ANALYSIS OF SHELL COLOR DETERMINISM IN THE MANILA CLAM RUDITAPES-PHILIPPINARUM

The Manila clam presents a large polymorphism for shell colours and patterns. No experiment has been conducted to study the determinism of such a polymorphism which has been well known for a long time. The aim of this study was firstly to determine the genetic variation for shell colour and secondly to produce some homozygous lines for the shell colouring. These lines could then be used as controls for laboratory experiments. The colouring is defined by three characters: asymmetry, ornamentation and hue. Clams with asymmetrical colouring are not fully pigmented, except for a posterior band on the left valve. Clams with symmetrical colouring are fully pigmented on the two valves. Four cross series were conducted between the different characters: Asymmetry versus Symmetry, and the Wave [V] and Zebra [Z] ornamentations. These results suggest that colouration is controlled by at least two genes. Asymmetry is controlled by one gene with two alleles; asymmetry (A) is dominant over symmetry (S). Asymmetrical clams are either homozygous or heterozygous and symmetrical ones are therefore always homozygous. Some asymmetrical homozygous clams were identified. A variegated symmetrical phenotype named Ri appeared in the offspring of the V X V and Z X Z crosses. These results suggest that V and Z are dominant over Ri. Nevertheless, other ornamentations (I phenotype for instance) also exist and the gene is probably multiallelic. Determinism of the hue is not defined. Results also show that colouring and sex are not linked. In nearly all the experiments, the observed proportions for the asymmetry are in departure from what was expected under Mendel's hypothesis. This difference is due to a chronic shortage in symmetrical homozygote clams which is still in unexplained.