Interestingly, no correlated response was observed between growth and survival in the spat.
Triploid S animals showed higher mortality (58 percent) than triploid R progenies (36 percent), while the control was 50 percent. In 2003, triploid progenies were produced by crossing unselected tetraploid males with G2 R or S diploid females. The unselected control was intermediate at 48 percent. In Rivière d’Auray, summer mortality of G3 S oysters was 73 percent, while it was only 27 percent for R progenies. Similar results were found in 2003, in a third generation (G3) that replicated the outbred G2s without further selection. 2: Variation of mean mortality in the outbred G2 families (divergent selection) in Brittany (P < 0.001). The unselected control was intermediate at 24 percent. 2), summer mortality of outbred G2 S oysters was 43 percent, while it was only 7 percent for R progenies. “High-selected” families, named “R” for “resistant,” showed much higher survival than “low-selected” ones called “S” for “sensitive.” For example, in Rivière d’Auray (Fig. Results confirmed the high heritability of survival of spat. 1: Variation of mean mortality among G1 half-sib families (HSF), and identification of selected “high” and “low” families for breeding of the second generation (P < 0.001). Their selection was strictly at the family level. Both inbred and outbred family crosses were performed using G1 oysters that were protected from field mortality pressures. To confirm this result and assess whether selection could be efficient, a second generation (G2) was produced in 2002 by breeding the best and worst G1 half-sib families (Fig. The heritability estimate for survival during the first summer was remarkably high. A variance component analysis revealed that among the factors studied, 45 percent of the observed variance was due to variation among families. The families were monitored at three sites – Normandy at Baie des Veys, Brittany at Rivière d’Auray, and the Atlantic Coast in the Marennes-Oléron Bay at Ronce – previously known to show different survival rates for juvenile oysters. Due to the resulting variable larval survival and settlement success, heritability estimation of the spat survival in the field was determined using 17 half-sib families, which corresponds to 43 full-sib families. For each set, six males were crossed with four females each following a nested, half-sib design. In 2001, 72 biparental families (G1) were produced in three sets of 24. Within this program, a multisite field experiment was designed to assess the usefulness of selecting for improved survival of oyster spat during its first summer period. Morest, a national multidisciplinary program coordinated by Ifremer, was initiated in France to understand the causes of the summer mortalities in Pacific oysters and propose solutions. A combination of environmental and internal parameters provide a more likely explanation.
However, in most cases, the mortality cannot be explained by a single factor. Factors such as temperature, pathogens and physiological imbalances have been suggested to explain the summer mortality phenomena. gigas has been reported for many years and is a major production problem. In France and other countries, summer mortality in C. gigas juveniles, family-based selective breeding has been initiated to improve growth and yield. Recently in the United States, Australia, and New Zealand, where hatcheries are a major source of C. However, the limited extent of hatchery propagation and various technical difficulties and biological characteristics of the species have hampered the development of selective breeding programs. Consequently, significant gains could be obtained by selective breeding of diploid oysters. Several quantitative genetics studies have suggested that significant heritable variation exists for traits of aquaculture interest in oysters, such as disease resistance and growth. Triploid oysters have become increasingly important, as they yield better production performance than their natural diploid counterparts, mainly as a consequence of their high level of infertility.
To date, the most significant genetic advance for the production of this species resulted from the development of polyploids. Temperature, pathogens and physiological imbalances linked to summer mortalities Field tests of the Morest Pacific oyster families took place in the Marennes-Oléron Bay and two other sites in France.ĭespite its growing economic importance, aquaculture production of Pacific oysters ( Crassostrea gigas) has not benefited much from genetic improvement.
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