Health
Genetics
Genetic differentiation among marine shellfish populations in the natural environment is expected to be determined by the dispersal potential of gametes and/or the larval phase of the organism, which is heavily influenced by hydrodynamic forces that limit the distance and direction of movement1. In natural populations with restricted gene flow, the random effects of genetic drift cause genetic differences at neutral loci to accumulate among them over time.
In the aquaculture environment there is a driving need to produce large numbers of animals in the most economic way. Typically in shellfish aquaculture the broodstock population is collected from the wild, and may be renewed each year. However, in order to maximize production efficiencies, it has been customary to select individuals for particular traits, such as colour morphs or increased growth rates. As such, selective rearing programs often involve broodstock with small effective population sizes, thus creating a genetic bottleneck, followed by the uncontrolled crossing of closely related individuals (e.g. fathers and daughters), which together can lead to inbreeding depression2,3. Inbreeding decreases the genetic variability within a population, and may result in deleterious phenotypic effects, including reduced animal performance or viability. In the hatchery the effective number of spawners will dictate the genetic variation available to be passed to the offspring, which may then be compounded by the use of this new generation to produce the next and so on, dramatically reducing the presence of different genotypes. The loss of genetic diversity may be accompanied by the loss of valuable genetic resources that may confer fitness advantages; for example, disease resistance, heat tolerance or faster growth. In hatchery programs, inbreeding depression can be mitigated by maintaining moderate effective population sizes in each generation4.
In the hatchery environment, a genetic bottleneck occurs when a small subset of individuals from the original population is selected to found the broodstock population. Therefore the cultivated population begins with only a portion of the extant genetic diversity represented in the wild. The genetic differentiation will be dependent upon the selected individuals thereby creating a subdivision of the original population. The most common measure of genetic diversity is heterozygosity, which describes the proportion of individuals that are heterozygous at a genetic location, called a locus or loci (pl). At each locus there are two alleles, which are alternative forms of a gene. Heterozygosity for a locus means there are two different forms (alleles) of the genes present, whereas homozygosity for a locus means that there are two of the same alleles present. Heterozygosity is therefore a measure of genetic diversity, and multi-locus heterozygosity has been found to be positively correlated to size in juvenile bivalves5,6. An allele alone, or in combination with other alleles may carry a valuable trait, such as a colour variant.
Genetic Population Analysis (M. edulis) |
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Background map (courtesy of http://www.annwatley.com) |
In 1992 a population of Mytilus edulis was brought into British Columbia from Ellerslie and St. Peters Bay in Prince Edward Island and the F1 generation were approved for transfer to Genoa Bay. This population is currently being used by Blue Frontier Adventures Inc. to qualitatively breed a blond colour morph mussel over successive generations, and is currently spawning the F13 generation. At Taylor Shellfish Canada M. edulis broodstock from Okeover Inlet are sent to the Hawaiian hatchery for spawning and the seed returned for on-growing, successively breeding from the same broodstock pool. At Island Sea Farms Inc. M. edulis are used to generate hybrids with M. galloprovincialis, and are kept as broodstock for subsequent crosses.
Genetic Analysis Project Outline |
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We aim to compare multiple measures of genetic diversity, including heterozygosity, from Mytilus edulis in three aquaculture populations in British Columbia and one wild population from its native range in Prince Edward Island. We aim to determine whether inbreeding, either through regular hatchery methods or through trait selection, has led to a decrease in the genetic diversity of these populations. We will examine to what extent the genetic diversity may have been altered, and what implications this has for subsequent mussel aquaculture farming in B.C.
Project in collaboration with Dr. Cathryn Abbott of the Department of Fisheries and Oceans Canada
This project is funded by
