should be no barrier to the making of fertile hybrids 

 between the species within these two genera or even 

 between these two genera of oysters. No species of 

 the third oyster genus, Pycnodonte, has yet been 

 examined chromosomally probably because of the 

 difficulties of obtaining these noncommercial forms 

 which occur as singles in deep water. 



To date, there have been no indications for any 

 chromosome polymorphism in any of the popula- 

 tions of C . virginka in Long Island Sound nor in any 

 other population thus far examined from as far north 

 as Prince Edward Island, Canada, to' as far south as 

 the State of Virginia. Ahmed and Sparks (1970) may 

 have uncovered a chromosome polymorphism in 

 marine mussels; Staiger (1957) found extensive 

 polymorphism of chromosome numbers in a marine 

 gastropod mollusk: and Battaglia (1970) has re- 

 ported extensively on genetic polymorphisms in 

 marine copepods. Chromosome polymorphisms aid 

 the population in making rapid adjustments to fluc- 

 tuations in the local environment. 



In contrast to the population, species, and generic 

 constancy of the chromosomes of the oyster, early 

 developmental stages of C. \iri>inica are marked by 

 a high frequency of variation in chromosome number 

 (Stiles and Longwell, unpublished data). Fertilized 

 and cleaving eggs from a series of about 15 mass- 

 spawned groups, with a total of 835 spawning oys- 

 ters, had an average of about 269f postfertilization 

 genetic abnormalities. Abnormalities of chromo- 

 some number were present in 129? of the eggs. It 

 appears that the sensitivity of this oyster egg to all 

 kinds of environmental distresses results in acute 

 effects at the chromosome level. This should make 

 the oyster an excellent assay species for detecting 

 genetically damaging and zygote-destructive pol- 

 lutants (Stiles and Longwell, in press). 



INBREEDING C. VIRGINICA, ITS EFFECTS 

 AND THE SPECIES MATING SYSTEM 



Inbreeding to some extent will accompany mass 

 selection in commercial hatcheries. More extreme 

 forms will eventually be used to develop lines for 

 subsequent hybridization in hopes of so obtaining 

 hybrid vigor. 



Full-sib crosses of C.riri^inica made over a period 

 of a year gave consistently poor development to the 

 straight-hinge larval stage. An investigation into this 

 revealed that marked fertilization and early de- 

 velopmental failures were occurring in these crosses 

 (Longwell and Stiles, 1973). In 5 of 9 cultures thus 



far studied carefully, an average of 63*^ of the sib- 

 crossed eggs remained unfertilized. Only \Wc of the 

 eggs of the contemporary between-line crosses or 

 outcrosses to unrelated wild oysters remained unfer- 

 tilized. Only 39? of cleavages in the sib-crossed eggs 

 were normal. In the controls 70% of the cleavages 

 were normal. Parthenogenesis averaged 109? in the 

 inbreeding crosses and 0.59f in the controls. See 

 Table 1. Some of the inbreeding crosses were 

 characterized by polyspermy. In others there was a 

 degeneration of the one or more sperm that had 

 penetrated the egg. 



A second, more extensive series of sib crosses 

 with their contemporary control interline crosses 

 showed essentially the same crossing difficulties. 

 The incidence of ineffective fertilization was higher 

 than in the first series. Only 469? of the fertilizations 

 actually achieved with sibling sperm activated the 

 eggs to normal development, contrasted to 989? 

 when the sperm of nonrelated oysters of other lines 

 was used. Parthenogenesis was also higher; only 7% 

 in the controls but 299? in the sib crosses. 



Prolonged fertilization attempts increased the 

 number of eggs fertilized in sib crosses by 20%. It, 

 however, also led to more polyspermy, and there 

 was more degeneration of sperm in the cytoplasm of 

 the eggs. Such late fertilizations seldom seem to be 

 effective. 



These crossing barriers are interpreted as meaning 

 that a strong outbreeding system in C. viiffinica 

 must, at least in some individuals in some popula- 

 tions, be reinforced by a system of gamete cross 

 incompatibility with a basis in genetic factors. This 

 system must operate to prevent the crossing of 

 gametes of closely related oysters with similar 

 genes. Inbreeding is thereby discouraged, and out- 

 breeding promoted. Incompatibility genes are 

 known to be highly mutable. An increased cross- 

 ability of C. virf^inica full- and half-sibs originating 

 from irradiated gametes so supports this interpre- 

 tation. 



Genetic systems of cross incompatibility prevent- 

 ing inbreeding exist in an estimated 3,000 higher 

 plants (Brewbaker, 1964; Williams. 1964). The only 

 carefully studied case in animals though has been in a 

 marine organism — the hermaphroditic ascidians 

 (Morgan. 1924, 1942a. 1942b). Some of these are 

 self-fertile, others self-sterile, with some variability 

 between geographic races. Other groups having in- 

 breeding incompatibility often shou interspecies 

 crossing barriers as well. From the pioneering work 

 of Imai in Japan ( Imai and Sakai, 1961 ) and the later 



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