Cytogenetic study of spawned eggs X-irradiated 

 at a rale of 164 R/15 sec and then crossed with 

 untreated sperm showed chromosome damage to 

 begin at 500 R. By 2. (KM) R such damage was pro- 

 nounced. The highest dose used on the eggs was 

 4,000 R. 



In terms of the numberofset produced and surviv- 

 ing for I yr from cultures of 500.000 fertilized eggs. 

 even the lowest dose of 125 R of X-rays administered 

 at a rate of 199 R/lOsec to pooled sperm crossed with 

 untreated eggs had a slight effect. By 2,000 R the 

 number of these spat was reduced appreciably. At 

 3.000 R there was hardly any. and none at all at doses 

 above this. From 4.000 R on there was a clear, strong 

 drop in percent development of the eggs to some 

 cleavage stage and percent development to the 

 straight-hinge larval stage. There was an increased 

 percent of abnormal straight-hinge or 2-day-old lar- 

 vae. The highest dose used on the sperm was 10,000 

 R. See Table 2. 



The overall performance of the larval and spat 

 cultures from the Fi of the gamma-irradiated oysters 

 and from the Fa of the irradiated sperm lines has 

 been characterized by an increased incidence of ab- 

 normal larvae at the straight-hinge stage, increased 

 mortality at all the early larval stages, heterosis of 

 the surviving larvae, and possibly some heterosis 

 also in the young spat. This extra vigor must be due 

 to the increased genetic heterozygosity resulting 



from the irradiation. Irradiation lines are also 

 characterized by a greater crossability of sibs. as 

 mentioned earlier, and also by a greater success of 

 the inbreds. Improved crossability of sibs can 

 likewise be attributed to the increased heterozygos- 

 ity that results from induced mutations and to muta- 

 tions of the cross-compatibility genes themselves as 

 well. There has been very little morphological 

 change in either the larvae or spat as a result of the 

 irradiation. 



Fertilization and early cleavage stages of these 

 crosses have increased polyspermy, more spindle 

 disturbances, more abnormalities of chromosome 

 number, more chromosome rearrangements, ab- 

 normal coiling of the chromosomes, and abnormal 

 nuclei. These are all classic signs of irradiation dam- 

 age to the genetic material. 



LITERATURE CITED 



AHMED, M.. and A. K. SPARKS. 



1970. Chromosome number, structure and autosomal 

 polymorphism in the marine mussels \t\liliis ectiilis and 

 Mytilus ctilifornianus. Biol. Bull. (Woods Hole) 

 138:1-13. 



BATTAGLIA, B. 



1970. Cultivation of marine copepods for genetic and 

 evolutionary research. Helgolaender wiss. Meeresun- 

 ters. 20:385-392. 



Table 2. — Summary of the culture history of crosses of I wild adult x pooled irradiated 

 sperm of 5 wild adult Cntsso.strea virginka. 



'.Accidental loss of l;ir\;ie in culture process. 



86 



