Reciprocal Recurrent Selection 



adaptations to local conditions, as well as subtle 

 genetically based morphological and biochemical 

 differences in the different populations (Stauber, 

 1947. 1950; Loosanoff and Nomejko, 1951: Hillman, 

 1964: Numachi, 1962: Loosanoff, 1969). 



Old, well-established subspecies usually have 

 gene combinations so well adapted to the environ- 

 ments they occupy that any new gene combinations 

 created by hybridization will nearly always be less 

 favorable. If, however, hybridization takes place in 

 an unstable environment, some of the vast array of 

 segregates appearing in later generations will very 

 likely be better adapted to the new field environ- 

 ment than any individual of the parental group. 

 They could also be better adapted to the artificial 

 environment of a commercial hatchery or experi- 

 mental shellfish laboratory. 



Inclusion of nonlocal C. virginica in several of the 

 individual mass-spawned groups that made up the 

 foundation stock of the selection experiment already 

 referred to had no measurable adverse effects in 

 respect to percentage fertilization and percentage 

 development of the eggs to the straight-hinge larval 

 stage. Numbers of dead larvae and abnormal larvae 

 at this stage were not increased— just as many larvae 

 reached setting. There was an increase in the inci- 

 dence of polyspermy, but this seemed to have no 

 adverse effects. 



WildC. virginica sampled from approximately 25 

 different sites — from areas farmed commercially and 

 from nonfarmed areas, from Prince Edward Island. 

 Canada, to Greenwich, Conn. — are being test- 

 hybridized. 



To date, individual hybrid crosses along with ap- 

 propriate local and nonlocal control crosses have 

 been made between Long Island Sound oysters and 

 oysters from: 



Prince Edward Island Martha's Vineyard, Mass. 

 Maine Niantic. Conn. 



New Hampshire Greenwich, Conn. 



Some spat have been obtained from all these cross- 

 es. Fj segregates have been obtained from Niantic 

 and Prince Edward Island hybrids. At least these 

 hybrids are fully fertile. 



Fertilization records, larval culture histories, and 

 spat records of all these hybrids and control crosses 

 are being studied. Data accumulated up to the pres- 

 ent time appear to show to following: 



1. Lack of any crossing barriers between Long 

 Island Sound oysters and the populations thus far 

 crossed with the exception of some oysters from 

 Maine which may be developing an incipient cross- 

 ing barrier. 



2. Good performance of all the hybrids as larvae. 



83 



