Melanie L. Marine

Genetic adaptation to captivity can occur in a single generation

Captive breeding programs are widely used for the conservation and restoration of threatened and endangered species. Nevertheless, captive-born individuals frequently have reduced fitness when reintroduced into the wild. The mechanism for these fitness declines has remained elusive, but hypotheses include environmental effects of captive rearing, inbreeding among close relatives, relaxed natural selection, and unintentional domestication selection (adaptation to captivity). We used a multigenerational pedigree analysis to demonstrate that domestication selection can explain the precipitous decline in fitness observed in hatchery steelhead released into the Hood River in Oregon. After returning from the ocean, wild-born and first-generation hatchery fish were used as broodstock in the hatchery, and their offspring were released into the wild as smolts. First-generation hatchery fish had nearly double the lifetime reproductive success (measured as the number of returning adult offspring) when spawned in captivity compared with wild fish spawned under identical conditions, which is a clear demonstration of adaptation to captivity. We also documented a tradeoff among the wild-born broodstock: Those with the greatest fitness in a captive environment produced offspring that performed the worst in the wild. Specifically, captive-born individuals with five (the median) or more returning siblings (i.e., offspring of successful broodstock) averaged 0.62 returning offspring in the wild, whereas captive-born individuals with less than five siblings averaged 2.05 returning offspring in the wild. These results demonstrate that a single generation in captivity can result in a substantial response to selection on traits that are beneficial in captivity but severely maladaptive in the wild.

Who are the missing parents? Grandparentage analysis identifies multiple sources of gene flow into a wild population

In order to increase the size of declining salmonid populations, supplementation programmes intentionally release fish raised in hatcheries into the wild. Because hatchery‐born fish often have lower fitness than wild‐born fish, estimating rates of gene flow from hatcheries into wild populations is essential for predicting the fitness cost to wild populations. Steelhead trout (Oncorhynchus mykiss) have both freshwater resident and anadromous (ocean‐going) life history forms, known as rainbow trout and steelhead, respectively. Juvenile hatchery steelhead that ‘residualize’ (become residents rather than go to sea as intended) provide a previously unmeasured route for gene flow from hatchery into wild populations. We apply a combination of parentage and grandparentage methods to a three‐generation pedigree of steelhead from the Hood River, Oregon, to identify the missing parents of anadromous fish. For fish with only one anadromous parent, 83% were identified as having a resident father while 17% were identified as having a resident mother. Additionally, we documented that resident hatchery males produced more offspring with wild anadromous females than with hatchery anadromous females. One explanation is the high fitness cost associated with matings between two hatchery fish. After accounting for all of the possible matings involving steelhead, we find that only 1% of steelhead genes come from residualized hatchery fish, while 20% of steelhead genes come from wild residents. A further 23% of anadromous steelhead genes come from matings between two resident parents. If these matings mirror the proportion of matings between residualized hatchery fish and anadromous partners, then closer to 40% of all steelhead genes come from wild trout each generation. These results suggest that wild resident fish contribute substantially to endangered steelhead ‘populations’ and highlight the need for conservation and management efforts to fully account for interconnected Oncorhynchus mykiss life histories.

Effective size of a wild salmonid population is greatly reduced by hatchery supplementation

Many declining and commercially important populations are supplemented with captive-born individuals that are intentionally released into the wild. These supplementation programs often create large numbers of offspring from relatively few breeding adults, which can have substantial population-level effects. We examined the genetic effects of supplementation on a wild population of steelhead (Oncorhynchus mykiss) from the Hood River, Oregon, by matching 12 run-years of hatchery steelhead back to their broodstock parents. We show that the effective number of breeders producing the hatchery fish (broodstock parents; Nb) was quite small (harmonic mean Nb=25 fish per brood-year vs 373 for wild fish), and was exacerbated by a high variance in broodstock reproductive success among individuals within years. The low Nb caused hatchery fish to have decreased allelic richness, increased average relatedness, more loci in linkage disequilibrium and substantial levels of genetic drift in comparison with their wild-born counterparts. We also documented a substantial Ryman–Laikre effect whereby the additional hatchery fish doubled the total number of adult fish on the spawning grounds each year, but cut the effective population size of the total population (wild and hatchery fish combined) by nearly two-thirds. We further demonstrate that the Ryman–Laikre effect is most severe in this population when (1) >10% of fish allowed onto spawning grounds are from hatcheries and (2) the hatchery fish have high reproductive success in the wild. These results emphasize the trade-offs that arise when supplementation programs attempt to balance disparate goals (increasing production while maintaining genetic diversity and fitness).

