Evidence for high levels of genetic divergence between populations of the bivalve Mytilaster minimus from a brackish environment and two adjacent marine sites

© The Malacological Society of London 2001 Mytilaster minimus (Poli, 1795) (Mytilidae) is a small bivalve (maximum shell length 20 mm) distributed on many hard bottoms of the intertidal zone in the Mediterranean and eastern Atlantic. The species has a planktonic larval stage, but larval morphology and life-span are unknown, so that its potential for dispersal cannot reliably be inferred. Individuals from brackish and marine waters are morphologically indistinguishable. The purpose of the present study was to investigate the genetic structure of M. minimus from the Orbetello Lagoon (Tuscany, Italy) and two adjacent marine sites. The Orbetello Lagoon is one of the largest western Mediterranean brackish basins, constituting a shallow virtually tideless pond that communicates with the sea through two narrow channels. One of the characteristics affecting the composition of the lagoon community is the instability of the physico-chemical parameters of the water, above all salinity and organic pollution due to human activities such as agriculture, aquaculture, urbanisation and tourism. Genetic structure can result from the effect of genetic drift and/or natural selection. The relative importance of each of these factors in shaping the pattern of genetic structure is a central question in population genetics. Evidence for the correlation of changes in allele frequencies with different habitat types or heterogeneous environmental parameters has been used to infer the effect of selection in natural bivalves populations. The present study investigates the relationship between genetic structure and habitat type in M. minimus estimating the degree of allozyme genetic divergence between lagoon and adjacent marine populations. In the period between July 1998 and February 1999 a total of 200 individuals of M. minimus (40 from each site) were collected at three sites in the Orbetello lagoon (42 26N, 11 12 E) and two adjacent marine sites, located at about 300m from the mouth of the channel between the lagoon and sea. Distances between sampling sites were less than 4 km. Specimens were transferred alive to the laboratory and stored at 80 C. Cellulose acetate electrophoresis was used to determine allele frequencies in the following enzymatic systems: dihydrolipoamide dehydrogenase (DDH, E.C. 1.8.1.4); glycerol-3phosphate dehydrogenase (G3PDH, E.C. 1.1.1.8); glucose6-phosphate dehydrogenase (G6PDH, 1.1.1.49); glucose-6phosphate isomerase (GPI, E.C. 5.3.1.9); isocitrate dehydrogenase (NADP ) (IDH, E.C. 1.1.1.42); L-lactate dehydrogenase (LDH, E.C. 1.1.1.27); leu-tyr peptidase (LTP, E.C. 3.4.*.*); malate dehydrogenase (MDH, E.C. 1.1.1.37); malate dehydrogenase (NADP ) (MDHP, E.C. 1.1.1.40); phosphogluconate dehydrogenase (PGDH, E.C. 1.1.1.44); phosphoglucomutase (PGM, E.C. 5.4.2.2); phe-pro peptidase (PPP, E.C. 3.4.*.*); triose-phosphate isomerase (TPI, E.C. 5.3.1.1). For electrophoretic runs and staining, slight modifications of the procedures reported in Pasteur et al. were applied. Allele frequencies and population genetic statistics were calculated using the computer package FSTAT 2.9. 1. Within sample genetic variability was estimated by using the average expected unbiased heterozygosity. F-statistics were applied in order to quantify the withinand betweensample genetic heterogeneity. Unbiased estimators were obtained by jack-knifing across loci and tested for difference from zero by permutation tests. All statistics were applied to different data sets: first, analyses were performed on the total data set; second, marine and lagoon samples were pooled separately. Finally, on the basis of pairwise Nei’s unbiased genetic distances, a UPGMA dendrogram was generated and subjected to bootstrapping with 5000 replicates using GDA. Our analysis revealed a high level of within-sample genetic variability in both lagoon and sea samples. The 13 enzyme systems surveyed yelded 19 putative gene loci, 17 of which were polyallelic in at least one sample (Table 1). Heterozygosity values revealed a fairly high degree of within-sample genetic variability (Table 2). Such levels were generally comparable with those detected in other mytilids, such as Mytilus edulis (H 0.115), M. trossulus and M. galloprovincialis (H 0.31 to 0.41 and H 0.24 to 0.43, respectively) and in other bivalves from brackish habitats, such as, Ruditapes decussatus (H 0.229 to 0.282), Potamocorbula amurensis (H 0.363 to 0.384), Cerastoderma edule (H 0.092 to 0.728), Crassostrea angulata (H 0.292 to 0.382). A bulk of literature reported that brackish water species present lower levels of genetic variability as compared to their marine counterparts, as a result of their adaptation to unpredictable environments. Bivalves seem to represent an exception to this trend, as the results of our study and previous papers suggest that genetic variability in brackish water bivalves is fairly high, with values for brackish water populations comparable to their marine counterparts. The populations studied showed many rare alleles, suggesting that the populations apparently have not been subjected to recent founder events or recurrent bottlenecks. Hence the dystrophic crises in 1992 and 1993 that caused extensive mortality among many other benthic species do not seem to have substantially affected the lagoon population of M. minimus. The positive and significant FIS values indicate a general departure from Hardy-Weinberg equilibrium due to a deficit of heterozygotes (Table 2). Explanations for that may take RESEARCH NOTES