Mitochondrial Variability of Sand Smelt Atherina Boyeri Populations from North Mediterranean Coastal Lagoons
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MARINE ECOLOGY PROGRESS SERIES Vol. 297: 233–243, 2005 Published August 1 Mar Ecol Prog Ser Mitochondrial variability of sand smelt Atherina boyeri populations from north Mediterranean coastal lagoons L. Astolfi1, I. Dupanloup2, R. Rossi1, P. M. Bisol3, E. Faure4, L. Congiu1,* 1Department of Biology, University of Ferrara, Via Borsari 46, 44100 Ferrara, Italy 2Center for Integrative Genomics, University of Lausanne, 1015 Lausanne, Switzerland 3Department of Biology, University of Padua, Via Colombo 3, 35121 Padua, Italy 4EA Biodiversité, Université de Provence, 13331 Marseille cedex 3, France ABSTRACT: The role of transition environments, such as lagoons or deltas, in shaping the geograph- ical pattern of genetic variability may be investigated through the analysis of population structure in the species living in these habitats. Several fish species are strictly bound to deltas or coastal lagoons for at least part of their life cycle. This paper reports the analysis of mitochondrial DNA variability in several populations of Atherina boyeri (Risso, 1810) sampled along the north Mediterranean coasts from the Atlantic Ocean to the Black Sea. The results show a high genetic structure among popula- tions, probably due to the deep fragmentation of the lagoon environments in which this species spends most of its life cycle. On the other hand, a clear phylogeographic pattern and a significant pos- itive correlation between genetic and geographical distances support the hypothesis of gene flow among populations, probably due to an imprecise anadromic behaviour from the lagoons to the close coastal sea, which allows for an exchange of individuals between contiguous populations. Our results are in good agreement with those previously obtained on the same species by RAPD techniques. Our analysis among different Atherina genera are also compatible with the most recent classification of these species based on morphological and genetic characters. KEY WORDS: D-loop · Atherina boyeri · Coastal lagoons · Population genetics · Phylogeography Resale or republication not permitted without written consent of the publisher INTRODUCTION Many fish species are closely linked to lagoon envi- ronments for at least part of their life cycle. Their sub- The role of physical and geographical factors in division in genetically distinct populations is the result shaping the distribution of genetic variability is espe- of 2 opposite influences: the tendency to isolation from cially relevant for species closely bound to discon- habitat fragmentation and the tendency to homogene- tinuous habitats such as coastal lagoons (Sinclair & ity due to migrations, which are related to the dispersal Iles 1988, 1989). Besides geographical fragmentation, ability of the species (Ward et al. 1994). A relevant role these environments are characterised by relevant vari- may also be played by oceanographic features, such as ability of chemical and physical parameters: the spe- physical-chemical parameters or currents, which may cies inhabiting them are exposed to strong evolution- ease or obstruct migratory movements, thus contribut- ary pressures (Bamber & Henderson 1988). All these ing to shape phylogeographic patterns. factors may result in a high degree of population struc- The sand smelt Atherina boyeri is a very common ture, and the genetic analysis of lagoon species may species in the Mediterranean Sea. Individuals of this help to describe the phylogeographic effects of these species spend a relevant part of their life cycle within environments. brackish coastal environments (Bamber & Henderson *Corresponding author. Email: [email protected] © Inter-Research 2005 · www.int-res.com 234 Mar Ecol Prog Ser 297: 233–243, 2005 1988) where they represent a relevant resource for i.e. sequences of the mtDNA control region, to deter- predators and, mainly in the Adriatic Sea, for fisheries. mine whether the results previously obtained with Adults spawn in inshore shallow waters, and the eggs RAPD markers could find additional support (Congiu are attached to seaweed or rocks to avoid dispersion. et al. 2002). The application of RAPD analysis to popu- During larval and juvenile stages the fishes reside lation-genetic studies has often been criticised be- within lagoons. Only adults, during cold months, leave cause of limited data reproducibility and because of the inshore waters and move toward the sea until the the ‘dominant’ nature of its markers (MacPherson et al. next reproductive season (Henderson & Bamber 1987, 1993, Micheli et al. 1994, Rieseberg 1996). The com- Henderson et al. 1988, Mistri & Colombo 1988). Due to parison of the results obtained by mitochondrial mark- the low dispersal