Consequences of hybridization during invasion on establishment success

Novel Mito-Nuclear Combinations Facilitate the Global Invasion of a Major Agricultural Crop Pest

A fundamental understanding of the underlying mechanisms involved in biological invasions is crucial to developing effective risk assessment and control measures against invasive species. The fall armyworm (FAW), Spodoptera frugiperda, is a highly invasive pest that has rapidly spread from its native Americas into much of the Eastern Hemisphere, with a highly homogeneous nuclear genetic background. However, the exact mechanism behind its rapid introduction and propagation remains unclear. Here, a systematic investigation is conducted into the population dynamics of FAW in China from 2019 to 2021 and found that FAW individuals carrying "rice" mitochondria (FAW-mR) are more prevalent (>98%) than that with "corn" mitochondria (FAW-mC) at the initial stage of the invasion and in newly-occupied non-overwintering areas. Further fitness experiments show that the two hybrid-strains of FAW exhibit different adaptions in the new environment in China, and this may have been facilitated by amino acid changes in mitochondrial-encoded proteins. FAW-mR used increases energy metabolism, faster wing-beat frequencies, and lower wing loadings to drive greater flight performance and subsequent rapid colonization of new habitats. In contrast, FAW-mC individuals adapt with more relaxed mitochondria and shuttle energetics into maternal investment, observed as faster development rate and higher fecundity. The presence of two different mitochondria types within FAW has the potential to significantly expand the range of damage and enhance competitive advantage. Overall, the study describes a novel invasion mechanism displayed by the FAW population that facilitates its expansion and establishment in new environments.

The ecological and evolutionary consequences of tropicalisation

Tropicalisation is a marine phenomenon arising from contemporary climate change, and is characterised by the range expansion of tropical/subtropical species and the retraction of temperate species. Tropicalisation occurs globally and can be detected in both tropical/temperate transition zones and temperate regions. The ecological consequences of tropicalisation range from single-species impacts (e.g., altered behaviour) to whole ecosystem changes (e.g., phase shifts in intertidal and subtidal habitats). Our understanding of the evolutionary consequences of tropicalisation is limited, but emerging evidence suggests that tropicalisation could induce phenotypic change as well as shifts in the genotypic composition of both expanding and retracting species. Given the rapid rate of contemporary climate change, research on tropicalisation focusing on shifts in ecosystem functioning, biodiversity change, and socioeconomic impacts is urgently needed.

Population structure and interspecific hybridisation of two invasive blowflies (Diptera: Calliphoridae) following replicated incursions into New Zealand

Rates of biological invasion are increasing globally, with associated negative effects on native biodiversity and ecosystem services. Among other genetic processes, hybridisation can facilitate invasion by producing new combinations of genetic variation that increase adaptive potential and associated population fitness. Yet the role of hybridisation (and resulting gene flow) in biological invasion in invertebrate species is under-studied. Calliphora hilli and Calliphora stygia are blowflies proposed to have invaded New Zealand separately from Australia between 1779 and 1841, and are now widespread throughout the country. Here, we analysed genome-wide single nucleotide polymorphisms (SNPs), generating genotyping-by-sequencing data for 154 individuals collected from 24 populations across New Zealand and Australia to assess the extent of gene flow and hybridisation occurring within and between these blowflies and to better understand their overall population structure. We found that New Zealand populations of both species had weak genetic structure, suggesting high gene flow and an absence of dispersal limitations across the country. We also found evidence that interspecific hybridisation is occurring in the wild between C. hilli and C. stygia in both the native and invasive ranges, and that intraspecific admixture is occurring among populations at appreciable rates. Collectively, these findings provide new insights into the population structure of these two invasive invertebrates and highlight the potential importance of hybridisation and gene flow in biological invasion.

Identification of hybrids between the Japanese giant salamander (Andrias japonicus) and Chinese giant salamander (Andrias cf. davidianus) using deep learning and smartphone images

Human-mediated hybridization between native and non-native species is causing biodiversity loss worldwide. Hybridization has contributed to the extinction of many species through direct and indirect processes such as loss of reproductive opportunity and genetic introgression. Therefore, it is essential to manage hybrids to conserve biodiversity. However, specialized knowledge is required to identify the target species based on visual characteristics when two species have similar features. Although image recognition technology can be a powerful tool for identifying hybrids, studies have yet to utilize deep learning approaches. Hence, this study aimed to identify hybrids between the native Japanese giant salamander (Andrias japonicus) and the non-native Chinese giant salamander (Andrias cf. davidianus) using EfficientNetV2 and smartphone images. We used smartphone images of 11 individuals of native A. japonicus (five training and six test images) and 20 individuals of hybrids between A. japonicus and A. cf. davidianus (five training and 15 test images). In our experimental environment, an AI model constructed with EfficientNetV2 exhibited 100% accuracy in identifying hybrids. In addition, gradient-weighted class activation mapping revealed that the AI model was able to classify A. japonicus and hybrids between A. japonicus and A. cf. davidianus on the basis of the dorsal head spot patterning. Our approach thus enables the identification of hybrids against A. japonicus, which was previously considered difficult by non-experts. Furthermore, since this study achieved reliable identification using smartphone images, it is expected to be applied to a wide range of citizen science projects.

