Noureddine Chatti

Mitochondrial DNA variation in the caramote prawn Penaeus (Melicertus) kerathurus across a transition zone in the Mediterranean Sea

In this study we analysed mitochondrial DNA variation in Penaeus kerathurus prawns collected from seven locations along a transect across the Siculo–Tunisian region in order to verify if any population structuring exists over a limited geographical scale and to delineate the putative transition zone with sufficient accuracy. Partial DNA sequences of COI and 16S genes were analysed. In contrast to the highly conservative 16S gene, the COI sequences exhibited sufficient diversity for population analysis. The COI gene revealed low levels of haplotype and nucleotide diversities. The size of the annual landings of this commercial species suggests large population sizes. Hence, the low genetic diversity detected in this study could indicate a possible reduction in effective population sizes in the past. We detected significant genetic differentiation between eastern and western populations likely due to restricted gene flow across the Siculo–Tunisian boundary. We discuss the different evolutionary forces that may have shaped the genetic variation and suggest that the genetic divide is probably maintained by present-day dispersal limitation.

DEVELOPMENTAL INSTABILITY IN WILD CHROMOSOMAL HYBRIDS OF THE HOUSE MOUSE

In wild populations of the house mouse from Tunisia, fluctuating asymmetry and character size of tooth traits were compared between chromosomal races (2n = 40, all acrocentric standard karyotype, and 2n = 22, with nine fixed Robertsonian fusions) and their natural hybrids. Developmental stability was impaired in hybrids compared to both parental groups. Because genetic divergence measured by allozyme markers was low, genomic incompatibilities were not expected between the chromosomal races. This suggests that differentiation of gene systems specifically involved in development may have occurred between the chromosomal races. Support for the latter was found in the study of character size which showed that the 2n = 22 mice had smaller teeth than either the hybrid or the standard mice. The study of Tunisian chromosomal races thus shows that chromosomal evolution may lead to important changes in coadapted gene systems without involving extensive genic differentiation.

Genetic variation and population structure of the carpet shell clam Ruditapes decussatus along the Tunisian coast inferred from mtDNA and ITS1 sequence analysis

Surveys of allozyme polymorphisms in the carpet shell clam Ruditapes decussatus have revealed sharp genetic differentiation of populations. Analysis of population structure in this species has now been extended to include nuclear and mitochondrial genes. A partial sequence of a mitochondrial COI gene and of the internal transcribed spacer region (ITS-1) were used to study haplotype distribution, the pattern of gene flow, and population genetic structure of R. decussatus. The samples were collected from twelve populations from the eastern and western Mediterranean coasts of Tunisia, one from Concarneau and one from Thau. A total of twenty and twenty-one haplotypes were detected in the examined COI and ITS1 regions respectively. The study revealed higher levels of genetic diversity for ITS1 compared to COI. The analysis of haplotype frequency distribution and molecular variation indicated that the majority of the genetic variation was distributed within populations (93% and 86% for COI and ITS1 respectively). No significant differentiation was found among eastern and western groups on either side of the Siculo-Tunisian strait. However, distinct and significant clinal changes in haplotypes frequencies between eastern and western samples were found at the most frequent COI haplotype and at three out of five major ITS1 haplotypes. These results suggest the relative importance of historical processes and contemporary hydrodynamic features on the observed patterns of genetic structure.

Morphological differentiation among geographically close populations of the green crab Carcinus aestuarii Nardo, 1847 (Brachyura, Carcinidae) from the Tunisian coast

In the present study, we assessed patterns of morphological variation among populations of the Mediterranean green crab Carcinus aestuarii Nardo, 1847 from the Tunisian coast. Eleven morphometric traits were measured on 464 specimens from nine sites, and the pattern of morphological variation among sites was assessed using multivariate approach. Non-parametric MANOVA (NPMANOVA) and non-metric Multidimensional Scaling (MDS) analyses revealed statistically significant differences among the studied locations for both sexes proving that natural variability of the measured traits differs clearly between locations. Canonical discriminant analysis (CDA), on the other hand, showed that over 36% of individuals could be assigned to their location of origin on the basis of their morphology. Correct assignment of individuals to their location was much higher (over 63%) when sites were combined into three regional groups and marked morphological differentiation was found in both sexes. A positive correlation was noted between the squared Mahalanobis distances (D2) and the geographic distances for both male and female green crab populations suggesting isolation by distance. The results obtained indicate that morphological divergence among populations of the green crab can occur over shorter distances than expected based on dispersal potential. Possible drivers of the observed pattern of morphological variation are discussed.

