GENETICS OF THE AMERICAN OYSTER, 

 CRASSOSTREA VIRGINICA GMELIN 



A. CROSBY LONGWELL' 



INTRODUCTION 



The development of refined culture techniques 

 for the commercial East Coast American oyster, 

 Crassostrea virginica, at the U.S. government's 

 Milford Laboratory (Loosanoff and Davis, 1963), 

 along with the opening of experimental and com- 

 mercial shellfish hatcheries in the United States, 

 have led to an interest and modest support of genet- 

 ic research on the oyster (Longwell, 1969; Long- 

 well and Stiles, 1970). Our growers ofC. virginica, 

 whether dealing with a wild oyster set in the field or 

 with their own hatchery products, are faced with 

 the dilemma of cultivating an organism which does 

 either exceedingly well producing a superabun- 

 dance or does very poorly, both for reasons seldom 

 known. Part of the hoped for process of bringing 

 this oyster under some greater measure of control 

 by the cultivator necessitates some information on 

 its genetic and breeding system. This is particularly 

 so for profitable hatchery production and for the 

 best pond culture. Knowledge of oyster genetics 

 would also be of value for the special stocking of 

 decimated beds or for the introduction of stock to 

 previously uncultivated beds in the wild. 



CHROMOSOME BASIS OF C. VIRGINICA'S 

 BREEDING SYSTEM 



An examination of the male gametogenesis and of 

 the spawned and fertilized eggs of C. virginica 

 revealed that its meiotic system, fertilization, and 

 cleavage are of the typical type found in most higher 

 plants and animals (Longwell and Sfiles, 1968a). The 

 oyster is not characterized by any anomalous 

 chromosome behavior as, for example, is the honey 

 bee, which would frustrate breeding plans based on 



' Milt'ord Laboratory. Middle Atlantic Coastal Fisheries 

 Center. National Marine Fisheries Service. NOAA. Milford. 

 CT 06460. 



successes in higher organisms. Interestingly, the 

 fate of the fertilizing sperm can be followed from the 

 time of its entry into the egg until the fusion of its 

 chromosomes with the chromosomes of the female 

 gamete on the first cleavage spindle (Fig. 1). It 

 should be noted that any cytogenetic examination of 

 the eggs of at least this species of oyster must be 

 preceded by treatment of the eggs with methyl al- 

 cohol and chloroform in a micro-Soxhlet apparatus 

 to remove interfering yolk granules (Longwell and 

 Stiles, 1968b). 



There are 10 pairs of chromosomes at metaphase I 

 of meiosis in the mature spawned eggs of C. virgini- 

 ca (Longwell, Stiles, and Smith, 1967). Genes are 

 linked together then in 10 different groups. At least 

 in the female the chromosomes indicate that there is 

 a small amount of recombination or crossing-over of 

 the genes making for genetic variability without 

 which there can be no improvement by selective 

 breeding. There is no evidence for any sex chromo- 

 somes. This is in keeping with the protandric nature 

 of this oyster. See Figure 2. 



The chromosomes of C. virginica are all short, 

 about the same length, and hardly any of them read- 

 ily distinguishable from the others on any basis. This 

 makes for difficulty in their detailed study ( Fig. 3.4). 



Also studied were the chromosomes of the 

 Japanese oyster, C. gigas; the Puerto Rican oyster, 

 C rhizophorae: the European flat oyster, Ostreu 

 editlis; the U.S. West Coast small Olympia oyster, 

 O. liirida; and the horse oyster, O. equestris. All of 

 these species of both the viviparous Crassostrea 

 genus and of the larviparous Ostrea species have the 

 same number of chromosomes as C virginica. Also, 

 insofar as can be discerned, the chromosomes of all 

 these species are metrically and morphologically like 

 those of C virginica (Longwell et al., 1967). Menzel 

 (1968b) in Florida has examined several other 

 species of these two oyster genera. He further found 

 no variation in chromosome number and morphol- 

 ogy. At least at the chromosome level then there 



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