Catherine Thiriot-Quiévreux

Chromosomal study of spatial variation of the prevalence of a gill neoplasia in Macoma balthica (L.) from the Gulf of Gdansk (Baltic Sea)

Abstract Spatial variation of the prevalence of a gill neoplasia in the bivalve Macoma balthica was investigated by chromosomal study of individuals from ten different sites in the Gulf of Gdansk. The prevalence of gill neoplasia, individually identified by the occurrence of abnormal metaphases with higher chromosome numbers than normal metaphases (2n = 38), ranges from 0 to 94% of individuals, according to the site studied. Sites Hel 45 m and Vistula 30 m, located in the zone of the thermocline and halocline were the most affected, probably related to very unstable hydrological conditions at these sites. Out of a total of 152 animals studied from the Gulf of Gdansk, 33% showed the occurrence of gill neoplasia. A chromosome number from 59 to 109 was scored in 29 abnormal metaphases. Karyotypes were constructed from 14 abnormal metaphases and showed a similar general pattern. This study showed that chromosomal abnormalities involving numerical and morphological changes imply similar disorders among the sites studied. Etiology of this disease is still unknown but the seriously polluted Gulf of Gdansk can be used as a model basin to assess the relationship between the incidence of neoplasia and the possible controlling factors of environmental instability and/or level of environmental contaminants.

A 'G' chromosome banding study of three cupped oyster species: Crassostrea gigas, Crassostrea angulata and Crassostrea virginica (Mollusca: Bivalvia)

The G-banding technique was performed on chromosomes from gill tissue of three cupped oyster species: Orassostrea gigas, Cmssostrea angulata and Cmssostrea virginiea. Identification of the ten individual chromosome pairs was obtained. Comparative analysis of G-banded karyotypes of the three species showed that their banding patterns generally resembled each other, with chromosome pair 3 being similar in ail three species. However, differences from one species to another were also observed. The G-banding pattern highlighted greater similarities between C. gigas and C. angl!lata than between these two species and C. virginiea, thus providing an additional argument for genetic divergence between these two evolutionary lineages. C. gigas and C. angulata showed a different G-banding patterns on the two arms of chromosome pair 7, which agrees with their taxonomic separation. The application of this banding technique offers a new approach to specifie problems in oyster taxonorny and genetics.

Experimental evidence for a genetic basis to differences in aneuploidy in the Pacific oyster (Crassostrea gigas)

Aneuploidy has been previously reported in wild and cultivated Pacific oysters, Crassostrea gigas, and has been shown to be negatively correlated with growth. This is especially important since high variability of growth rate is one of the major problems in the aquacultural production of this species. The existence of a genetic basis for the observed differences in aneuploidy was first investigated through the comparative study of six full-sib families, with mean individual weights ranging from 0.59 to 1.49 g. The slowest growing family was found to have the highest level of aneuploidy. Significant differences in aneuploidy were also found among families when individuals with the same growth rate were sampled. This supports the hypothesis of the existence of a genetic basis for the control of aneuploidy level. Additionally, the possible inheritance of the level of aneuploidy was studied in four full-sib progenies originating from crosses within and between two different populations with contrasting levels of aneuploidy. The limited number of parental oysters (N = 6) in which the scoring of aneuploidy was possible did not allow demonstration of the inheritance of the level of aneuploidy. However, a genetic difference in aneuploidy could be attributed to the origin of the parental populations. As in the first experiment, significant differences in aneuploidy were observed between progenies when sampling individuals of the same weight. Thus, the results of our study of full-sib progenies of C. gigas lend support to the hypothesis of a genetic basis for the level of aneuploidy.

Genetic differences and environments of mussel populations in Kerguelen Islands

SummarySamples of subantarctic mussels, Mytilus desolationis, from 12 locations in the Kerguelen Islands (South Indian Ocean) were analysed electrophoretically at 5 polymorphic loci to study correlations between genotype frequencies and environmental factors. Thirty-one out 48 pairs (population-locus) were not in Hardy-Weinberg equilibrium proportions, and 41 out 60 showed heterozygote deficiency. Spatial heterogeneity analysed with correspondence analysis was observed at 3 loci (Lap2, Pgd and Pgm). The genetic differentiation of M. desolationis could be explained through both geographical origin of populations and adaptation to environmental conditions. Within each geographical group, the first parameter influencing spatial structure is salinity. Additional differentiation is related to a factor linked to maximal size and to a lesser extent to shore exposure. The higher genetic differentiation occurring within the Kerguelen populations than among continental populations could correspond to incomplete panmixia due to a reduced gene flow versus constant external immigration in continental populations.

Preliminary data on the genetic differentiation of Mytilus desolationis Lamy 1936 and Aulacomya ater regia Powell 1957 (Bivalvia, Mytilidae) in the Kerguelen Islands (terres australes et antarctiques françaises)

SummaryMussel populations in the Kerguelen Islands constitute an excellent model in which to study genetic differentiation, because of the exceptional geographic position of the islands with consequent very low external gene flow, the variety of biotopes that they occupy and their abundance there. Enzyme polymorphism and the electrophoretic variability of some loci have been studied in four populations of Mytilus desolationis Lamy 1936 and in four other populations of Aulacomya ater regia Powell 1957. In both species 67% of the loci were polymorphic. The electrophoretic variability of ACP-2, LAP-1, LAP-2, PGI and PGM in M. desolationis populations, and of ACP-2, LAP-1, LAP-2 and MDH in A. ater regia populations was analyzed by correspondence analysis. For M. desolationis the four populations studied were found to be heterogeneous; four loci contribute clearly to this differentiation. The populations from sheltered and exposed areas were distinguishable on the basis of the allelic frequencies of LAP-1 and LAP-2. The allelic frequencies of PGI and PGM allow further differentiation of the populations. For A. ater regia the four populations studied were distinguishable on the basis of the allele frequencies of ACP-2 and LAP-1 and the differences could not actually be related to their environmental conditions.