. 

Abstract Sensory drive can lead to the evolution of signals that are optimized to the environment in which they are perceived. However, when environmental conditions change, the interactions between signal, environment, and receiver may also shift, leading to the evolution of a new signal optimum or more categorical shifts in sexual signals (gains or losses). We evaluated how visual systems have evolved following a change in environment and male signal, and whether visual system divergence contributes to reproductive isolation between ancestral and derived types in red and black morphs of Pacific Northwest freshwater threespine stickleback. We found that opsin sequence was tuned to enhance the perceived contrast of black fish on a red-shifted light background, whereas opsin expression was not. Further, we found no evidence for homotypic preferences or assortative mating between colour morphs; males of both morphs were equally successful in no-choice mating contexts, perhaps because black males are more vigorous courters. Together, our results suggest that habitat transitions in black stickleback have led to a shift in sensory-drive dynamics with some aspects of the visual system and behaviour evolving in response to other factors (foraging or predation) or lagging behind the evolution of opsin sequences in red-shifted environments. 

Abstract Competition for mates can play a critical role in determining reproductive success, shaping phenotypic variation within populations, and influencing divergence. Yet, studies of the role of sexual selection in divergence and speciation have focused disproportionately on mate choice. Here, we synthesize the literature on how mate competition may contribute to speciation and integrate concepts from work on sexual selection within populations—mating systems, ecology, and mate choice. Using this synthesis, we generate testable predictions for how mate competition may contribute to divergence. Then, we identify the extent of existing support for these predictions in the literature with a systematic review of the consequences of mate competition for population divergence across a range of evolutionary, ecological, and geographic contexts. We broadly evaluate current evidence, identify gaps in available data and hypotheses that need testing, and outline promising directions for future work. A major finding is that mate competition may commonly facilitate further divergence after initial divergence has occurred, e.g., upon secondary contact and between allopatric populations. Importantly, current hypotheses for how mate competition contributes to divergence do not fully explain observed patterns. While results from many studies fit predictions of negative frequency-dependent selection, agonistic character displacement, and ecological selection, results from ~30% of studies did not fit existing conceptual models. This review identifies future research aims for scenarios in which mate competition is likely important but has been understudied, including how ecological context and interactions between mate choice and mate competition can facilitate or hinder divergence and speciation. 

Abstract Hybrid sterility is a complex phenotype that can result from the breakdown of spermatogenesis at multiple developmental stages. Here, we disentangle two proposed hybrid male sterility mechanisms in the house mice, Mus musculus domesticus and M. m. musculus, by comparing patterns of gene expression in sterile F1 hybrids from a reciprocal cross. We found that hybrid males from both cross directions showed disrupted X chromosome expression during prophase of meiosis I consistent with a loss of meiotic sex chromosome inactivation (MSCI) and Prdm9-associated sterility, but that the degree of disruption was greater in mice with an M. m. musculus X chromosome consistent with previous studies. During postmeiotic development, gene expression on the X chromosome was only disrupted in one cross direction, suggesting that misexpression at this later stage was genotype-specific and not a simple downstream consequence of MSCI disruption which was observed in both reciprocal crosses. Instead, disrupted postmeiotic expression may depend on the magnitude of earlier disrupted MSCI, or the disruption of particular X-linked genes or gene networks. Alternatively, only hybrids with a potential deficit of Sly copies, a Y-linked ampliconic gene family, showed overexpression in postmeiotic cells, consistent with a previously proposed model of antagonistic coevolution between the X- and Y-linked ampliconic genes contributing to disrupted expression late in spermatogenesis. The relative contributions of these two regulatory mechanisms and their impact on sterility phenotypes await further study. Our results further support the hypothesis that X-linked hybrid sterility in house mice has a variable genetic basis, and that genotype-specific disruption of gene regulation contributes to overexpression of the X chromosome at different stages of development. Overall, these findings underscore the critical role of epigenetic regulation of the X chromosome during spermatogenesis and suggest that these processes are prone to disruption in hybrids. 

