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Thousands of small populations are at increased risk of extinction because genetics and evolutionary biology are not well‐integrated into conservation planning–a major lost opportunity for effective actions. We propose that if the risk of outbreeding depression is low, the default should be to evaluate restoration of gene flow to small inbred populations of diploid outbreeding organisms that were isolated by human activities within the last 500 years, rather than inaction. We outline the elements of a scientific‐based genetic management policy for fragmented populations of plants and animals, and discuss the reasons why the current default policy is, inappropriately, inaction.

Hybridization, defined as breeding between two distinct taxonomic units, can have an important effect on the evolutionary patterns in cross-breeding taxa. Although interspecific hybridization has frequently been considered as a maladaptive process, which threatens species genetic integrity and survival via genetic swamping and outbreeding depression, in some cases hybridization can introduce novel adaptive variation and increase fitness. Most studies to date focused on documenting hybridization events and analyzing their causes, while relatively little is known about the consequences of hybridization and its impact on the parental species. To address this knowledge gap, we conducted a systematic review of studies on hybridization in mammals published in 2010–2021, and identified 115 relevant studies. Of 13 categories of hybridization consequences described in these studies, the most common negative consequence (21% of studies) was genetic swamping and the most common positive consequence (8%) was the gain of novel adaptive variation. The total frequency of negative consequences (49%) was higher than positive (13%) and neutral (38%) consequences. These frequencies are biased by the detection possibilities of microsatellite loci, the most common genetic markers used in the papers assessed. As negative outcomes are typically easier to demonstrate than positive ones (e.g., extinction vs hybrid speciation), they may be over-represented in publications. Transition towards genomic studies involving both neutral and adaptive variation will provide a better insight into the real impacts of hybridization.

When divergent populations interbreed, their alleles are brought together in hybrids. In the initial F1 cross, most divergent loci are heterozygous. Therefore, F1 fitness can be influenced by dominance effects that could not have been selected to function well together. We present a systematic study of these F1 dominance effects by introducing variable phenotypic dominance into Fisher’s geometric model. We show that dominance often reduces hybrid fitness, which can generate optimal outbreeding followed by a steady decline in F1 fitness, as is often observed. We also show that “lucky” beneficial effects sometimes arise by chance, which might be important when hybrids can access novel environments. We then show that dominance can lead to violations of Haldane’s Rule (reduced fitness of the heterogametic F1) but strengthens Darwin’s Corollary (F1 fitness differences between cross directions). Taken together, results show that the effects of dominance on hybrid fitness can be surprisingly difficult to isolate, because they often resemble the effects of uniparental inheritance or expression. Nevertheless, we identify a pattern of environment-dependent heterosis that only dominance can explain, and for which there is some suggestive evidence. Our results also show how existing data set upper bounds on the size of dominance effects. These bounds could explain why additive models often provide good predictions for later-generation recombinant hybrids, even when dominance qualitatively changes outcomes for the F1.

The increase in habitat fragmentation impacts plant-pollinator interactions and threatens the sustainability of plant species. Astragalus tragacantha (Fabaceae), is a rare endangered plant species along the coastal habitats where the plant populations have undergone considerable fragmentation and decline of size. Controlled pollination treatments, the observation of pollinator activity, and pollinator captures, have been conducted to study: (1) the mating system of A. tragacantha and the potential for inbreeding depression and/or outbreeding depression based on controlled pollination treatments, (2) the pollinator composition among populations using a correspondence analysis and a hierarchical clustering, and (3) the link between pollinators and the plant reproductive success using a path-analysis model. In this study, we demonstrated that this plant was not autogamous self-pollinating and depended on pollinators for its reproduction. The absence of difference between manual and open pollinations regarding the reproductive success showed an absence of pollen limitation in our populations. We showed that populations differed in the composition of their pollinator guilds. Some pollinator species were predominant in certain populations. The pollination treatments revealed the existence of a mixed mating system in A. tragacantha populations. We showed an inbreeding depression potentially linked to a predominant pollinator-facilitated selfing, and the existence of outbreeding depression between some distant populations. These differences in pollinator guild and plant mating systems among populations must be considered during the restoration of populations along the Mediterranean coastal habitats in order to enhance the reproductive success and sustainability of A. tragacantha.