Real-time geographic settling of a hybrid zone between the invasive winter moth (Operophtera brumata L.) and the native Bruce spanworm (O. bruceata Hulst)

Hybridization plays an important and underappreciated role in shaping the evolutionary trajectories of species. Following the introduction of a non-native organism to a novel habitat, hybridization with a native congener may affect the probability of establishment of the introduced species. In most documented cases of hybridization between a native and a non-native species, a mosaic hybrid zone is formed, with hybridization occurring heterogeneously across the landscape. In contrast, most naturally occurring hybrid zones are clinal in structure. Here, we report on a long-term microsatellite data set that monitored hybridization between the invasive winter moth, Operophtera brumata (Lepidoptera: Geometridae), and the native Bruce spanworm, O. bruceata, over a 12-year period. Our results document one of the first examples of the real-time formation and geographic settling of a clinal hybrid zone. In addition, by comparing one transect in Massachusetts where extreme winter cold temperatures have been hypothesized to restrict the distribution of winter moth, and one in coastal Connecticut, where winter temperatures are moderated by Long Island Sound, we found that the location of the hybrid zone appeared to be independent of environmental variables and maintained under a tension model wherein the stability of the hybrid zone was constrained by population density, reduced hybrid fitness, and low dispersal rates. Documenting the formation of a contemporary clinal hybrid zone may provide important insights into the factors that shaped other well-established hybrid zones.

Characteristics and Fitness Analysis through Interspecific Hybrid Progenies of Transgenic Brassica napus and B. rapa L. ssp

Interspecific hybridization between transgenic crops and their wild relatives is a major concern for transgene dispersal in the environment. Under controlled conditions, artificial hand pollination experiments were performed in order to assess the hybridization potential and the fitness of interspecific hybrids between Brassica rapa and genetically modified (GM) Brassica napus. Initially, six subspecies of B. rapa were hybridized with GM B. napus through hand pollination. In the resulting F₁ hybrids, the combination of B. rapa ssp. narinosa (♀) × GM B. napus (♂) had the highest crossability index (16.9 ± 2.6). However, the F₁ selfing progenies of B. rapa ssp. rapa (♀) × GM B. napus were found to be more effective in producing viable future generations with the highest crossability index (1.6 ± 0.69) compared to other subspecies. Consequently, they were used for the generation of F₂ and F₃ progenies. The 18 different morphological characteristics among the parental cross-combinations and F₁ hybrid progenies were measured and visualized through hierarchical clustering. Different generations were found to be grouped based on their different morphological characteristics. The chromosome numbers among the interspecific hybrids ranged from 2n = 29 to 2n = 40. Furthermore, the SSR markers revealed the presence of genomic portions in the hybrids in comparison with their parental lines. There is a high possibility of transgene flow between GM B. napus and B. rapa. The study concluded that the interspecific hybrids between B. napus and B. rapa can be viable and can actively hybridize up to F₃ generations and more. This suggests that the GM B. napus can disperse the transgene into B. rapa, and that it can pass through for several generations by hand pollination in a greenhouse environment.

Assessing drivers of localized invasive spread to inform large-scale management of a highly damaging insect pest

Studies of biological invasions at the macroscale or across multiple scales can provide important insights for management, particularly when localized information about invasion dynamics or environmental contexts is unavailable. In this study, we performed a macroscale analysis of the roles of invasion drivers on the local scale dynamics of a high-profile pest, Lymantria dispar dispar L., with the purpose of improving the prioritization of vulnerable areas for treatment. Specifically, we assessed the relative effects of various anthropogenic and environmental variables on the establishment rate of 8010 quadrats at a localized scale (5 × 5 km) across the entire L. dispar transition zone (the area encompassing the leading population edge, currently from Minnesota to North Carolina). We calculated the number of years from first detection of L. dispar in a quadrat to the year when probability of establishment of L. dispar was greater than 99% (i.e., waiting time to establishment after first detection). To assess the effects of environmental and anthropogenic variables on each quadrat's waiting time to establishment, we performed linear mixed-effects regression models for the full transition zone and three subregions within the zone. Seasonal temperatures were found to be the primary drivers of local establishment rates. Winter temperatures had the strongest effects, especially in the northern parts of the transition zone. Furthermore, the effects of some factors on waiting times to establishment varied across subregions. Our findings contribute to identifying especially vulnerable areas to further L. dispar spread and informing region-specific criteria by invasion managers for the prioritization of areas for treatment.