Karyological analysis of four Tunisian species of Anura (Amphibia)

ABSTRACT We present the results of a chromosomal study (C-, Ag-NOR-, endonuclease digestion, DAPI and CMA3 banding) carried out on four Tunisian species of amphibians: Bufo mauritanicus, B. viridis, Rana saharica and Discoglossus pictus auritus. For comparative purposes, our cytogenetic analysis was also extended to Moroccan specimens of B. mauritanicus and D. pictus scovazzi. Tunisian specimens of B. viridis display chromosome and chromatin characters very similar to those found in other specimens, both North African and Euro-Asian. By contrast, chromosomes of Tunisian specimens of B. mauritanicus differed from the Moroccan specimens in presenting a different Q-banding phenotype. Tunisian specimens of R. saharica displayed the plesiomorphic Pelophylax karyotype of 2n = 26 biarmed chromosomes and NORs on the eleventh pair. The Tunisian specimens of D. pictus auritus had the typical D. pictus 2n = 28 karyotype with the 13th pair of telocentric elements, in contrast to the Spanish auritus specimens that display the chromosomes of this pair shaped as metacentric. Similarities and differences of the four studied Tunisians anuran taxa are discussed with references to published data on their relatives.

Allozyme Variation and Population Genetic Structure in the Carpet Shell Clam Ruditapes decussatus Across the Siculo-Tunisian Strait

This study reports on the polymorphism of 15 allozyme loci in Ruditapes decussatus clams collected from 11 locations along the Tunisian coasts. We concentrated our sampling effort around the Siculo-Tunisian region to verify if any population structuring exists in this region and to identify the factors that have shaped this structure. Measurements of genetic diversity were quantified both within and between populations, and the geographic variability of gene frequencies was analyzed. Our study shows that the Siculo-Tunisian Strait is an important genetic boundary between eastern and western regions, which agrees with findings for a variety of other species. We suggest that vicariance is a predominant factor shaping the current distribution of genetic diversity of R. decussatus, and the mixing of divergent gene pools from the eastern and western regions still seems to be limited by some physical and/or biological factors.

Very Low Mitochondrial Diversity and Genetic Homogeneity in the Starfish Echinaster sepositus Along the Tunisian Coast

Phylogeographic structure in marine organisms is ascribed to contemporary and/or historical barriers to dispersal (Benzie and Williams 1997; Waters and Roy 2003). In addition, life history mode can exert a strong influence on the genetic structure of populations (Hart et al. 1997; Riginos and Victor 2001), as illustrated by the consequences of the extreme heterogeneity of dispersal capability on geographic distribution of genetic polymorphism (Kinlan and Gaines 2003). For instance, some species with long-lived planktotrophic larvae show vast geographic distributions and limited genetic structure (Booth and Ovenden 2000; Colgan et al. 2005). Conversely, taxa with brief larval stages or direct development often show marked phylogeographic structure (Collin 2001; Planes et al. 2001; Riginos and Victor 2001). Mitochondrial DNA is commonly used as a genetic marker to infer phylogeographic relationships in most marine organisms (Avise 2000). The gene coding for cytochrome c oxidase subunit I (COI) is among the most frequently used markers because of its high degree of variability (e.g., Avise 2000; Duran et al. 2004a; Lee 2000). Among marine invertebrates, few examples of almost negligible levels of intraspecific variation in COI have been reported for basal phyla such as porifera and anthozoa (Duran et al. 2004b; France and Hoover 2002; Shearer et al. 2002). In asteroids, the COI gene has proven to be highly polymorphic (Flowers and Foltz 2001; Waters et al. 2004).

