that some of these hybrid crosses would be sterile. 

 Sterile hybrids might, because of their sterility, have 

 a faster rate of growth or greater ultimate size than 

 fertile nonhybrids. 



Hybrid development did occur in all combina- 

 tions. There were, however, wide differences in sur- 

 vival rates. Hybrids between 32 combinations 

 survived until they reached the free-swimming 

 stage. Nine of these showed survival rates similar or 

 better than those of parental species. Those combi- 

 nations that did survive are currently being studied 

 at later growth stages. 



Among hybrid characteristics already ascertained 

 is a heterosis against disease. This is an important 

 feature since prevention of epidemic disease is a 

 serious problem in trout culture. 



Even some intergeneric hybridization was 

 achieved. These hybrids though presented special 

 difficulties m cultunng and rearing. 



Tohoku University School of Fisheries and 



Tohoku Regional Fisheries Research Laboratory — 



Radiation Effects on Oyster; Seaweeds 



The effects of ionizing radiation on Crassostrea 

 gigas are being studied at Tohoku University (L. 

 Maeda, pers. comm.). This work may be similar in 

 nature to some done on C. virginica (Longwell and 

 Stiles, 1972). 



Recent advances in cultural techniques for the 

 seaweeds, like Porphyra and Undaria, have brought 

 substantial benefit to a considerable portion of 

 fishermen engaged in seaweed production in Japan. 

 (At the same time once important nori grounds about 

 Tokyo have been irretrievably lost to pollution, and 

 nori products have come to contain mercury and 

 cadmium.) Little attention though has been given to 

 the possibility of artificially crossing seaweed types 

 to give a more productive, profitable strain. Since 

 1958, S. Suto has been making interspecies hybrids 

 always using a local commercial species as one par- 

 ent. This work has been done with a view of obtain- 

 ing a new seaweed with good characteristics for 

 culture (Suto. 1963; also see Suto, 1972). Fortunate- 

 ly, self-fertilization rates are low, facilitating artifi- 

 cial fertilization. However, much preliminary work 

 had to be done before artificial fertilization could 

 be achieved with certainty. Also, the relationship 

 between Porphyra species had to be clarified. 



Artificial crosses were attempted between five 

 species of the Porphyra genus, including four local 

 forms. Crosses occurred easily between any two 



species tested. Descendants from crosses of two 

 dioecious parents and from two monoecious parents 

 grew normally. Those from dioecious-monoecious 

 combinations died in mass at a young stage, but left a 

 few survivors. The three dioecious species studied 

 appear to be very closely related. At least in hybrids 

 of the dioecious species, there is normal reproduc- 

 tion with typical genetic segregation. 



Recently, some lines started from an interspecies 

 hybrid have yielded two and three succeeding gener- 

 ations. Some of the lines grow more vigorously than 

 did their native parents. 



This work demonstrates that new lavers, more 

 suitable for commercial production than the present 

 wild lavers, can be bred. Probably the F, inter- 

 species hybrids will first have to be subjected to a 

 program of artificial selection in the laboratory, or 

 one of natural selection in the wild before being used 

 commercially. 



Introduction to Japan of foreign species of 

 Porphyra, which may be disease resistant, for field 

 trials and for use in hybrids with Japanese species, is 

 regarded important. High quality species already 

 being utilized are, unfortunately, not the most pro- 

 ductive, and disease is a factor in lowering produc- 

 tivity. 



There is a likelihood of some natural hybridization 

 occurring on the wild beds between closely related 

 species. The genetic and other implications of this 

 are recognized. 



Some cytogenetic studies on P. yezoensis have 

 been done elsewhere at Nagasaki University Fa- 

 culty of Fisheries (Migita, 1967). There are three 

 chromosomes in vegetative and spermatogonial cells 

 in the leaf thalli, and six chromosomes in fertilized 

 carpogonia. The haploid chromosome number of 

 this alga is then three. Meiosis takes place in the cell 

 division of the conchospore at the time of spore 

 formation. Such information is pertinent to the mak- 

 ing of artificial crosses. Also for the artificial somatic 

 replication of a single plant for multiplication prior to 

 larger scale commercial production. 



Aside from the great commercial value of sea- 

 weeds in Japan (their most important fishery), the 

 Japanese probably have here one of the most in- 

 teresting areas of the future field of marine genetics 

 with most promise of great commercial application. 

 This is because plants, in general, lend themselves 

 so well to genetic study with subsequent genetic 

 manipulation and because so many genetic and 

 breeding techniques are so well worked out for 

 plants. 



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