being selected itself and spawned for the second 

 selected generation of spat. 



Larval data collected in the course of rearing these 

 progeny of the tlrst generation to be selected are 

 showing, as did the larvae from a prior spawning of 

 the same animals which failed to give set, a correla- 

 tion between selection for juvenile oyster size and 

 larval growth rate. This correlation could be a 

 nongenetic one or a significant genetic one. 



First realized heritability estimates are being 

 made on measurements of the spat from the first 

 selected generation of parents at 33 days postsetting. 

 Data are not yet fully analyzed. Nonetheless, pre- 

 liminary calculations indicate the heritability for fast 

 growth to this age to be high, 93%. 



Considering these estimates, and the possibility in 

 the oyster of very high selection differentials, hatch- 

 ery breeders should be able to improve growth rate 

 of their oysters. This could be done by mass selec- 

 tion without recourse, for some cycles of selection, 

 to family performance records or progeny testing 

 which are time consuming and costly. Problems will 

 probably arise from inbreeding. Commercial shell- 

 fish hatcheries seem prone to start selection pro- 

 grams with too few oysters. Because the spawn of a 

 single, excellent cross might fill even a good-sized 

 commercial hatchery, the problem is accentuated. 



Selection progress is being made for resistance to 

 the microsporidian MSX disease of C. virginica in 

 the U.S. mid-Atlantic states by both natural selec- 

 tion on the wild beds and in a small artificially 

 selected experimental stock (discussion at 63rd Joint 

 Annual Convention between Shellfish Institute of 

 North America. Pacific Oyster Growers Associa- 

 tion, and National Shellfisheries Association. June 

 1971. Seattle, Wash.). Overa period of years the C. 

 virginicu oysters of Malpeque Bay, Prince Edward 

 Island, Canada, slowly but certainly made them- 

 selves resistant to the Malpeque disease through the 

 agency of natural selection (Needier and Logic, 

 1947). 



HYBRIDIZATION OF C. VIRGIMCA 



Plants and animals are artificially crossbred or 

 hybridized in order to combine in the offspring some 

 of the desirable characteristics displayed by either 

 set of parents. Another purpose of crossbreeding or 

 hybridizing is to utilize the effects of hybrid vigor. In 

 different species, in addition to the general effects of 

 hybrid vigor, there can be an increase in size some- 

 times accompanied by partial or complete sterility; 



increased reproductive capacity sometimes accom- 

 panied by a reduction in another character: in- 

 creased environmental range, or ability to live in a 

 range in which either parent is unable to live: greater 

 uniformity among individuals. Often hybrid vigor is 

 concentrated in a critical stage of early life. Unfortu- 

 nately, there is no way of predicting for any species 

 just how to obtain hybrid vigor. Inbreeding lines 

 might be test-crossed with one another each genera- 

 tion so that selection can be practiced in terms of the 

 potential of the separate lines for producing 

 heterosis on crossing (see Fig. 8). 



Hybridization for the sake of combining the desir- 

 able genes of two different types sometimes takes 

 the form of upgrading practiced when the overall 

 performance of the import is better than the local, 

 but when the local is superior in some particularly 

 important traits related to local environmental con- 

 ditions. The initial hybridization is followed by 

 backcrosses to the import with either natural or arti- 

 ficial selection. Another way of combining the 

 characteristics of two types is the introgression of 

 special desirable foreign genes into the local stocks. 

 This is accomplished by backcrossing the hybrid to 

 the local type again with either natural or artificial 

 selection. 



Hybridization of C. virginicu might supply a less 

 sensitive, more vigorous larval form better able to 

 hold up to the vicissitudes of the hatchery. Hybrids 

 could furnish a diversification of the U.S. EastCoast 

 oyster crop. It is a fact that the close cultivation of 

 any species facilitates the spread of disease. Diver- 

 sification can break the wildfire spread of a disease 

 by the interdispersal of resistant types. It can assure 

 some marketable crop when the disease toll of the 

 susceptible type is greatest. Were more known about 

 the genetics of different geographic populations of 

 C rirginica, some of the fear might be diminished of 

 experimental transplantation of oyster set to com- 

 mercial beds badly in need of seed from areas where 

 it occurs in great abundance and goes to waste. 



C. rirginicti occurs extensively over an unusually 

 wide range of temperature, from cold to subtropical, 

 along the .Atlantic coast from the Gulf of St. Law- 

 rence to the Gulf of Mexico and farther south to 

 Panama, and around the West Indies. Ihere is evi- 

 dence for a number of physiological spawning races 

 within the species. Accumulated experience of 

 growers of C. virginicu indicates that these oysters 

 transferred from one region to another often fail to 

 thri\e in the new environment. No douht. as in C. 

 gigds (Imai and Sakai. 1961). there are real genetic 



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