A single generation of domestication heritably alters the expression of hundreds of genes

The genetic underpinnings associated with the earliest stages of plant and animal domestication have remained elusive. Because a genome-wide response to selection can take many generations, the earliest detectable changes associated with domestication may first manifest as heritable changes to global patterns of gene expression. Here, to test this hypothesis, we measured differential gene expression in the offspring of wild and first-generation hatchery steelhead trout (Oncorhynchus mykiss) reared in a common environment. Remarkably, we find that there were 723 genes differentially expressed between the two groups of offspring. Reciprocal crosses reveal that the differentially expressed genes could not be explained by maternal effects or by chance differences in the background levels of gene expression among unrelated families. Gene-enrichment analyses reveal that adaptation to the novel hatchery environment involved responses in wound healing, immunity and metabolism. These findings suggest that the earliest stages of domestication may involve adaptation to highly crowded conditions.

Hyperdiverse Gene Cluster in Snail Host Conveys Resistance to Human Schistosome Parasites

Schistosomiasis, a neglected global pandemic, may be curtailed by blocking transmission of the parasite via its intermediate hosts, aquatic snails. Elucidating the genetic basis of snail-schistosome interaction is a key to this strategy. Here we map a natural parasite-resistance polymorphism from a Caribbean population of the snail Biomphalaria glabrata. In independent experimental evolution lines, RAD genotyping shows that the same genomic region responds to selection for resistance to the parasite Schistosoma mansoni. A dominant allele in this region conveys an 8-fold decrease in the odds of infection. Fine-mapping and RNA-Seq characterization reveal a <1Mb region, the Guadeloupe Resistance Complex (GRC), with 15 coding genes. Seven genes are single-pass transmembrane proteins with putative immunological roles, most of which show strikingly high nonsynonymous divergence (5-10%) among alleles. High linkage disequilibrium among three intermediate-frequency (>25%) haplotypes across the GRC, a significantly non-neutral pattern, suggests that balancing selection maintains diversity at the GRC. Thus, the GRC resembles immune gene complexes seen in other taxa and is likely involved in parasite recognition. The GRC is a potential target for controlling transmission of schistosomiasis, including via genetic manipulation of snails.

Genome-Wide Scan and Test of Candidate Genes in the Snail Biomphalaria glabrata Reveal New Locus Influencing Resistance to Schistosoma mansoni

Background New strategies to combat the global scourge of schistosomiasis may be revealed by increased understanding of the mechanisms by which the obligate snail host can resist the schistosome parasite. However, few molecular markers linked to resistance have been identified and characterized in snails. Methodology/Principal Findings Here we test six independent genetic loci for their influence on resistance to Schistosoma mansoni strain PR1 in the 13-16-R1 strain of the snail Biomphalaria glabrata. We first identify a genomic region, RADres, showing the highest differentiation between susceptible and resistant inbred lines among 1611 informative restriction-site associated DNA (RAD) markers, and show that it significantly influences resistance in an independent set of 439 outbred snails. The additive effect of each RADres resistance allele is 2-fold, similar to that of the previously identified resistance gene sod1. The data fit a model in which both loci contribute independently and additively to resistance, such that the odds of infection in homozygotes for the resistance alleles at both loci (13% infected) is 16-fold lower than the odds of infection in snails without any resistance alleles (70% infected). Genome-wide linkage disequilibrium is high, with both sod1 and RADres residing on haplotype blocks >2Mb, and with other markers in each block also showing significant effects on resistance; thus the causal genes within these blocks remain to be demonstrated. Other candidate loci had no effect on resistance, including the Guadeloupe Resistance Complex and three genes (aif, infPhox, and prx1) with immunological roles and expression patterns tied to resistance, which must therefore be trans-regulated. Conclusions/Significance The loci RADres and sod1 both have strong effects on resistance to S. mansoni. Future approaches to control schistosomiasis may benefit from further efforts to characterize and harness this natural genetic variation.

Sequencing and characterization of the anadromous steelhead (Oncorhynchus mykiss) transcriptome.

