Table 1.— Fertilization failures, cleavage failures, and parthenogenesis in 



full-sib crosses of Crassostrea virginicci and in contemporary control 



outcrosses and between-line crosses. 



work of Menzel {1968a) we know that such crossing 

 barriers do exist between oyster species. 



Some of the difficulties of inbreeding C. vir- 

 ginicci in the face of gamete cross incompatibility 

 might somehow be compensated for by making prac- 

 tical use of the parthenogenesis induced in the incom- 

 patible matings. A variety of physical and chemical 

 agents and other means can be used to overcome 

 these crossing barriers, as done in other organisms. 



As for the less intensive, lower level of inbreeding 

 which will accompany mass selective breeding, in- 

 compatibility will tend to keep the level of inbreeding 

 lower than would otherwise occur. This will some- 

 times work in the breeder's favor, other times 

 against him. When too much inbreeding is practiced 

 too fast, fertilization failures should occur. 



In a third group of sib-inbred and between-line 

 crosses of C. virginica. food and water levels were 

 adjusted every other day to number of surviving 

 larvae (Longwell and Stiles. 1973). For the period 

 12 to 17 days, survival of the outbreds was 6 times 

 greater than for the inbreds. In a highly fecund out- 

 breeding species as the oyster a large number of 

 defective recessive genes can be expected to be har- 

 bored. On becoming homozygous with inbreeding 

 these will increase total mortality. [Battaglia (1970) 

 has found marine cope pods to be extremely sensitive 

 to protracted inbreeding, as are perhaps most highly 

 fecund outbreeding marine species.] 



Measurements were made on a total of 1,562 lar- 

 vae of this series and a least squares analysis of 

 variance done. Looking to the means and standard 

 errors which were quite small — 0.7 to 3.0% — 

 differences in larval lengths between the in- 

 breds and outbreds became apparent by day 6 with 

 differences becoming more pronounced as time 

 progressed (Fig. 5).- 



These measured differences between inbreds and 

 outbreds point to considerable genetic variation in 

 these oyster stocks. Judging from these results alone 

 there is a good basis for expecting improvements in 

 hatchery-produced C. virginicci by selective breed- 

 ing. 



Considering the high degree of inbreeding depres- 

 sion and the high mortality of inbred larvae thus far 

 encountered, it can be anticipated that commercial 

 breeders of C virginicci will probably find it neces- 

 sary to hybridize highly selected lines for marketing. 

 Certainly they will have to hybridize intensely in- 

 bred lines. No spat have yet been obtained from any 

 of the full-sib crosses made at the Milford Labora- 

 tory with the exception of some from the irradiated 

 lines. While some portion of this failure to obtain 



■' Statistical analysis was done by Ruel WiKon of the Biomelri- 

 cal Services. Livestock Research Staff. Agricultural Research 

 Center. ARS. U.S. Department of Agriculture. Bellsville. MD 

 20705. 



