ANALYSIS OF SHELL COLOR DETERMINISM IN TH E MANILA CLAM RUDITAPES-PHILIPPINARUM

The Manila clam presents a large polymorphism for shell colours and pa tterns. No experiment has been conducted to study the determinism of s uch 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 co louring. These lines could then be used as controls for laboratory exp eriments. The colouring is defined by three characters: asymmetry, orn amentation and hue. Clams with asymmetrical colouring are not fully pi gmented, except for a posterior band on the left valve. Clams with sym metrical colouring are fully pigmented on the two valves. Four cross s eries were conducted between the different characters: Asymmetry versu s Symmetry, and the Wave [V] and Zebra [Z] ornamentations. These resul ts suggest that colouration is controlled by at least two genes. Asymm etry is controlled by one gene with two alleles; asymmetry (A) is domi nant over symmetry (S). Asymmetrical clams are either homozygous or he terozygous and symmetrical ones are therefore always homozygous. Some asymmetrical homozygous clams were identified. A variegated symmetrica l 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. Neve rtheless, other ornamentations (I phenotype for instance) also exist a nd the gene is probably multiallelic. Determinism of the hue is not de fined. Results also show that colouring and sex are not linked. In nea rly all the experiments, the observed proportions for the asymmetry ar e in departure from what was expected under Mendel's hypothesis. This difference is due to a chronic shortage in symmetrical homozygote clam s which is still in unexplained.