MAPPING HETEROSIS QTL IN THE PACIFIC OYSTER Crassostrea gigas

¹ Department of Biological Science, University of Southern California, Los Angeles, CA 90089-0371

Progress in mapping QTL for growth heterosis in the Pacific oyster Crassostrea gigas is proceeding on two fronts, the construction of moderately dense linkage maps of AFLP and microsatellite DNA markers and the collection of data on individual growth rates and other phenotypes for F₂ families. Parents and 93 hatchery-reared offspring of one F₂ family (the 45 heaviest, mean live weight at 4 months = 2.9 g, and 48 lightest, mean live weight = 1.1 g, of 291) were typed for 80 AFLP primer combinations (EcoRI & MseI). Of 2356 AFLP markers produced, 1123 are polymorphic, including 335 segregating in a ratio of 1:1 and 594 segregating in a ratio of 3:1; 139 markers show significant (P < 0.01) distortion of Mendelian ratios, while 55 markers show a significant difference between heavy and light groups. Preliminary male and female linkage maps for this family, constructed using JoinMap and data for 186 AFLP and 51 microsatellite markers, reveal 216 markers in 11 linkage groups for the female and 189 markers in 10 linkage groups for the male. Linkage maps for this and two other QTL-mapping families are being integrated with maps obtained previously for three unrelated F₂ families, using 100 microsatellites, with attention paid to chromosomal rearrangements. QTL-mapping families are rearing at low densities in the field; we have weighed 500, tagged individuals from each family monthly since June 2003. The families show very low mortality (0.006 - 0.016) and multi-modal size-frequency distributions suggesting a small number of QTL affecting growth.