capability of these small fishes, the ers with those previously yielded by RAPD analysis percentage of animals re-entering the native lagoons is may provide interesting information about the useful- probably very high, as is also confirmed by morpholog- ness, reliability and consistency of the 2 approaches. ical studies (Berrebi & Britton-Davidian 1980); this may One relevant result obtained by RAPD analyses of have the same demographic effect of a homing behav- Italian sand smelt populations was the high genetic iour, limiting gene flow among sites. A clear genetic distance which separates the Marsala population structure among A. boyeri populations from different (southern Sicily) from all the other populations (Congiu lagoons was indeed recently observed along the Italian et al. 2002). A possible explanation for this finding coasts using random amplified polymorphic DNA could be the presence of migratory movement across (RAPD) markers (Congiu et al. 2002). However, a sig- the Sicily channel from Tunisia toward southern Sicil- nificant correlation between genetic and geographical ian coasts. This hypothesis is supported by the results distances among the sampled populations was found, observed in another lagoon fish species, Aphanius fas- thus suggesting the presence of some migratory move- ciatus, in which the population from the Marsala ments along the coastal line. Occasional exchanges of lagoon was found to be more similar to Tunisian popu- individuals among populations could be favoured by lations than to Italian ones (Maltagliati 1999). Since the annual migration of adults toward the coastal sea these 2 species exhibit low dispersal capabilities, these during the cold season, establishing a pattern of isola- migrations should be prompted by marine currents. To tion by distance along the coasts. test the above hypothesis, a few Atherina boyeri indi- This observation contrasts with the results obtained viduals from a Tunisian lagoon were included in some for another atherinid species (Odontesthes argentinen- analyses. If the genetic similarity between the north sis) inhabiting the Atlantic coasts of Brazil and African and the Marsala populations found in A. fas- Argentina, for which no significant correlation be- ciatus is observed also in the sand smelt, this may con- tween genetic and geographical distances was ob- firm the usefulness of the interspecific approach. served, when estuarine populations were compared Indeed, the same phylogeographic pattern observed in (Beheregaray & Sunnucks 2001). On the other hand, different species should represent a relevant insight marine populations, which are considered to have into the factors that shape the distribution of genetic recently colonised the estuaries, showed a clear geo- diversity across the Mediterranean Sea. graphical structure of genetic variants (Beheregaray & The taxonomic relationships within genus Atherina Sunnucks 2001). This pattern was considered the pro- are controversial due to the high morphological variabil- duct of the rapid adaptive divergence and consequent ity which characterises these fishes (Kiener & Spillmann reproductive isolation of the estuarine populations in 1969, 1972, Marfin 1982, Bamber & Henderson 1988, the face of potential gene flow along the coastal line Creech 1991, 1992). Phenotypic polymorphism is espe- (Beheregaray & Sunnucks 2001). cially high in A. boyeri. Before 1969 nearly 20 distinct The presence of an isolation-by-distance pattern in species were described in the literature until Kiener & the Italian populations of Atherina boyeri (Congiu et al. Spillman (1969, 1972) considered them all synonymous 2002) seems to indicate that the homogenising effect of with A. boyeri. The other 2 Atherina spp. known to be gene flow counteracts the reproductive isolation as a present in the Mediterranean Sea are A. hepsetus and consequence of adaptive divergence in this region. A. presbyter. Due to the high morphological variability of However, the reduced geographical scale of this study the species belonging to the genus Atherina, a wrong (Congiu et al. 2002), as well as the large size of the sam- classification of the samples could not be excluded. For pled populations, may have affected our conclusions. this reason, in order to verify the monophyly of the To investigate the genetic relationships among Athe- A. boyeri samples studied here, some individuals from rina boyeri populations at a larger geographical scale, different populations of A. hepsetus and A. presbyter we sampled 12 lagoons along the north Mediterranean were also analysed. Besides the above congeneric spe- coasts from the Atlantic Ocean to the Black Sea. We cies, well documented and formally recognised, 2 addi- also typed a different genetic marker in these samples, tional species, until now considered morphological vari- Astolfi et al.: