近交及其对水产养殖的影响

因大多数野生物种的捕捞已接近甚至超过了可开发的限度，因此必须大量增加水产品的养殖以满足人们对水产品的需要。目前，许多水产养殖种类的苗种来源于野生亲体或野生种苗，但是当野生种群被过度捕捞或数量大量减少时，越来越多的苗种将来源于人工蓄养的亲体群体。

Inbreeding and its impact on aquaculture

Inbreeding,which refers to situations where relatives are mating,is unavoidable in aquaculture populations because of their restricted population size.The rate of inbreeding in a population is roughly equal to one over twice the 'effective population size',1/(2N_e),but a more accurate 'inbreeding coefficient'(F) is usually used to quantify inbreeding in populations undergoing artificial and/or natural selection.Inbreeding usually leads to reductions of performance in aquaculture species.Such 'inbreeding depression' is mainly the result of recessive deleterious genes being unmasked and expressed in homozygous individuals.All aquaculture populations will accumulate recessive deleterious genes because of mutations in the DNA.It is assumed that most aquaculture species have a 'genetic load' of about 100 recessive deleterious genes,which would be more than enough to cause the extinction of that species if these genes were made homozygous through mating between relatives.A literature review confirmed that traits related to the overall fitness of aquaculture species are strongly affected by inbreeding(ranging between 3%-50% inbreeding depression per 10% increase of the inbreeding coefficient,F).Fast inbreeding through full-sib mating has resulted,on average,in three times higher inbreeding depression in growth compared to slow inbreeding in large-scale selective breeding programs.Studies with other species suggest that slow inbreeding usually lead to more effective purging of recessive deleterious genes than rapid full-sib inbreeding since fewer genes are fixed because of random drift.Inbreeding will also reduce the additive genetic variation within hatchery populations,but increases the additive genetic variation between isolated hatchery populations.Inbred populations usually show different responses in different environments due to the reduced additive genetic variance and their loss of ability to adapt to environmental changes.Therefore,efforts should be made to restrict inbreeding and its negative consequences on the production efficiency of aquaculture species farmed in China.It is recommended that hatcheries,which are maintaining their own broodstock,should restrict inbreeding by securing an effective population size(N_e) of minimum 50 individuals.Crossbreeding schemes,which are masking the negative consequences of inbreeding in different hatchery populations,could be used to secure a long-term conservation of aquaculture species.However, only selective breeding has the potential to genetically improve aquaculture species compared to the best performing wild populations.No matter which selection strategy is used,i.e.mass-selection,family selection,within-family selection,combined family and within-family selection or marker-assisted selection(MAS),accumulation of inbreeding should be restricted to about 1% per generation of selection.Such low inbreeding should be secured by producing full-sib families in separate rearing units,from which samples of individuals are tested and used as potential breeding candidates.