Comparisons between the genomes of salmon species reveal that they underwent extensive chromosomal rearrangements following whole genome duplication that occurred in their lineage 58−63 million years ago. Extant salmonids are diploid, but occasional pairing between homeologous chromosomes exists in males. The consequences of re-diploidization can be characterized by mapping the position of duplicated loci in such species. Linkage maps are also a valuable tool for genome-wide applications such as genome-wide association studies, quantitative trait loci mapping or genome scans. Here, we investigated chromosomal evolution in Chinook salmon (Oncorhynchus tshawytscha) after genome duplication by mapping 7146 restriction-site associated DNA loci in gynogenetic haploid, gynogenetic diploid, and diploid crosses. In the process, we developed a reference database of restriction-site associated DNA loci for Chinook salmon comprising 48528 non-duplicated loci and 6409 known duplicated loci, which will facilitate locus identification and data sharing. We created a very dense linkage map anchored to all 34 chromosomes for the species, and all arms were identified through centromere mapping. The map positions of 799 duplicated loci revealed that homeologous pairs have diverged at different rates following whole genome duplication, and that degree of differentiation along arms was variable. Many of the homeologous pairs with high numbers of duplicated markers appear conserved with other salmon species, suggesting that retention of conserved homeologous pairing in some arms preceded species divergence. As chromosome arms are highly conserved across species, the major resources developed for Chinook salmon in this study are also relevant for other related species.

Large genomic studies are becoming increasingly common with advances in sequencing technology, and our ability to understand how genomic variation influences phenotypic variation between individuals has never been greater. The exploration of such relationships first requires the identification of associations between molecular markers and phenotypes. Here, we explore the use of Random Forest (RF), a powerful machine-learning algorithm, in genomic studies to discern loci underlying both discrete and quantitative traits, particularly when studying wild or nonmodel organisms. RF is becoming increasingly used in ecological and population genetics because, unlike traditional methods, it can efficiently analyse thousands of loci simultaneously and account for nonadditive interactions. However, understanding both the power and limitations of Random Forest is important for its proper implementation and the interpretation of results. We therefore provide a practical introduction to the algorithm and its use for identifying associations between molecular markers and phenotypes, discussing such topics as data limitations, algorithm initiation and optimization, as well as interpretation. We also provide short R tutorials as examples, with the aim of providing a guide to the implementation of the algorithm. Topics discussed here are intended to serve as an entry point for molecular ecologists interested in employing Random Forest to identify trait associations in genomic data sets.

Captive breeding has the potential to rebuild depressed populations. However, associated genetic changes may decrease restoration success and negatively affect the adaptive potential of the entire population. Thus, approaches that minimize genetic risks should be tested in a comparative framework over multiple generations. Genetic diversity in two captive-reared lines of a species of conservation interest, Chinook salmon (Oncorhynchus tshawytscha), was surveyed across three generations using genome-wide approaches. Genetic divergence from the source population was minimal in an integrated line, which implemented managed gene flow by using only naturally-born adults as captive broodstock, but significant in a segregated line, which bred only captive-origin individuals. Estimates of effective number of breeders revealed that the rapid divergence observed in the latter was largely attributable to genetic drift. Three independent tests for signatures of adaptive divergence also identified temporal change within the segregated line, possibly indicating domestication selection. The results empirically demonstrate that using managed gene flow for propagating a captive-reared population reduces genetic divergence over the short term compared to one that relies solely on captive-origin parents. These findings complement existing studies of captive breeding, which typically focus on a single management strategy and examine the fitness of one or two generations.

A novel application of genomewide association analyses is to use trait‐associated loci to monitor the effects of conservation strategies on potentially adaptive genetic variation. Comparisons of fitness between captive‐ and wild‐origin individuals, for example, do not reveal how captive rearing affects genetic variation underlying fitness traits or which traits are most susceptible to domestication selection. Here, we used data collected across four generations to identify loci associated with six traits in adult Chinook salmon (Oncorhynchus tshawytscha) and then determined how two alternative management approaches for captive rearing affected variation at these loci. Loci associated with date of return to freshwater spawning grounds (return timing), length and weight at return, age at maturity, spawn timing, and daily growth coefficient were identified using 9108 restriction site‐associated markers and random forest, an approach suitable for polygenic traits. Mapping of trait‐associated loci, gene annotations, and integration of results across multiple studies revealed candidate regions involved in several fitness‐related traits. Genotypes at trait‐associated loci were then compared between two hatchery populations that were derived from the same source but are now managed as separate lines, one integrated with and one segregated from the wild population. While no broad‐scale change was detected across four generations, there were numerous regions where trait‐associated loci overlapped with signatures of adaptive divergence previously identified in the two lines. Many regions, primarily with loci linked to return and spawn timing, were either unique to or more divergent in the segregated line, suggesting that these traits may be responding to domestication selection. This study is one of the first to utilize genomic approaches to demonstrate the effectiveness of a conservation strategy, managed gene flow, on trait‐associated—and potentially adaptive—loci. The results will promote the development of trait‐specific tools to better monitor genetic change in captive and wild populations.

Understanding the genetic basis of repeated evolution of the same phenotype across taxa is a fundamental aim in evolutionary biology and has applications in conservation and management. However, the extent to which interspecific life‐history trait polymorphisms share evolutionary pathways remains underexplored. Here, we address this gap by studying the genetic basis of a key life‐history trait, age at maturity, in four species of Pacific salmonids (genus Oncorhynchus) that exhibit intra‐ and interspecific variation in this trait—Chinook Salmon, Coho Salmon, Sockeye Salmon, and Steelhead Trout. We tested for associations in all four species between age at maturity and two genome regions, six6 and vgll3, that are strongly associated with the same trait in Atlantic Salmon (Salmo salar). We also conducted a genome‐wide association analysis in Steelhead to assess whether additional regions were associated with this trait. We found the genetic basis of age at maturity to be heterogeneous across salmonid species. Significant associations between six6 and age at maturity were observed in two of the four species, Sockeye and Steelhead, with the association in Steelhead being particularly strong in both sexes (p = 4.46 × 10−9 after adjusting for genomic inflation). However, no significant associations were detected between age at maturity and the vgll3 genome region in any of the species, despite its strong association with the same trait in Atlantic Salmon. We discuss possible explanations for the heterogeneous nature of the genetic architecture of this key life‐history trait, as well as the implications of our findings for conservation and management.