Diversity of S-alleles and mate availability in 3 populations of self-incompatible wild pear (Pyrus pyraster)

Small populations of self-incompatible plants may be expected to be threatened by limitation of compatible mating partners (i.e. S-Allee effect). However, few empirical studies have explicitly tested the hypothesis of mate limitation in small populations of self-incompatible plants. To do so, we studied wild pear (Pyrus pyraster), which possesses a gametophytic self-incompatibility system. We determined the S-genotypes in complete samplings of all adult trees from three populations using a PCR-RFLP approach. We identified a total of 26 different S-alleles, homologous to S-alleles of other woody Rosaceae. Functionality of S-alleles and their Mendelian inheritance were verified in artificial pollination experiments and investigations of pollen tube growth. The smallest population (N = 8) harboured nine different S-alleles and showed a mate availability of 92.9%, while the two larger populations harboured 18 and 25 S-alleles and exhibited mate availabilities of 98.4% and 99.2%, respectivel...