STRUCTURAL AND FUNCTIONAL CONSERVATION OF S-SPECIFICITIES AMONG PYRINAE SPECIES

Gametophytic self-incompatibility (GSI) is the main mechanism that controls fertilization in many rosaceous species, including those belonging to the subtribe Pyrinae (formerly the Maloideae). In natural conditions, S-specificities are subject to frequency-dependent balancing selection; the genetic imprint left by this kind of selection is visible on the S-RNase gene in terms of high sequence diversity, evidence of positive selection, and shared ancestral polymorphisms: thus, some alleles have been maintained almost unaltered during evolution in different but related species, as is the case of Malus and Pyrus species. We have questioned whether the same expected features can be extended to whole S haplotypes, thus including not only the female S determinant (the S-RNase) but also its male counterpart, most likely provided by S-locus F-box Brothers (SFBB) genes - even though direct functional evidence is still needed. On the one side, coevolution between female and male S genes has been postulated several times, given that the generation of any new Sspecificity requires a coordinated change on both sides in order to maintain the full S-haplotypes functionality. But on the other side, recent models for S-RNase-based GSI suggest a key difference between the control of female and male S functions: while the first entirely depends on the single S-RNase gene, the second might likely be provided by multiple SFBB genes acting in a collaborative mode. Even though this hypothesis makes it necessary to re-discuss the mode of coevolution between the female and male S genes, it might provide a suitable explanation for the complex phylogenetic profiles of SFBB genes, which are only in part in agreement with that of the S-RNase. Phylogenetic and segregation analyses of S-locus genes, together with the increasing amount of genomic information available for apple and pear, provide a valuable tool for understanding both the molecular mechanism of SRNase-based GSI, and the complex evolutionary pattern of S-haplotypes.