Temporally balanced selection during development of larval Pacific oysters (Crassostrea gigas) inherently preserves genetic diversity within offspring

Integration of natural selection across the life cycle stabilizes a marine mussel hybrid zone

Hybridization among related species is now recognized as common but it remains unclear how hybrid zones persist for prolonged periods. Here, we test the hypothesis that selection in different components of the life cycle may stabilize a hybrid zone. A hybrid zone occurs in southwest England between the marine mussels Mytilus edulis and M. galloprovincialis. Previous studies have found strong directional selection against alleles from M. edulis occurs among hybrids in the adult stage. Traditional hybrid zone models argue that alleles that are selected within the hybrid zone are replaced by migration from neighboring parental population into the hybrid zone. In this system, however, migration occurs out of this hybrid zone into neighboring parental populations. This hybrid zone should therefore be unstable and dissipate, yet this zone has persisted for more than 30 years. We tested and rejected the hypothesis that differences in fecundity may select for M. edulis alleles within this hybrid zone and thus counter the selection observed against these alleles among adults. We also tested the hypothesis that selection during the larval stage may counter selection against M. edulis alleles in the adult stage. We found that selection favors M. edulis alleles during the veliger stage of larval development. The direction and strength of selection during the larval stage are sufficient to counter strong selection during the adult portion of the life cycle. This hybrid zone is stabilized by opposing forms of directional selection operating in different portions of the life cycle.

Natural selection and genetic diversity maintenance in a parasitic wasp during continuous biological control application

Aphidius gifuensis is a parasitoid wasp and primary endoparasitoid enemy of the peach potato aphid, Myzus persicae. Artificially reared, captive wasps of this species have been extensively and effectively used to control populations of aphids and limit crop loss. However, the consequences of large-scale releasing of captive A. gifuensis, such as genetic erosion and reduced fitness in wild populations of this species, remains unclear. Here, we sequence the genomes of 542 A. gifuensis individuals collected across China, including 265 wild and 277 human-intervened samples. Population genetic analyses on wild individuals recovered Yunnan populations as the ancestral group with the most complex genetic structure. We also find genetic signature of environmental adaptation during the dispersal of wild populations from Yunnan to other regions. While comparative genomic analyses of captive wasps revealed a decrease in genetic diversity during long-term rearing, population genomic analyses revealed signatures of natural selection by several biotic (host plants) or abiotic (climate) factors, which support maintenance of the gene pool of wild populations in spite of the introduction of captive wasps. Therefore, the impact of large-scale release is reduced. Our study suggests that A. gifuensis is a good system for exploring the genetic and evolutionary effects of mass rearing and release on species commonly used as biocontrol agents.

Signatures of selection in Mulinia lateralis underpinning its rapid adaptation to laboratory conditions

The dwarf surf clam, Mulinia lateralis, is considered as a model species for bivalves because of its rapid growth and short generation time. Recently, successful breeding of this species for multiple generations in our laboratory revealed its acquisition of adaptive advantages during artificial breeding. In this study, 310 individuals from five different generations were genotyped with 22,196 single nucleotide polymorphisms (SNPs) with the aim of uncovering the genetic basis of their adaptation to laboratory conditions. Results revealed that M. lateralis consistently maintained high genetic diversity across generations, characterized by high observed heterozygosity (H o: 0.2733-0.2934) and low levels of inbreeding (F is: -0.0244-0.0261). Population analysis indicated low levels of genetic differentiation among generations of M. lateralis during artificial breeding (F st <0.05). In total, 316 genomic regions exhibited divergent selection, with 168 regions under positive selection. Furthermore, 227 candidate genes were identified in the positive selection regions, which have functions including growth, stress resistance, and reproduction. Notably, certain selection signatures with significantly higher F st value were detected in genes associated with male reproduction, such as GAL3ST1, IFT88, and TSSK2, which were significantly upregulated during artificial breeding. This suggests a potential role of sperm-associated genes in the rapid evolutionary response of M. lateralis to selection in laboratory conditions. Overall, our findings highlight the phenotypic and genetic changes, as well as selection signatures, in M. lateralis during artificial breeding. This contributes to understanding their adaptation to laboratory conditions and underscores the potential for using this species to explore the adaptive evolution of bivalves.

