**7. Conclusions**

392 DNA Repair

In the present study, we sequenced the coding regions of the Scots pine putative *RAD51* (GeneBank accession number: JN566226) and *KU80* (GeneBank accession number: JN566225) genes. The predicted amino acid sequences of the Scots pine RAD51 and KU80 proteins showed 77% and 41% identity to the Arabidopsis RAD51 and KU80 proteins, respectively. Blast searches in NCBI databases (http://www.ncbi.nlm.nih.gov) were performed for *recA/RAD51*-like genes as well as for KU80-like genes from various organisms, particularly from the species whose genomes have been completely sequenced. The nucleotide sequences were used for the reconstruction of the evolutionary history of the *recA/RAD51* gene family and *KU80* genes. In the case of other conifers, for which no unigene sequences were available, expressed sequence tag (EST) information was employed to reconstruct a contig containing the complete coding sequence. The nucleotide sequence alignments were performed with ClustalX (Thompson et al. 1997). Phylogenetic analyses were conducted in MEGA4 (Tamura et al. 2007) using the maximum parsimony (MP) method with close-neighbor-interchange algorithm (Nei and Kumar 2000). The bootstrap method (Felsenstein 1985) with 500 replicates was used to

In the phylogenetic tree, *recA* and *RAD51* sequences formed separate branches that were supported by 100% of the bootstraps (Fig. 2). Thus, the result supported the view that eukaryotic *recA* and *RAD51* genes have different evolutionary histories. The phylogenetic analysis suggested a common eukaryotic ancestor for *RAD51* genes, whereas eukaryotes seem to have acquired *recA* genes through horizontal gene transfer from bacteria. Endosymbiotic transfer of *recA* genes may have occurred from mitochondria and chloroplasts to nuclear genomes of ancestral eukaryotes (Lin et al., 2006). Both *RAD51* and *KU80* sequence-based phylogenies (Fig. 2 and 3) were in accordance with the current view

Fig. 3. Phylogenetic analysis of *KU80* sequences.

evaluate the confidence of the reconstructed trees.

Plants are sessile organisms, which are continuously exposed to a wide variety of biotic, abiotic or developmental stresses, which can cause DNA damages directly or indirectly via generation of ROS. In pines, the mechanisms maintaining genomic integrity must be efficient because the observed mutation rates in somatic cells are not high despite the long lifetime of the organisms. In pines, seed development includes developmentally programmed stresses as well as the strictly co-ordinated action of several cell death programs. Furthermore, pine seed represents an interesting inheritance of seed tissues and anatomically well-described sequences of embryogenesis. Thus, the pine seed provides a favorable model for the study of the effects of a variety of endogenous DNA damaging agents as well as developmentally regulated and environmental stresses on genome integrity. Due to the high evolutionary conservation of the DNA repair mechanisms, the pine seed, as a model system, may also shed light on the mechanisms that contribute to longevity and ageing in eukaryotes in general – things of great interest also with regard to the health of human beings.
