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Title: Analysis of Malus S-RNase gene diversity based on a comparative study of European wild apple, old and modern apple cultivars

item Fazio, Gennaro

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2009
Publication Date: 2/19/2010
Citation: Dreesen, R., Vanholme, B., Luyten, K., Van Wynsberghe, L., Fazio, G., Roldan-Ruiz, I., Keulemans, J. 2010. Analysis of Malus S-RNase gene diversity based on a comparative study of European wild apple, old and modern apple cultivars. Molecular Ecology. Molecular Breeding DOI: 10.1007/s11032-010-9405-5.

Interpretive Summary: Apple flowers cannot self fertilize. This is due to the presence of genes (S-RNAses) that prevent pollen from fertilizing apple flowers that have the same genes. Because of this phenomenon, new forms of these genes are always being promoted through mutation and natural selection. This study has identified new forms of this gene in wild apple species and shown that there has been a significant loss in diversity due to domestication. This study also elucidates the functional structures of the S-RNAse protein by modeling tertiary structures.

Technical Abstract: Malus S-RNase genetic diversity was analyzed under natural circumstances (Malus sylvestris) and controlled mating (Malus x domestica). By PCR-based methods the S-RNase genotype of 196 European wild apple trees (M. sylvestris) from 12 sites, 140 M. x domestica cultivars and 27 interspecific hybrids was determined. Thirty-six different S-alleles were identified in M. sylvestris, compared to 27 and 17 in old and modern M. x domestica cultivars indicating the negative influence of domestication and breeding on S-RNase diversity. Study of S-allele diversity within the M. sylvestris geographical subgroups showed that Malus S-allele differentiation is high and that S-allelic frequencies are in equilibrium. A significant part (12.5%) of the M. sylvestris S-RNase alleles remains unknown. The majority of the known S-alleles are shared by both species which illustrates their close relatedness. 16 potentially novel allelic sequences, mostly unique for M. sylvestris, were identified. Genealogical analysis shows that 5 of these novel alleles form a separate clade with S5 and S34. By comparison of Malus S-RNase sequence information, S-RNase genetic diversity was further explored. Despite extreme length polymorphisms, the extremities of the single intron region tend to be much conserved. Alignment of deduced partial protein sequences shows that only one residue –being a cysteine- is conserved in the Malus S-RNase PS1-region. Based on a modeled tertiary structure of S4-RNase, we concluded that this is essential for the anchoring of a loop to the protein core providing additional stability to the secreted protein.