A genetic study of unilateral incompatibility between diploid (1EBN) Mexican species Solanum pinnatisectum and S-cardiophyllum subsp cardiophyllum

Many angiosperms have developed mechanisms to prevent self pollination and inbreeding. The most widespread system of self incompatibility is gametophytic, in which pollen tube growth is inhibited in the style by the action of a single locus (S-locus). Similar inhibition of pollen tube growth can also be observed in interspecific crosses, where successful pollinations occur in only one direction, termed unilateral incompatibility. We observed unilateral incompatibility in crosses between Solanum pinnatisectum and S. cardiophyllum, and viable interspecific hybrid seed was generated only when S. pinnatisectum was the pistillate parent. Segregations in the backcross to S. cardiophyllum revealed two independently segregating putative loci controlling interspecific crossability. Both putative loci were inherited independently of the S-locus. Segregation ratios in the backcross to the S. pinnatisectum parent did not agree with those observed in the backcross to the S. cardiophyllum parent. These reciprocal backcross families revealed that unilateral incompatibility is not conditioned by a single locus, and is inherited independently of the S-locus. Segregation disparity between the two backcross families may be due to distorted transfer of specific alleles or the presence of different mechanisms controlling pistil and pollen recognition systems in the two species.