Potential applications of next generation sequencing to the genomics of Posidonia oceanica

MOLECULAR STUDIES ON Posidonia oceanica ARE STILL LIMITED Despite of its paramount ecological importance in shallow shoreline habitats in the Mediterranean sea, basic knowledge of Posidonia oceanica genome is still limited, with regard to both functional and structural genomics. Concerning protein-coding sequences, a survey of NCBI nucleotide database resulted in finding only 103 putative genes (on November 5, 2012). Of these, many are putative metallothionein (MT)or aquaporin (AQP)-encoding genes. These sequences have been extensively studied, since involved in the adaptation of Posidonia to endangered environments, at both physiological and genetic levels. Sequences encoding putative type2 MTs belong to a multigene family with at least two subgroups (Giordani et al., 2000; Cozza et al., 2006). Northern hybridizations indicated that MT transcripts accumulation is constitutive and seasonally regulated. However, MT transcripts accumulated after rhyzome harvesting and after 15 days of cultivation in an aquarium. As for animal MTs, transcripts accumulation was observed also after exposure to trace metals such as copper and cadmium. The two MT subgroups showed differences in their histological expression, i.e., in proliferative tissues or in lignified or suberized cells. Concerning AQP gene family, two genes encoding AQPs of the plasmalemma and the tonoplast were isolated (Maestrini et al., 2004). Both genes were constitutively expressed in the leaves, with higher levels of transcripts in young than in differentiated leaf tissues. Variations of salt concentration in aquarium determined different AQPs transcript accumulation, suggesting that AQPs are involved in osmotic balance maintenance in seagrasses (Maestrini et al., 2004; Serra et al., 2011). When surveying the Posidonia repetitive DNA, i.e., the intergenic, putatively non-coding portion of the genome, one can realize that this genome component is even less studied than coding portion. The 2C-genome size of Posidonia was established in 5–6 pg DNA (Cavallini et al., 1995; Koce et al., 2003), posing this species in the middle range of monocots, being genome much larger than in other seagrasses as Cymodocea nodosa (1.1 pg), Zostera nolti (1.5 pg), and Z. marina (1.2 pg) (Koce et al., 2003). Such differences are largely independent on chromosome number and hence on polyploidization: P. oceanica has 2n = 20 and Zostera species have 2n = 12 (Koce et al., 2003). P. oceanica genome has been characterized biochemically (Cavallini et al., 1995; Maestrini et al., 2002). At molecular level, only a few repetitive DNA sequences are found in the NCBI nucleotide database.