Gene expression has a key role in reproductive isolation, and studies of hybrid gene expression have identified mechanisms causing hybrid sterility. Here, we review the evidence for altered gene expression following hybridization and outline the mechanisms shown to contribute to altered gene expression in hybrids. Transgressive gene expression, transcending that of both parental species, is pervasive in early generation sterile hybrids, but also frequently observed in viable, fertile hybrids. We highlight studies showing that hybridization can result in transgressive gene expression, also in established hybrid lineages or species. Such extreme patterns of gene expression in stabilized hybrid taxa suggest that altered hybrid gene expression may result in hybridization‐derived evolutionary novelty. We also conclude that while patterns of misexpression in hybrids are well documented, the understanding of the mechanisms causing misexpression is lagging. We argue that jointly assessing differences in cell composition and cell‐specific changes in gene expression in hybrids, in addition to assessing changes in chromatin and methylation, will significantly advance our understanding of the basis of altered gene expression. Moreover, uncovering to what extent evolution of gene expression results in altered expression for individual genes, or entire networks of genes, will advance our understanding of how selection moulds gene expression. Finally, we argue that jointly studying the dual roles of altered hybrid gene expression, serving both as a mechanism for reproductive isolation and as a substrate for hybrid ecological adaptation, will lead to significant advances in our understanding of the evolution of gene expression. 

How barriers to gene flow arise and are maintained are key questions in evolutionary biology. Speciation research has mainly focussed on barriers that occur either before mating or after zygote formation. In comparison, postmating prezygotic (PMPZ) isolation – a barrier that acts after gamete release but before zygote formation – is less frequently investigated but may hold a unique role in generating biodiversity. Here we discuss the distinctive features of PMPZ isolation, including the primary drivers and molecular mechanisms underpinning PMPZ isolation. We then present the first comprehensive survey of PMPZ isolation research, revealing that it is a widespread form of prezygotic isolation across eukaryotes. The survey also exposes obstacles in studying PMPZ isolation, in part attributable to the challenges involved in directly measuring PMPZ isolation and uncovering its causal mechanisms. Finally, we identify outstanding knowledge gaps and provide recommendations for improving future research on PMPZ isolation. This will allow us to better understand the nature of this often-neglected reproductive barrier and its contribution to speciation. 

In hybrid zones, whether barrier loci experience selection mostly independently or as a unit depends on the ratio of selection to recombination as captured by the coupling coefficient. Theory predicts a sharper transition between an uncoupled and coupled system when more loci affect hybrid fitness. However, the extent of coupling in hybrid zones has rarely been quantified. Here, we use simulations to characterize the relationship between the coupling coefficient and variance in clines across genetic loci. We then re-analyze 25 hybrid zone data sets and find that cline variances and estimated coupling coefficients form a smooth continuum from high variance and weak coupling to low variance and strong coupling. Our results are consistent with low rates of hybridization and a strong genome-wide barrier to gene flow when the coupling coefficient is much greater than 1, but also suggest that this boundary might be approached gradually and at a near constant rate over time. 

The theory and practice of evolutionary tree-thinking is pervasive through many scientific fields and is a critical component of biological literacy. Many elements of tree-thinking are introduced early in undergraduate biology education. However, basic concepts are often not revisited/reinforced and are assumed to have been fully conceptually grasped in upper-level courses and beyond. Here, we present a project-based activity that we developed to aid upper-level biology students to learn, conceptualize, and practice tree-thinking. This approach allows them to identify the misconceptions that they may have about tree-thinking, while reinforcing the theories and concepts that they may have encountered in introductory courses. It also integrates several pedagogical styles (instructor-led and student-centered), along with an organismal case study to make concepts concrete and realistic to students. 

Patterns of morphological divergence across species’ ranges can provide insight into local adaptation and speciation. In this study, we compared phenotypic divergence among 4,221 crickets from 337 populations of two closely related species of field cricket,Gryllus firmusandG. pennsylvanicus, and their hybrids. We found that these species differ across their geographic range in key morphological traits, such as body size and ovipositor length, and we directly compared phenotype with genotype for a subset of crickets to demonstrate nuclear genetic introgression, phenotypic intermediacy of hybrids, and essentially unidirectional mitochondrial introgression. We discuss how these morphological traits relate to life history differences between the two species. Our comparisons across geographic areas support prior research suggesting that cryptic variation withinG. firmusmay represent different species. Our study highlights how variable morphology can be across wide-ranging species and the importance of studying reproductive barriers in more than one or two transects of a hybrid zone.