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Abstract: Following the results of a series of investigations carried out to estimate the degree of marine pollution by utilizing certain marine filter feeders, such as the blue musselMytilus galloprovincialis, research has been planned to detect possible genetic effects of pollutants, with special attention to those acting at the population level. The possible selective role of pollutants has been studied both in natural (Mytilus) and in experimental (Tisbe holothuriae) populations by utilizing some electrophoretically-detected gene-enzyme systems as genetic markers. For some of the seven polymorphic loci studied inMytilus (AP, LAP, 6-PGD, IDHs, IDHm, PGI, PGM) significant changes in gene frequencies have been detected which can be related to the degree of pollution in the sampling areas. In the more polluted areas these changes were accompanied by a decrease in the frequency of heterozygotes. Similar changes in gene frequencies also occurred in laboratory populations of the copepodTisbe, reared under various experimental conditions. In particular, certain alleles of two loci, PGI-1 and AP-1, exhibited an increase in frequency, especially in populations cultured at various levels of oil pollution. This trend appeared more significant for the locus PGI. The fact that equilibria are reached and that the less favoured alleles are nevertheless maintained in the populations, even at extremely low frequencies, suggests the balanced nature of these enzyme polymorphisms. The significance of the above findings is briefly discussed.

Antarctic krill (Euphausia superbaDana) is a key species in the Antarctic food web and occurs on a circum-continental scale. Population genetic structure of this species was investigated by sequence analysis of the ND1mitochondrial gene in four population samples collected at different geographical localities around the Antarctic continent. Results indicate the existence of significant genetic differences between samples, and we suggest that oceanographic barriers could be sufficiently strong and temporally stable to restrict gene flow between distinct areas. Moreover, our data indicate that Antarctic krill is not at mutation–drift equilibrium and that the species possibly has a low effective population size as compared to the census size.

Most fishes of the perciform suborder Notothenioidei are endemic to the subzero marine waters of Antarctica. A number of remarkable physiological attributes allow them to inhabit this extreme environment; for example, the blood of almost all notothenioid species contains antifreeze glycopeptides. To establish a molecular phylogenetic hypothesis for these fishes, DNA sequences from two mitochondrial genes, portions of the 12S and 16S ribosomal genes (928 base pairs [bp]), were determined for 18 species. These belong to 15 genera in five families of the suborder. The DNA data suggest that two of these families are unnatural groups and consequently that the classification and phylogeny of this suborder is in need of revision. In terms of DNA variation, the Bovichtidae are a distantly related sister group to the other families of the suborder that includes the icefishes, the only vertebrates without hemoglobin. The fishes of the suborder (except the Bovichtidae) seem to have speciated rapidly, forming an adaptive radiation in the Antarctic waters. A phylogenetic analysis of published hemoglobin amino acid sequences for other notothenioid fishes supports these results from mtDNA. On the basis of molecular phylogeny, the evolution of antifreeze glycopeptides was studied. The age of the radiation of notothenioid fishes had been estimated to be at least 38 Mya. However, the level of mtDNA variation detected in notothenioid fishes appears to be too low to agree with this date of origin and might instead suggest a younger age (10-15 Mya). Alternatively, the low level of detected mtDNA variation would agree with the traditional old-age estimate if an extremely slow rate of mtDNA evolution is postulated for this group. This slow-rate hypothesis, if true, could be explained by decreased metabolic rates slowing down the tempo of molecular evolution.

Studies concerning the evaluation of the genetic structure and the mechanisms of genetic adapation to the Antarctic environment were carried out on two population samples (A and B) of the amphipod Paramoera walkeri Stebbing collected in two different areas of the Terranova Bay during the Italian Antarctic Expedition 1987/1988. The analyses of the level of genetic variability were conducted on starch gel. The electrophoretic results on the twenty two loci examined reveal a very low amount of genetic polymorphism in both populations with an average observed heterozygosity of 0.6% in population A and 0.7% in population B. The two populations can be considered part of a single breeding unit on the basis of the Student test for paired observations since the differences in single-locus heterozygosity for seven polymorphic loci are not significant (P>0.70). A possible explanation of the observed low genetic polymorphism is discussed in terms of genetic adaptation, as suggested by some results obtained in studies on enzyme activity carried out at the level of the PGI locus.