Patterns of genetic and morphometric diversity in the marbled crab (Pachygrapsus marmoratus, Brachyura, Grapsidae) populations across the Tunisian coast

The present study reports on population structure analysis of the marbled crab Pachygrapsus marmoratus (Fabricius, 1787) from the Tunisian coast, an appropriate location to study biogeographical processes because of the presence of a well-known discontinuous biogeographic area (the Siculo-Tunisian Strait). Patterns of morphological and genetic variation of this highly dispersive and continuously distributed decapod species were assessed among its geographically close populations which cover almost the entire Tunisian coastline. A total of 386 specimens from nine sites were collected and examined for morphometric variability at 14 morphometric traits. The results of multivariate analyses of linear morphometric traits showed the existence of sexual dimorphism in this species by PERMANOVA (Permutational multivariate analysis of variance). In addition, both CDA (Canonical discriminant analysis) and NPMANOVA (Non parametric MANOVA test) analyses revealed statistically significant differences among the studied locations for both sexes. Overall, the outcome of CDA analysis showed that over 87% of individuals could be assigned correctly to three regional groups in both sexes (North, Center and South). Specifically, SIMPER (Similarity Percentages) analysis showed that carapace length, carapace width and merus length were major contributors to the morphometric separation between populations. The pattern of phenotypic variation suggested by morphometric analyses was found to be highly discordant with that suggested by the analysis of a mitochondrial marker (cytochrome oxidase I, COI). Indeed, the results inferred from restriction fragment analysis of the COI in 180 crabs, suggested high genetic homogeneity. Very low levels of haplotype diversity (h) were found in almost all the studied populations, associated with non significant genetic distances for nearly all population comparisons. Explanations to these morphometric and mtDNA patterns as well as the discrepancy between them are discussed.

Phylogeography of the marbled crab Pachygrapsus marmoratus (Decapoda, Grapsidae) along part of the African Mediterranean coast reveals genetic homogeneity across the Siculo-Tunisian Strait versus heterogeneity across the Gibraltar Strait

ABSTRACT We investigate the influence of previously postulated biogeographic barriers in the Mediterranean Sea on the population genetic structure of a highly dispersive and continuously distributed coastal species. In particular, we examine nuclear and mitochondrial genetic variation in the marbled crab, Pachygrapsus marmoratus, across part of the African Mediterranean coast in order to assess the influence of the Siculo-Tunisian Strait on its population genetic structure. Four polymorphic microsatellite loci were genotyped for 110 individuals, collected from eight locations covering parts of the Algerian, Tunisian and Libyan coasts. In addition, mtDNA corresponding to the Cox1 gene was sequenced for 80 samples. The corresponding results show contrasting patterns of genetic differentiation. While mtDNA results revealed a homogeneous haplotype composition in our study area, microsatellite data depicted genetic differentiation among populations, but not associated with any geographic barrier. This pattern, already recorded for this species from different geographic regions, may hint at the involvement of a complex series of abiotic and biotic factors in determining genetic structure. Demographic history reconstruction, inferred from mtDNA data, supports demographic and spatial expansion for the North African metapopulation dating back to the Mid-Pleistocene and following an historical bottleneck. Comparison of these African mitochondrial sequences with new sequences from a Turkish population and previously published sequences revealed a weak but significant separation of Atlantic and Mediterranean populations across the Gibraltar Strait, which was not recorded in previous studies of this grapsid species.

High genetic connectivity among morphologically differentiated populations of the black sea urchin Arbacia lixula (Echinoidea: Arbacioida) across the central African Mediterranean coast

The present study represents the first population structure analysis of the black sea urchin Arbacia lixula across part of its African Mediterranean distribution range, the Tunisian coast. This location is appropriate for the study of biogeographical processes given the presence of a well-known discontinuous biogeographic area (the Siculo-Tunisian Strait). Patterns of morphological and genetic variation in this highly dispersive echinoid species were assessed among its populations from the western and eastern Mediterranean coasts of Tunisia. A total of 109 specimens from six sites were collected and examined for morphometric variability in seven morphometric traits. Concordant results, inferred from CDA analyses, pairwise NPMANOVA comparisons and MDS plot, showed significant inter-population differences in the measured traits among the studied populations. In addition, UPGMA clustering and discriminant/Hotelling analysis enabled the delineation of two morphologically differentiated groups assigned to the western and eastern Mediterranean basins. SIMPER analysis showed that total dry weight and test diameter were major contributors to the morphometric separation between locations and among groups. Despite the extensive morphological variation found in A. lixula, genetic analysis of the mitochondrial COI marker recovered only a single evolutionary lineage and showed a lack of population structure as inferred from the results of one-level AMOVA and pairwise comparisons of genetic differentiation. Possible explanations for this genetic pattern are discussed. Notably, the lack of COI divergence patterns, highlighted by shallow genealogy associated with high haplotype diversity and low nucleotide diversity, together with a recent demographic expansion event retrieved from mismatch distribution and BSP analysis could be considered a residual effect of a recent evolutionary history of the species in the Mediterranean.