marine contaminants as they affect the lethal gene 

 load and reproductive cells of breeding fish popula- 

 tions. Recently, workers at this National Institute of 

 Genetics have developed a very sensitive test sys- 

 tem for the detection of chemical mutagens in pol- 

 luted environments. 



The Department of Cytogenetics is studying 

 chromosome polymorphism in populations of the 

 black rat around the world. With emphasis on the 

 Japanese population, the aetiology of Down's syn- 

 drome in humans is being studied. Expertise is cer- 

 tainly available then for breeding-related cytogenet- 

 ic work. 



Microbial genetics is also being pursued. Training 

 in such a field could be the basis for studying disease 

 organisms of aquatic plants and animals. 



In the Department of Applied Genetics, rice is 

 most extensively investigated - aquatic, marine or- 

 ganisms not at all. A worldwide collection of rice 

 species is maintained; there is a complete collection 

 of wheat and its relative species. 



OYSTERS OF JAPAN, SPECIFIC USE 

 OF HYBRIDS AND HYBRID VIGOR 



There is a very real separation of Crassostrea 

 gigcis. commonly called the Japanese oyster, into 

 several distinct races (Imai and Sakai, 1961). 



There may even be a variation in the chromosome 

 number in some Japanese populations of C gigas. 

 This is at variance with all the recent reports of 

 American workers (review, Longwell and Stiles, in 

 press). However, all the C. gigiis sampled in the 

 United States for chromosome analysis derive from 

 seed imported from Japan to the U.S. West Coast, 

 and most of the spat exported from Japan has been 

 taken from one particular region. 



C. gigas is collected widely in Japan in the shallow 

 waters on the coast from Hokkaido to Kyushu. Seed 

 were, until recenth . collected most actively in 

 Hiroshima, where oyster culture is said to have 

 begun 400 yr ago. The Miyagi district is next in 

 production to Hiroshima. Main seed-producing beds 

 in Japan are Hiroshima, Miyagi, Mie, and 

 Kumamoto Prefectures. The Miyagi Prefecture is 

 famous for their export of oyster seed. This is most 

 likely where most seed imported to the U.S. West 

 Coast has been originating. Hiroshima culturists sel- 

 dom bought oyster seed and seldom exported their 

 seed to other prefectures or countries. 



There are 20 different species of oysters in Japan, 

 all edible. Only three of these are of direct economic 



importance. C gigas, C. rivitlaris. and Ostreci nip- 

 poini . 



It is believed that C. lapcioitsi and C. ariaken- 

 sis. native Japanese oysters, reported some years 

 ago by Kobayashi (1954) to have four more chromo- 

 somes than C. gigas, are really C. riviilaris. The 

 species C. riviilaris is not cross-compatible with C 

 gigas (Imai and Sakai, 1961). Confusing here is the 

 fact that American malacologists regard C. laper- 

 oiisi and C. ariakensis as merely forms of the vari- 

 able C. gigas. 



C. riviilaris. imported with seed of C. gigas. has 

 been planted in Puget Sound and, according to Galts- 

 off (1964), has established itself there. 



Some of this taxonomic confusion in classifying 

 oysters may stem from the existence already of sev- 

 eral hybrid-type populations in the wild. Nothing 

 was learned in Japan regarding the opinion of some 

 U.S. workers that C. angiilata, the Portuguese oys- 

 ter, and C . gigas are the same species. 



Lest an understanding of the nature of these 

 species be regarded as esoteric, it should be pointed 

 out that such much publicized benefits of genetics, 

 as miracle rice, resulted from a combination of two 

 species or varieties. The characteristics of each of 

 these types of rice were clearly known and under- 

 stood by the breeders who test-hybridized them 

 initially along with many others which were not 

 "miracles" at all. 



Part of the vigor of C gigas relative to the more 

 sensitive commercial East Coast American oyster, 

 C. virginica, may derive from a slow though steady 

 rate of introgressive hybridization. That is. by the 

 addition of other species genes to C. gigas on a small 

 scale over a long period. It could also derive from 

 larger scale hybridization in various oyster popula- 

 tions in the more distant past. Both of these prob- 

 abilities can be checked experimentally. This is 

 basic research, of course, but it would provide some 

 information on how important hybridization pro- 

 grams are in developing vigorous hatchery stocks. 

 Such would remove some of the present unavoidable 

 guesswork as to the usefulness of wide species 

 crosses. 



Spat of C gigas are now being shipped to France 

 from Japan for planting on oyster beds once popu- 

 lated by C angiiUiiii. There is a chance of natural 

 hybridization on the wild beds in France between 

 imported C. gigas and the remains of the once plenti- 

 ful C . angiilata populations. These two species read- 

 ily hybridize in the laboratory, the hybrids survive 

 and are fertile (Imai and Sakai, 1961). 



127 