Active coral restoration typically involves two interventions: crossing gametes to facilitate sexual larval propagation; and fragmenting, growing, and outplanting adult colonies to enhance asexual propagation. From an evolutionary perspective, the goal of these efforts is to establish self-sustaining, sexually reproducing coral populations that have sufficient genetic and phenotypic variation to adapt to changing environments. Here, we provide concrete guidelines to help restoration practitioners meet this goal for most Caribbean species of interest. To enable the persistence of coral populations exposed to severe selection pressure from many stressors, a mixed provenance strategy is suggested: genetically unique colonies (genets) should be sourced both locally as well as from more distant, environmentally distinct sites. Sourcing three to four genets per reef along environmental gradients should be sufficient to capture a majority of intraspecies genetic diversity. It is best for practitioners to propagate genets with one or more phenotypic traits that are predicted to be valuable in the future, such as low partial mortality, high wound healing rate, high skeletal growth rate, bleaching resilience, infectious disease resilience, and high sexual reproductive output. Some effort should also be reserved for underperforming genets because colonies that grow poorly in nurseries sometimes thrive once returned to the reef and may harbor genetic variants with as yet unrecognized value. Outplants should be clustered in groups of four to six genets to enable successful fertilization upon maturation. Current evidence indicates that translocating genets among distant reefs is unlikely to be problematic from a population genetic perspective but will likely provide substantial adaptive benefits. Similarly, inbreeding depression is not a concern given that current practices only raise first-generation offspring. Thus, proceeding with the proposed management strategies even in the absence of a detailed population genetic analysis of the focal species at sites targeted for restoration is the best course of action. These basic guidelines should help maximize the adaptive potential of reef-building corals facing a rapidly changing environment.

Hybridization may drive rare taxa to extinction through genetic swamping, where the rare form is replaced by hybrids, or by demographic swamping, where population growth rates are reduced due to the wasteful production of maladaptive hybrids. Conversely, hybridization may rescue the viability of small, inbred populations. Understanding the factors that contribute to destructive versus constructive outcomes of hybridization is key to managing conservation concerns. Here, we survey the literature for studies of hybridization and extinction to identify the ecological, evolutionary, and genetic factors that critically affect extinction risk through hybridization. We find that while extinction risk is highly situation dependent, genetic swamping is much more frequent than demographic swamping. In addition, human involvement is associated with increased risk and high reproductive isolation with reduced risk. Although climate change is predicted to increase the risk of hybridization‐induced extinction, we find little empirical support for this prediction. Similarly, theoretical and experimental studies imply that genetic rescue through hybridization may be equally or more probable than demographic swamping, but our literature survey failed to support this claim. We conclude that halting the introduction of hybridization‐prone exotics and restoring mature and diverse habitats that are resistant to hybrid establishment should be management priorities.

Assisted gene flow (AGF) between populations has the potential to mitigate maladaptation due to climate change. However, AGF may cause outbreeding depression (especially if source and recipient populations have been long isolated) and may disrupt local adaptation to nonclimatic factors. Selection should eliminate extrinsic outbreeding depression due to adaptive differences in large populations, and simulations suggest that, within a few generations, evolution should resolve mild intrinsic outbreeding depression due to epistasis. To weigh the risks of AGF against those of maladaptation due to climate change, we need to know the species' extent of local adaptation to climate and other environmental factors, as well as its pattern of gene flow. AGF should be a powerful tool for managing foundation and resource-producing species with large populations and broad ranges that show signs of historical adaptation to local climatic conditions.

Heterostyly represents a fascinating adaptation to promote outbreeding in plants that evolved multiple times independently. While L-morph individuals form flowers with long styles, short anthers, and small pollen grains, S-morph individuals have flowers with short styles, long anthers, and large pollen grains. The difference between the morphs is controlled by an S-locus “supergene” consisting of several distinct genes that determine different traits of the syndrome and are held together, because recombination between them is suppressed. In Primula, the S locus is a roughly 300-kb hemizygous region containing five predicted genes. However, with one exception, their roles remain unclear, as does the evolutionary buildup of the S locus. Here we demonstrate that the MADS-box GLOBOSA2 (GLO2) gene at the S locus determines anther position. In Primula forbesii S-morph plants, GLO2 promotes growth by cell expansion in the fused tube of petals and stamen filaments beneath the anther insertion point; by contrast, neither pollen size nor male incompatibility is affected by GLO2 activity. The paralogue GLO1, from which GLO2 arose by duplication, has maintained the ancestral B-class function in specifying petal and stamen identity, indicating that GLO2 underwent neofunctionalization, likely at the level of the encoded protein. Genetic mapping and phylogenetic analysis indicate that the duplications giving rise to the style-length-determining gene CYP734A50 and to GLO2 occurred sequentially, with the CYP734A50 duplication likely the first. Together these results provide the most detailed insight into the assembly of a plant supergene yet and have important implications for the evolution of heterostyly.

