Reference: Biol Bull 188: 117-119.(April, 1995) Evidence for Selection Against Heterozygotes: Post-Settlement Excess of Allozyme Homozygosity in a Cohort of the Chilean Oyster, 1845 Ostrea chilensis Philippi, J. E. TORO AND A. M. VERGARA Instituto de Biologia Manna, L'niversidadAustral de Chile. Casilla 567, I'aldivia. Chile Reports ofheterozygote deficiencies in electrophoretic heterozygote deficiency was found in the carbonic an- survevs carried mil in marine bivalves abound in the lit- hydrase(CA)locusfrom asouthern population (Melinka, erature(1-6), but themechanism ormechanismsproduc- 4353'S) (1). The time at which heterozygote deficiency ingthisphenomenon havenot been welldefined. H'ereport first appears in the population can help distinguish caus- that, in theChilean oyster(Ostreachilensis), heterozygote ative mechanisms(16). In laboratory studieswith mussels, deficiencies in a cohort obtainedhy mass spawning in the an overall significant deficiency of heterozygotes was laboratory are not randomly distributed in lime among found at thejuvenile stage but not at the spat stage (4). genotypes. The eggs ofthe Chilean oyster are internally In the present study, we used a cohort ofO. chilensis fertilised, and the larvae, which are brooded within the settledon artificial collectorsin theQuempillen hatchery, mantlecavitv. havelimiteddispersalcapabilities because Ancud, Chiloe (4552'S, 7346'W). The parental stock oftheir extremely short pelagic stage (7). Thesefeatures was a cohort of O. chilensis collected during December could allow mechanisms such as inbreeding or Wahlwnl 1987 from a natural spatfall in the wild population at effect to produce heterozygote deficiencies. However, we Quempillen estuary. The Chilean oysterbecomes sexually observed no significant heteroiygote deficiencies injuve- mature at the beginning ofthe second year oflife with a mm niles at 6 months ofage; insteadallozyme heterozygosiiy shell length of about 27 (7). After three years of decreased over time. Inbreeding. H'ahlund effect, aneu- growth under uniform conditions, 800 randomly chosen ploidy. and null alleles are unlikely to be main causes of oysters were mass spawned in the laboratory. The tem- the heterozygosity deficiency in this cohort; ifthey were, perature and salinity used in the experiment were within the deficiency should be evidentfrom the juvenile stage the range ofthose in the natural environment (10-18C and would not necessarily increase over time (2, 5. 8, 9. and 27-32 ppt). Although the use ofmass spawning pre- 10). Wesuggest that selection against heterozygotes isthe ventsone from knowinghow many individualscontribute most probable cause ofthe increasing degree ofhetero- genes to the offspring obtained, the female contribution zvgotedeficiency with age in this cohort ofO. chilensis, a can be estimated by keeping track ofthe number in each proposition that accords with dataforothermarinebivalve brood of eyed larvae. Fecundity in O. chilensis ranges species (2, 4. 11). between 10.000 and 1 15,000, with an average of60,000 Populations of marine bivalves exhibit deficiency of (7). The numberoflarvae released, more than 8.2 X 106, allozyme heterozygotes. This deficiency has been dem- indicates that at least 130 femalescontributed larvae. This onstrated in laboratory studies ofmussels and clams (2. number of females may be an underestimation because 3), in studies using wild populations (8, 12) of Mytilus someoftheeyed larvaereleased set within 5 min (7);thus edu/is. and in several studies ofoysters (Crassoslrea vir- this cohort cannot be treated as a product of restricted ginica) (13-15). In theChilean oyster(Ostrea chilensis) a matings. From an initial population size of 4050 randomly Received 10 May 1994; accepted 26January 1995. taggedjuvenilesgrown at the Hueihue location (4158'S, 117 18 J. E. TORO AND A. M. VERGARA Table I Helero:ygi>teIm/iu-iieyandD values torlourloci atthreestagesofthe litecvcleotOstreachilensistf>. IX. and30months ofage) Age (months) HETEROZYGOTE DEFICIENCY IN OYSTERS 19 10. Thriot-Quievreux, C, T. Noel, S. Bougrier, and S. Dalloi. 15. Zouros, E.,S. M.Singh,and II. E. Miles. 1980. Anoverdominant 1988. Relationship between aneuploidy and growth rate in pair phenotypeand itspossibleexplanations. Evolution34: 856-867. matingsoftheoysterCrassostreagigas. Ayuaculture75: 89-91. 16. Zouros, E.,and D.\V.Foltz. 1984. Minimalselectionrequirements 1 1 Ahlevtaerreozz,ygGo.s,iCt.yZaatpaplrao,teRi.nAlmocairoa,ndanfditAn.esGsueinrrtah.e19E8u9r.opeMaunltoiylsotceurs. dtoerfitchieenccoyrroeflahteitoenrobzeytgwoeteens,heinteoryosztyegrospiotpyulaantdiognrso.wtDhe,v.anGdenfeortth4e: Oslmicilulis L. Heredity63: 359-372. 393-405. 12. Gpoossleidn-gs,hoEr.eMm.u,ssaenlds,DM.ytMiclGursastphp..a1l9o9n0g.anGeennveitriocnmvaernitabailligtryadiinenetx.- 17. gdKeiofnefehetnrie,cnstRio.aftWmio.an,riiRnn.etMhpieellbeklcmuyaepnom,duss.asnIedVl.JM.SyeBll.ieclMtuiisolntc,odunml.iisg1.r9a7Et6vi.oolnutaPinoodpnugl3ea0nt:eito2in-c Mar. liii'l 104: 413-418. 32. 13. Singh, S. M., and E. Zouros. 1978. Genetic variation associated 18. Shaw, C. H., and R. Prasad. 1970. Starch gel electrophoresis of with growth rate in the American oyster (Crassostrea virginica}, enzymes: acompilation ofrecipies. Biochem. Genet- 4: 297-320. Evolution32: 342-353. 19. Selander, R. K., M. II. Smith, S. Y. Yang, W. E. Johnson, and 14. Zouros,E.,S.M.Singh,D.W.Foltz,andA.L.Mallei. 1983. Post- J. B. Gentry. 1971. Biochemical polymorphism andsystematics in settlement viability in the American oyster(Crassostrea virginica): thegenusPeromysenx 1. Variationintheoldfieldmouse(Peromyseti.'i anoverdommant phenotype. Genet Res 41: 259-270. polionotus). Stud. Genet. 6:49-90.