Genetics of growth rate variation in bivalves: aneuploidy and heterozygosity effects in a Crassostrea gigas family  <B>Genetics of growth rate variation in bivalves: aneuploidy and heterozygosity effects in a Crassostrea gigas family</B><P>C. THIRIOT-QUIEVREUX</B><I><P> Observatoire Oceanologique, Universite P. et M. Curie - CNRS, B.P. 28, 06230, Villefranche-sur-Mer, France</I><B><P> G. H. POGSON</B><I><P> Department of Biology and Marine Gene Probe Laboratory, Dalhousie University, Halifax, N.S., Canada, B3H 4J1</I><B><P> AND E. ZOUROS</B><I><P> Department of Biology and Marine Gene Probe Laboratory, Dalhousie University, Halifax, N.S., Canada B3H 4J1</I><I><BR> and Institute of Marine Biology of Crete and Department of Biology, University of Crete, Iraklion, Crete, Greece</I><P> Enzyme homozygosity and somatic aneuploidy are both known to adversely affect juvenile growth rate in marine bivalves. We have examined the joint effects of these two factors by scoring genotypes at nine segregating allozyme loci and counting the numbers of chromosomes lost in 30 cells in each of 83 full sibs of the Pacific oyster. A highly significant negative correlation was observed between the number of chromosomes missing and shell length in full sibs of the same age. No relationship was seen, however, between allozyme heterozygosity and either shell length or chromosome loss, nor was there any difference in the distribution of aneuploidy among genotypes at any given enzyme locus. Thus, the effects of homozygosity and aneuploidy on growth rate appear to have different genetic bases. Even in the most aneuploid oysters, more than half the cells examined had a complete chromosome complement of 2<I>n</I> = 20. This eliminates somatic aneuploidy as an explanation for the excess of enzyme homozygosity frequently observed in populations of marine molluscs. Significant deviations from Mendelian expectations, favoring homozygotes at some loci and heterozygotes at others, were recorded at eight of the nine allozyme loci, but these occurred independently of the aneuploidy observed. Our results suggest that within families a much larger component of variation in growth rate is due to aneuploidy than to allozyme genotype, but this conclusion cannot, at present, be extended to natural populations.<I></I><B><P> Key words:</B><I> </I>aneuploidy, heterozygosity, growth rate, oysters.<P> L'homozygotie enzymatique et le taux d'aneuploidie somatique sont tous deux connus pour avoir un effet defavorable sur le taux de croissance juvenile chez les bivalves marins. Nous avons examine les effets conjuges de ces deux facteurs chez 83 descendants d'un croisement (un male x une femelle) chez l'Huitre <I>Crassostrea gigas, </I>en determinant les genotypes de neuf loci allozymiques et en quantifiant le nombre de chromosomes manquants dans 30 cellules. Une correlation negative hautement significative a ete observee entre le nombre de chromosomes manquants et la longueur de la coquille dans la lignee ("full sibs") du meme age. Neanmoins, aucune relation n'a ete observee entre l'heterozygotie allozymique et la longueur de la coquille ou la perte de chromosome, ni de difference dans la distribution de l'aneuploidie parmi les genotypes a chaque locus enzymatique etudie. Ainsi, les consequences de l'heterozygotie et de l'aneuploidie sur le taux de croissance semblent avoir des bases genetiques differentes. Meme dans les cas les plus extremes d'aneuploidie, plus de la moitie des cellules examinees montraient une garniture chromosomique complete. Ceci elimine l'aneuploidie somatique en tant que cause de l'exces d'homozygotie observe dans les populations de Mollusques marins. Dans huit loci sur les neuf etudies, des deviations significatives de l'heridite mendelienne, favorisant les homozygotes a certains loci et les heterozygotes a d'autres, ont ete notees, mais independamment de l'aneuploidie observee. Nos resultats suggerent que, au sein de familles, une plus importante composante de variation du taux de croissance serait due a l'aneuploidie plutot qu'au genotype allozymique, mais cette conclusion ne peut etre etendue pour le moment aux populations naturelles.<I><P> Mots cles </I>: aneuploidie, heterozygotie, taux de croissance, huitres.<P> Genome, <B>35: </B>39-45. 1992<P>