Targeted gene flow is an emerging conservation strategy. It involves translocating individuals with favorable genes to areas where they will have a conservation benefit. The applications for targeted gene flow are wide-ranging but include preadapting native species to the arrival of invasive species. The endangered carnivorous marsupial, the northern quoll (Dasyurus hallucatus), has declined rapidly since the introduction of the cane toad (Rhinella marina), which fatally poisons quolls that attack them. There are, however, a few remaining toad-invaded quoll populations in which the quolls survive because they know not to eat cane toads. It is this toad-smart behavior we hope to promote through targeted gene flow. For targeted gene flow to be feasible, however, toad-smart behavior must have a genetic basis. To assess this, we used a common garden experiment, comparing offspring from toad-exposed and toad-naïve parents raised in identical environments, to determine whether toad-smart behavior is heritable. Offspring from toad-exposed populations were substantially less likely to eat toads than those with toad-naïve parents. Hybrid offspring showed similar responses to quolls with 2 toad-exposed parents, indicating the trait may be dominant. Together, these results suggest a heritable trait and rapid adaptive response in a small number of toadexposed populations. Although questions remain about outbreeding depression, our results are encouraging for targeted gene flow. It should be possible to introduce toad-smart behavior into soon to be affected quoll populations. El flujo génico dirigido es una estrategia de conservación emergente. Este involucra la reubicación de individuos con genes favorables a áreas en donde proporcionarán un beneficio para la conservación. Las aplicaciones del flujo génico dirigido son de una extensa variedad pero incluyen la pre-adaptación de las especies nativas a la llegada de especies invasoras. El carnívoro marsupial en peligro de extinción, el cuol del norte (Dasyurus hallucatus), ha declinado rápidamente desde la introducción del sapo de la caña (Rhinella marina), el cual envenena fatalmente a los cuoles que lo atacan. Aun así existen unas cuantas poblaciones de cuol invadidas por sapos en las cuales los cuoles sobreviven porque saben que no deben comerse a los sapos. Es este comportamiento el cual buscamos promover por medio del flujo génico dirigido. Sin embargo, para que el flujo génico dirigido sea factible, el comportamiento indicado debe tener una base genética. Para valorar esto, usamos un experimento de jardín común: comparamos la descendencia de padres expuestos y de padres no expuestos criados en ambientes idénticos para determinar si el comportamiento anti-sapos es heredable. Las crías de poblaciones expuestas a los sapos tuvieron una probabilidad sustancialmente menor de comerse a los sapos que aquellas con padres no expuestos. La descendencia híbrida mostró respuestas similares a la descendencia de dos padres expuestos a los sapos, lo que indica que el carácter puede ser dominante. Este conjunto de resultados sugiere que existe un carácter heredable y una respuesta rápida de adaptación en un número pequeño de poblaciones afectadas por sapos. Aunque todavía existen dudas sobre la depresión exogámica, nuestros resultados son alentadores para el flujo génico dirigido. Debería ser posible introducir el comportamiento anti-sapos dentro de poblaciones de cuoles que serán afectadas en un futuro cercano. 目标基因流操控是ー种新兴的保护策略，涉及将携带优势基因的个体转移到对其保护有利的地区。目标 基因流的应用很广泛，包括使本地种提前适应即将到来的人侵种。瀕危的有袋类食肉动物北澳袋鼬（Dasyums hallucatus)自从海蟾蜍(Rhindla maxim)被引人后数量迅速减少，因为海蟾蜍可以毒死攻击它们的袋鼬。然而， 有些遭到蟾蜍人侵的袋鼬种群却可以存活下來因为它们知道不能去吃海蟾蜍。我们希望能通过目标基因流操 控提升这种面对蟾蜍时聪明的行为。然雨輕向基因流可行的前提是这种行为有遗传基础。为了评估这一点，我 们利用同质园实验比较了在相同环境中长大、父母分別接触过蟾蜍和没有接触过蟾蜍的个体，以确定不吃蟾餘 的行为是否可以遗传。结果显示，接触过蟾蜍的种群的后代去吃蟾蜍的可能性大大低于那些没有接触过蟾蜍的 父母的后代。杂交后代的反应与父母双方都接触过蟾蜍的后代相似，表明这可能是ー种显性性状。综上这些结 果表明受到蟾蜍影响的少数袋鼬种群中存在相应的可遗传性状和快速适应。尽管还存在远交衰退的问题，我们 的结果还是可以支持目标基因流操控。接下来可以在即将受到蟾餘影响的袋鼬种群中引人具有这种行为基因的 个体。

Outbreeding response, the phenotypic differences observed between selfed parental lines and their outcrossed offspring, can influence the evolution of selfing strategies. However, such effects remain poorly understood in noncrop species. We explored the phenotypic outbreeding response variation across ploidy levels in Erysimum incanum , a predominantly selfing plant complex with diploid, tetraploid, and hexaploid populations distributed across the Iberian Peninsula and Morocco. We performed controlled within‐population crosses to generate offspring with varying heterozygosity levels across ploidy types. We quantified individual, flower, and reproductive traits, and we estimated fitness components, and assessed trait modularity and phenotypic integration to see how heterozygosity affects trait coordination. Tetraploids showed the strongest and most consistently positive outbreeding responses, particularly in gamete production. Trait‐specific outbreeding responses were positively associated with fitness across ploidy levels. Increasing heterozygosity was linked to a reduction in phenotypic integration, suggesting a loosening of trait correlations. Results show that outbreeding response is ploidy‐dependent and functionally connected to fitness and it may act as a selective force promoting outcrossing in highly inbred lineages. We suggest that outbreeding response is a dynamic and evolvable trait, with implications for mating system transitions and diversification in selfing plant populations.