Identifying the traits that differ between hatchery and wild fish may allow for pragmatic changes to hatchery practice. To meet those ends, we sequenced, assembled, and characterized the anadromous steelhead (Oncorhynchus mykiss) transcriptome. Using the Illumina sequencing platform, we sequenced nearly 41million 76-mer reads representing 3.1 Gbp of steelhead transcriptome. Upon final assembly, this sequence data yielded 86,402 transcript scaffolds, of which, 66,530 (77%) displayed homology to proteins of the non-redundant NCBI database. Gene descriptions and gene ontology terms were used to annotate the transcriptome resulting in 4030 unique gene ontology (GO) annotations attributed to the assembled sequences. We also conducted a comparative analysis that identified homologous genes within four other fish species including zebrafish (Danio rerio), stickleback (Gasterosteus aculeatus), and two pufferfish species (Tetraodon nigroviridis and Takifugu rubripes). Comparing our steelhead reference assembly directly to the transcriptome for rainbow trout (the fresh water life-history variant of the same species) revealed that while the steelhead and rainbow trout transcriptomes are complementary, the steelhead data will be useful for investigating questions related to anadromous (ocean-going) fishes. These sequence data and web tools provide a useful set of resources for salmonid researchers and the broader genomics community (available at http://salmon.cgrb.oregonstate.edu).

How Much Does Inbreeding Contribute to the Reduced Fitness of Hatchery-Born Steelhead (Oncorhynchus mykiss) in the Wild?

Many declining populations are supplemented with captive-born individuals that are released directly into the wild. Because captive-born individuals can have lower fitness in the wild than their wild-born counterparts, a comprehensive understanding of the mechanisms responsible for the reduced fitness of these individuals is required for appropriate conservation and management decisions. Inbreeding among captive-born individuals is one plausible mechanism because captive breeding programs frequently use small numbers of breeders to create large numbers of siblings that are subsequently released together into the wild. We tested this hypothesis in a supplementation program for steelhead (Oncorhynchus mykiss) from the Hood River, Oregon, for which first-generation hatchery fish were demonstrated to have lower fitness in the wild than their wild-born counterparts. To determine the contribution of inbreeding to this fitness decline, we first assigned 11 run-years of hatchery steelhead (3005 fish) back to their broodstock parents (462 fish) using 8 polymorphic microsatellite loci. By combining pedigree analyses with species-specific estimates of genetic load, we found that inbreeding could at most account for a 1-4% reduction in the fitness of hatchery fish relative to wild fish. Thus, inbreeding alone cannot adequately explain the 15% average fitness decline observed in first-generation hatchery fish from this population.

Spawn date explains variation in growth rate among families of hatchery reared Hood River steelhead (Oncorhynchus mykiss)

Body size at release for hatchery-reared steelhead is positively correlated with probability of survival to return as an adult. Although the relationship between body size and survival is well documented, little is known about what factors influence growth in the hatchery at the family level. We test if parent length, parent type (hatchery or natural), parent run date, or date of spawning correlate with among-family variation in offspring growth in the hatchery. Using Hood River winter steelhead (Oncorhynchus mykiss), two experiments were performed over two brood years. No effect of father length, parental type, or parental run date was found on offspring length in either year. Mother length was positively correlated with offspring length during both brood years and egg size data suggest this result occurs because longer females have larger eggs (i.e., it is a maternal effect). Although run date of parents was not correlated with offspring size, the date of spawning was negatively correlated with offspring size in a year with a protracted spawning season. Families spawned later in the season were smaller than those spawned earlier even though all fish began feeding on the same date. The spawn date effect lasted surprisingly long, being correlated with offspring size one year after spawning. A possible explanation for the spawn date effect is that eggs from early-spawned families were reared in chilled water to slow development so all families would begin feeding on the same date (a common hatchery practice). Colder water could have reduced metabolic costs and provided a surplus of energy to be used for somatic growth. This study highlights a little-studied environmental effect that could have large effects on long-term growth rates for hatchery-reared O. mykiss.

Reply to Matos: Maternal effects cannot explain documented patterns of reproductive success

Matos (1) suggests that maternal effects could also explain the results we presented in a recent paper (2). However, from the outset of our study, we were aware that environmental maternal effects could be a competing explanation for the reduced fitness of hatchery-origin fish. In figure 4 of ref. 2, for example, we presented the results for male and female broodstock separately. The broodstock were either wild-born or hatchery-born individuals (0 and 1 generations in captivity, respectively). As described in the paper, each hatchery-born broodstock fish was mated with only a wild-born partner. Thus, the male hatchery broodstock were mated with wild female broodstock. As such, there is no way that the results presented for males could be attributable to an environmental maternal effect.