The Biological Hierarchy, Time, and Temporal 'Omics in Evolutionary Biology: A Perspective

Sequencing data-genomics, transcriptomics, epigenomics, proteomics, and metabolomics-have revolutionized biological research, enabling a more detailed study of processes, ranging from subcellular to evolutionary, that drive biological organization. These processes, collectively, are responsible for generating patterns of phenotypic variation and can operate over dramatically different timescales (milliseconds to billions of years). While researchers often study phenotypic variation at specific levels of biological organization to isolate processes operating at that particular scale, the varying types of sequence data, or 'omics, can also provide complementary inferences to link molecular and phenotypic variation to produce an integrated view of evolutionary biology, ranging from molecular pathways to speciation. We briefly describe how 'omics has been used across biological levels and then demonstrate the utility of integrating different types of sequencing data across multiple biological levels within the same study to better understand biological phenomena. However, single-time-point studies cannot evaluate the temporal dynamics of these biological processes. Therefore, we put forward temporal 'omics as a framework that can better enable researchers to study the temporal dynamics of target processes. Temporal 'omics is not infallible, as the temporal sampling regime directly impacts inferential ability. Thus, we also discuss the role the temporal sampling regime plays in deriving inferences about the environmental conditions driving biological processes and provide examples that demonstrate the impact of the sampling regime on biological inference. Finally, we forecast the future of temporal 'omics by highlighting current methodological advancements that will enable temporal 'omics to be extended across species and timescales. We extend this discussion to using temporal multi-omics to integrate across the biological hierarchy to evaluate and link the temporal dynamics of processes that generate phenotypic variation.

Two parallel chromosome‐level reference genomes to support restoration and aquaculture of European flat oyster Ostrea edulis

This volume of Evolutionary Applications sees the publication of two genomes for the European native flat oyster Ostrea edulis, a species of significant evolutionary, ecological and commercial value. Each is a highly contiguous chromosome‐level assembly from individuals of different genetic backgrounds, which have been benchmarked against one another. This situation has resulted from the serendipitous discovery that two independent research groups were both deep into the process of building, annotating and investigating separately produced assemblies. Due to constraints with funder requirements and the need to recognize early career researchers for their work, alongside the technical challenge of integrating assemblies from two very different genomes, there was limited capacity to merge the sequences into one publication at the stage of discovery. This issue is likely to become very common over the next few years until the technologies for working with multiple genomes at once, for example, graph genomes, become commonplace in nonmodel species. Consequently, both of our teams have decided to collaborate rather than compete, recognizing the benefit to copublishing two separate genome resources for the research community, each with distinct scientific investigations, and working collaboratively to benchmark the assemblies.

Temporally balanced selection during development of larval Pacific oysters (Crassostrea gigas) inherently preserves genetic diversity within offspring

Balancing selection is one of the mechanisms which has been proposed to explain the maintenance of genetic diversity in species across generations. For species with large populations and complex life histories, however, heterogeneous selection pressures may create a scenario in which the net effects of selection are balanced across developmental stages. With replicated cultures and a pooled sequencing approach, we show that genotype-dependent mortality in larvae of the Pacific oyster (Crassostrea gigas) is largely temporally dynamic and inconsistently in favour of a single genotype or allelic variant at each locus. Overall, the patterns of genetic change we observe to be taking place are more complex than what would be expected under classical examples of additive or dominant genetic interactions. They are also not easily explained by our current understanding of the effects of genetic load. Collectively, temporally heterogeneous selection pressures across different larval developmental stages may act to maintain genetic diversity, while also inherently sheltering genetic load within oyster populations.

Larval development in the Pacific oyster and the impacts of ocean acidification: Differential genetic effects in wild and domesticated stocks

The adaptive capacity of marine calcifiers to ocean acidification (OA) is a topic of great interest to evolutionary biologists and ecologists. Previous studies have provided evidence to suggest that larval resilience to high pCO2 seawater for these species is a trait with a genetic basis and variability in natural populations. To date, however, it remains unclear how the selective effects of OA occur within the context of complex genetic interactions underpinning larval development in many of the most vulnerable taxa. Here we evaluated phenotypic and genetic changes during larval development of Pacific oysters (Crassostrea gigas) reared in ambient (~400 µatm) and high (~1600 µatm) pCO2 conditions, both in domesticated and naturalized "wild" oysters from the Pacific Northwest, USA. Using pooled DNA samples, we determined changes in allele frequencies across larval development, from early "D-stage" larvae to metamorphosed juveniles (spat), in both groups and environments. Domesticated larvae had ~26% fewer loci with changing allele frequencies across developmental stages and <50% as many loci affected by acidified culture conditions, compared to larvae from wild broodstock. Functional enrichment analyses of genetic markers with significant changes in allele frequency revealed that the structure and function of cellular membranes were disproportionately affected by high pCO2 conditions in both groups. These results indicate the potential for a rapid adaptive response of oyster populations to OA conditions; however, underlying genetic changes associated with larval development differ between these wild and domesticated oyster stocks and influence their adaptive responses to OA conditions.