Kurzfassung Einige benthische Copepoden eignen sich ausgezeichnet für das Studium genetischer Aspekte evolutiver Vorgänge im Meer. Dazu gehören speziell die Arten der GattungTisbe, weil sie (1) im Labor mit Hilfe einfacher Methoden kultiviert werden können, (2) einen kurzen Entwicklungszyklus haben, (3) eine hohe Vermehrungsrate und (4) oft genetischen Polymorphismus aufweisen. Es hat sich herausgestellt, daß dieser Polymorphismus in einigen Fällen adaptativ und balanciert ist, d. h. durch natürliche Selektion, die auf die einzelnen Genotypen verschieden einwirkt, aufrechterhalten wird. Auf diese Weise sind die Populationen befähigt, verschiedene ökologische Nischen auszunützen. Versuche haben gezeigt, daß gewisse ökologische Faktoren, wie Salzgehalt und Temperatur, stark selektiv wirken. Veränderungen in der Frequenz gewisser Genotypen einer polymorphen Art unter verschiedenen Bedingungen — in der Natur und im Labor — und die unterschiedliche Größe der genetischen Last in marinen und Brackwasser-Populationen derselben Art beweisen den Einfluß abiotischer Umweltbedingungen auf den sichtbaren Polymorphismus einerseits und auf die latente genetische Variabilität andererseits. Es darf allerdings nicht vergessen werden, daß in einer biologischen Gemeinschaft ebenfalls Faktoren, wie Anzahl und Art der miteinander lebenden Organismen, eine wichtige Rolle spielen. Die ersten Ergebnisse einer Reihe von Versuchen haben erkennen lassen, daß in gewissen Fällen die biotischen Faktoren eine mindestens ebenso große selektive Wirkung haben können wie die abiotischen. Weitere Experimente befassen sich mit der Frage der reproduktiven Isolation geographischer Populationen derselben Art und der dabei beteiligten Mechanismen.

The results of various crosses have shown that the colour patterns of the Copepod Tisbe reticulata living in the Lagoon of Venice is due to a series of at least three allelomorphs at the same locus, two of which dominant over a common recessive. The Same two allelomorphs do not exhibit dominance when occurring together; consequently this heterozygote is phenotypically detectable. There is some evidence that, under laboratory conditions, this heterozygote is more viable than both the respective homozygotes.Balanced polimorphism in Tisbe seems to play a role, together with other mechanisms described in previous papers on the subject, in the maintenance of genetical variability in this species.

Within the genus Tisbe (Copepoda: Harpacticoida), there is a wide range of reproductive isolation. In the European area, in addition to species where (between geographic populations) the reproductive barrier is practically absent, there are others which show a varying degree of incompatibility. The present paper deals with the results of cross-breeding experiments carried out between American and European populations. The respective intrapopulation crosses served as controls. Transatlantic crosses have been attempted with the following species: T. lagunaris, T. bulbisetosa, T. holothuriae, T. battagliai, and T. clodiensis. The American and European populations of the first 4 species are interfertile, and the F1 hybrids produce viable offspring. Moreover, the interpopulation crosses, compared to the controls, exhibit a slight superiority whose nature is, however, still unclear. The fifth species, T. clodiensis, behaves differently. The absence of viable offspring from interpopulation crosses, or the death of the hybrids before reaching the adult stage, indicate that, in this species, the barrier to gene flow is complete. Populations of T. clodiensis show, in addition, signs of “relative intraspecific incompatibility”. The problem of the different degree of reproductive isolation in the genus Tisbe, with special regard to the role played by ecological factors in determining the isolation patterns, is discussed.