dattolo@szn.it LITERATURES CITED Here we explored variations existing, in the daily phase, along a bathymetrical cline in gene expression of circadian clock genes and phytochromes in the in the marine angiosperm Posidonia oceanica (L.) Delile in natural condition via RT-qPCR. We choose P. oceanica to its peculiarity in life style (Aires et al. 2011) and central role for marine Mediterranean biodiversity conservation (Marbà and Duarte 2009). Shoots of P. oceanica were collected by Scuba diving at two different depths (-5m and -20m) in a continuous meadow located in the Bay of Calvi, Corsica (thanks to the ESF Cost Action 0906). Sampling was performed at 06:00, 09:00, 11:30, 15:00, 18:30 and 00:00. Gene expression studies were performed on three replicate shoots for each sampling point. Targets gene were selected according to their role in the circadian rhythm pathway in plants, according to KEGG classification (Kanehisa and Goto, 2000). Primers were designed using the software UGENE (ver. 1.12) on Contigs selected from P. oceanica ESTs, obtained from the same meadow and depths via RNA-seq (D'Esposito PhD Thesis). All RT-qPCR reactions were carried out in triplicate, to capture intra-assay variability, in a Viia7 Real Time PCR System (Applied Biosystem), using the ribosomal protein L23 as reference gene (Serra et al., 2012). Expression levels for each target gene was assessed using REST-MCS © tool (Pfaffl et al., 2002) and statistical analyses were performed using Unpaired t test and GraphPad Prism ver.6 (GraphPad Software). Spearman's Rho method, implemented in PAST ver.2.17c (Hammer et al., 2001), was used to correlate gene expression of circadian genes and genes belong to photosynthetic processes previously studied using the same samples (Ruocco Master Thesis). Daily variation in -5m and -20m plants Gene expression of selected genes has been evaluated at the different time points, and for each sampled depth, using the mean of the C T for each gene overall all hours of the day as control condition. Gene expression -5m Vs.-20m Comparison of differences in gene expression between -5m and -20m samples, for each single time point. The -5m condition is used as control condition, -20m as tester condition. In this study, we investigated differences in the daily-expression of genes implied in circadian rhythm in P. oceanica in natural condition between two sampling depths. Overall, our data showed large differences in expression profiles along the bathymetrical cline, both comparing the gene expression at each single sampling-time and in the daily trend for each depth. The large part of genes investigated were differentially regulated between depths at 11:30 and 15:00, when plants experienced the maximum differences in light condition between depths. Genes related with the maintenance of the molecular clock, displayed very similar expression profiles between depths, with some exceptions. Indeed, the expression of some members of the ARR-like family (APRR7) showed the typical circadian waves, but shifted along the day between the two depths. For what concern phytochromes, the higher differences in expression profiles were showed by PHYC, CRY1 and CRY2. Indeed cryptochromes, which are blue light photoreceptors, react at the differences in light quality along the water column. A significant correlation, ranging from 0.6 to 0.8 Spearman's Rho (p < 0,05) was observed between circadian and photosynthetic genes. As displayed by the correlation network, ZTL showed the highest number of significant correlations with photosynthetic genes: this fact suggests that ZTL has a central role in the circadian regulation of photosynthesis, as already reported in the literature (Dodd et al., 2005). The variation in expression of circadian genes along the depth-cline showed by these data support further analysis on the potential adaptive value of these traits in P. oceanica response to light conditions.