Genetic diversity analysis of the invasive gall pest Leptocybe invasa (Hymenoptera: Apodemidae) from China

Leptocybe invasa Fisher et LaSalle is a global invasive pest that seriously damages Eucalyptus plants. Studying the genetic diversity, genetic structure and introgression hybridization of L. invasa in China is of great significance for clarifying the breeding strategy, future invasion and diffusion trends of L. invasa in China and developing scientific prevention and control measures. Genetic diversity and phylogenetic analyses of 320 L. invasa female adults from 14 geographic populations in China were conducted using 10 polymorphic microsatellite loci (SSRs) and mitochondrial DNA cytochrome oxidase I gene sequences (COIs). (1) The Bayesian phylogenetic tree and haplotype network diagram showed that only haplotype Hap3 existed in L. invasa lineage B in China, while haplotypes Hap1 and Hap2 existed in lineage A, among which haplotype Hap2 was found for the first time. The nucleotide and haplotype diversities of lineage A were higher than those of lineage B. (2) The SSR genetic diversity of the Wuzhou Guangxi, Ganzhou Jiangxi and Panzhihua Sichuan populations was higher than that of the other 11 populations, and the SSR genetic diversity of lineage A was higher than that of lineage B. (3) The AMOVA analysis of mitochondrial COI data showed that 75.55% of the variation was among populations, and 99.86% of the variation was between lineages, while the AMOVA analysis of nuclear SSR data showed that 35.26% of the variation was among populations, and 47.04% of the variation was between lineages. There were obvious differences in the sources of variation between the COI and SSR data. (4) The optimal K value of COI and SSR data in structure analysis was 2, and PCoA analysis also divided the dataset into two obvious categories. The UPMGA phylogenetic tree based on SSR data clustered 14 geographic species into two groups. The results of genetic structure analysis supported the existence of two lineages, A and B, in China. (5) Structural analysis showed that there was obvious introgressive hybridization in Wuzhou Guangxi, Ganzhou Jiangxi, Panzhihua Sichuan and other populations. These results suggest that lineage introgressive hybridization has occurred in the L. invasa population in China. The introgressive hybridization degree and genetic diversity of lineage A are obviously higher than those of lineage B. Lineage introgressive hybridization may be the driving force for further L. invasa invasion and diffusion in China in the future.

Introgressive hybridization between two non-native apple snails in China: widespread hybridization and homogenization in egg morphology

BACKGROUND

Apple snails from the genus Pomacea have spread widely in paddy fields and other wetlands of southern China since their introduction in the 1980s. Pomacea spp. are commonly identified using mitochondrial COI sequences. However, sequencing the nuclear elongation factor 1-alpha (EF1α) gene revealed genetic introgression between field populations of P. canaliculata and P. maculata, which produce surviving hybrids in laboratory crossbreeding experiments.

RESULTS

In this study, we sequenced 1054 EF1α clones to design specific primers and established a fast and accurate multiplex polymerase chain reaction (PCR) method for genotyping EF1α. Combined with genotyping P. canaliculata and P. maculata based on mitochondrial COI and nuclear EF1α, we revealed the genetic introgression patterns of 30 apple snail populations in China. Purebred and hybrid individuals of P. canaliculata were widely distributed, while pure maculata-EF1α type was detected only in a few individuals identified as P. canaliculata based on COI sequences. Each egg clutch had one to three genetic patterns, indicating multiple paternity or segregation in the progeny of hybrids. The higher percentages of hybrids in both wild populations and progeny than the homozygotes indicated a potential heterosis in the apple snail populations. Additionally, egg size and clutch size of the apple snails became homogeneous among the non-native populations exhibiting introgression hybridization.

CONCLUSION

Our findings emphasize the value of apple snails as a model to study the mechanisms and impacts of introgressive hybridization on fitness traits. © 2020 Society of Chemical Industry.

Heterospecific mating interactions as an interface between ecology and evolution

Reproductive interference (costly interspecific sexual interactions) is well-understood to promote divergence in mating-relevant traits (i.e. reproductive character displacement: RCD), but it can also reduce population growth, eventually leading to local extinction of one of the species. The ecological and evolutionary processes driven by reproductive interference can interact with each other. These interactions are likely to influence whether the outcome is coexistence or extinction, but remain little studied. In this paper, we first develop an eco-evolutionary perspective on reproductive interference by integrating ecological and evolutionary processes in a common framework. We also present a simple model to demonstrate the eco-evolutionary dynamics of reproductive interference. We then identify a number of factors that are likely to influence the relative likelihoods of extinction or RCD. We discuss particularly relevant factors by classifying them into four categories: the nature of the traits responding to selection, the mechanisms determining the expression of these traits, mechanisms of reproductive interference and the ecological background. We highlight previously underappreciated ways in which these factors may influence the relative likelihoods of RCD and local extinction. By doing so, we also identify questions and future directions that will increase our holistic understanding of the outcomes of reproductive interference.