MOLECULAR DIVERSITY AT THE SELF-INCOMPATIBILITY LOCUS IS A SALIENT FEATURE IN NATURAL-POPULATIONS OF WILD TOMATO (LYCOPERSICON-PERUVIANUM)

A cDNA encoding a stylar protein was cloned from flowers of self-incompatible wild tomato (Lycopersicon peruvianum). The corresponding gene was mapped to the S locus, which is responsible for self-incompatibility. The nucleotide sequence was determined for this allele, and compared to other S-related sequences in the Solanaceae. The S allele was used to probe DNA from 92 plants comprising 10 natural populations of Lycopersicon peruvianum. Hybridization was conducted under moderate and permissive stringencies in order to detect homologous sequences. Few alleles were detected, even under permissive conditions, underscoring the great sequence diversity at this locus. Those alleles that were detected are highly homologous. Sequences could not be detected in self-incompatible Nicotiana alata, self-compatible L. esculentum (cultivated tomato) or self-compatible L. hirsutum. However, hybridization to an individual of self-incompatible L. hirsutum revealed a closely related sequence that maps to the S locus in this reproductively isolated species. This supports the finding that S locus polymorphism predates speciation. The extraordinarily high degree of sequence diversity present in the gametophytic self-incompatibility system is discussed in the context of other highly divergent systems representing several kingdoms.