Biological Performances And Genetic Diversity Of Inbred Families In The Pacific Oyster (Crassostrea Gigas)

Inbreeding is the production of offspring from the mating or breeding or individuals or organisms that are closely related genetically. Inbreeding can result in inbreeding depression by increasing the frequency of recessive harmful gene. At the same time, inbreeding can be seen as a useful tool to produce pure lines and broadly used in breeding program. The enormous fecundity and high level of genetic diversity and various mating systems of bivalves make them favorable species for the study of inbreeding. The Pacific oyster (Crassostrea gigas) is the most cultivated shellfish worldwide, and a traditional cultured bivalve species of great economic importance in China. In recent years, some amount of inbreeding appears in cultured stocks. The oyster industry in China faces lots of questions and hot problems, such as the high mortality rate and decrease of yield and growth, therefore, the objective of this dissertation was to study the inbreeding effects on Pacific oyster, with the goal of laying a foundation to oyster selective breeding in inbred family.The effects of inbreeding on phenotypic traits and genetic diversity of Pacific oyster was investigated using inbreeding families which was established by first generation of full-sib mating (F1, F=0.250) and second successive generation of full-sib mating (F2, F=0.375). Control group (F=0) was produced by random mating of wild oysters. The main results of these studies are as follows:1. Biological performances of inbred families in Pacific oysterThe fertilized rate, hatching rate, survival rate, growth were compared between different experimental groups. The results indicated that all the three experimental groups had high fertilized rates, and hatching rates were all above 90% except group F2. The growth of F1 and F2 showed significant inbreeding depression after 12 days of breeding at larval stage. An increasing inbreeding depression of shell height was observed with the increase of inbreeding coefficient in group F1 and F2 at larval stage. The inbreeding depression of larval survival was obvious and increasing with age. A significant inbreeding depression in average shell height of spat was observed all the time in families with F=0.250 and F=0375, and the depression become serious with the increase of inbreeding coefficient. However, no significant difference of average shell length of spat was found between control group and the other two inbreeding groups until 300 days later. Inbreeding depression of survival existed all the time in inbreeding families with F=0.250 and F=0.375 at juvenile stage, and both of them increased with age. This study demonstrates that inbreeding has negative effects on economic traits. This negative influence even still exist in the family with second successive generation of full-sib mating, This results verify the theory that a high genetic load existed in the Pacific oyster, suggesting that purging the genetic load of Pacific oyster need a long-term inbreeding program especially when the load is due to detrimentals of relatively small effect.2. Genetic diversity of inbred families in Pacific oysterAmplified fragment length polymorphism (AFLP) technique was used to detect the genetic variation among a total of 90 individuals representing group F1, groupF2 and control group. Nine pairs of selective EcoRl and Msel primers produced a total of 215,204 and 200 bands in control group, group F1 and group F2, respectively, and 67, 58 and 55 bands were polymorphic markers. The percentage of polymorphic loci was 31.16%,28.43% and 27.5% in control group, group F1 and group F2 respectively. On the average, each primer set had detected 7.4,6.4 and 6.1 polymorphic markers in control group, group F1 and group F2 respectively. Nei’s gene diversity and Shannon information indexes were decreased with increasing inbreeding coefficient. The results indicated that the Pacific oyster had a high level of the genetic diversity. With increasing inbred generations, the inbred family of the Pacific oyster showed a reduction in genetic diversity.