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Biol. Mar. Mediterr. (20 12), 19 {1): 45-48

D. D 'EsPosrTo, E. DATTOLoi, F. BADALAMENTI1, L. 0RsrNr2, G. PROCACCINI

Stazione Z oologica Anton Dohrn, Villa Comunale - 80121 Napoli, ltaly.
desposito@szn.i t

1IAMC-CNR, Istituto per l'Ambiente Marino Costiero,
Via Luigi Vaccara - 91026 Mazara del Vallo (TP), Italy.
2Katholieke Un iv. Leuven, Lab. Aquat. Ecol.& Evolutiona ry Biol.,

C h. Deberiotstraat 32 - 3000 Leuven, Belgium .

COMPARATIVE ANALYSIS OF GENETIC DIVERSITY
OF POS/DON/A OCEAN/CA ALONG A DEPTH GRADIENT

USING NEUTRAL AND SELECTIVE/NON NEUTRAL
MICROSATELLITES MARKERS

ANALISI COMPARATIVA DELLA DIVERSITÀ GENETICA
DI POSIDONIA OCEANICA LUN GO IL GRADIEN TE BATIMETRICO

USANDO MARCATORI MICROSATELLITI
NEUTRALI E SOTTO SELEZIONE

Abstract - Posidonia oceanica is an endemie seagrass of the Mediterranean Sea with relevant
conservation and ecologica/ value. In an effort to understand its adaptive potential to environmental
changes we screened the genome of P. ocean ica to identify outlier foci correlated with adaptive responses
to dijferent depths, where tlze p/ant experiences different light and temperature regimes. Depth-related
genetic diversity was analyzed in six P. oceanica (Posidoniaceae) meadows with anonymous and EST-
Iinked microsatellites markers. Our results show that EST-Iinked microsatellites give a better resolution
in separating the two analyzed deptlzs. We identified eleven putative outlier fo ci linked to depth adaptation
in the set of sixty-four microsatel/ites screened.

Key-words: Posidonia oceanica, depth, microsatellites, adaptation, genome-scan.

Introductioo - Posidonia oceanica (Posidoniaceae) forms continuous meadows
along the coasts of the Mediterranean Sea, extending from O to 40 m in depth.
Differences in morphological, phenological and physiological traits have been shown
among plants growing along the depth gradient (Buia & Mazzella, 1991). Studies
employing microsatellite loci evidenced spatial genetic differentiation in this species,
both among geographic areas and between different depth s (Serra et al., 2010).
Diversity in genetic patterns among populations can result from both neutra} and
adaptive variation. The aim of this work is to identify adaptive variation in natura!
P. oceanica populations using a genome-scan approach, a technique that allows the
identification of loci under selection or outlier loci, i.e. loci which shown an higher
or lower differentiation among populations than expected under the hypothesis
of neutrality (Storz, 2005). The genome-scan implies the use of a large number of
markers. In our analysis we used microsatellites or simple sequence repeats (SSR),
which are among the most widely used molecular markers in population genetics due
to their co-dominance, relative a bundance and high mutation rate. Besides anonymous
microsatellites, for which the precise location in the genome is unknown, SSR
located in the flanking regions of Expressed Sequence Tags (ESTs) are also utilized
in genome-scan studies. Being linked to coding sequences via genetic hitchhiking,
in fact, this markers can more li kely be under positive selection. Here we used both
anonymous and EST-linked microsatellites and we compared the results obtained
with the two classes of markers, in order to identify population-specific outliers.

Materials and methods - P. oceanica shoots were sampled for each of two depth s
(- 5 m and - 25 m) in six distinct meadows (Meloria (PI), Piombino (LI), Lacco