The variable lymphocyte receptors (VLRs) of jawless vertebrates such as lamprey and hagfish are composed of highly diverse modular leucine-rich repeats. Each lymphocyte assembles a unique VLR by rearrangement of the germline gene. In the lamprey genome, we identify here about 850 distinct cassettes encoding leucine-rich repeat modules that serve as sequence templates for the hypervariable VLR repertoires. The data indicate a gene conversion-like process in VLR diversification. Genomic analysis suggested a link between the VLR and platelet glycoprotein receptors. Lamprey lymphocytes express two putative deaminases of the AID-APOBEC family that may be involved in VLR diversification, as indicated by in vitro mutagenesis and recombination assays. Vertebrate acquired immunity could have therefore originated from lymphocyte receptor diversification by an ancestral AID-like DNA cytosine deaminase.

In many flowering plants, self-fertilization is prevented by an intraspecific reproductive barrier known as self-incompatibility (SI), that, in most cases, is controlled by a single multiallelic S locus. So far, the only known S locus product in self-incompatible species from the Solanaceae, Scrophulariaceae and Rosaceae is a class of ribonucleases called S RNases. Molecular and transgenic analyses have shown that S RNases are responsible for pollen rejection by the pistil but have no role in pollen expression of SI, which appears to be mediated by a gene called the pollen self-incompatibility or Sp gene. To identify possible candidates for this gene, we investigated the genomic structure of the S locus in Antirrhinum, a member of the Scrophulariaceae. A novel F-box gene, AhSLF-S2, encoded by the S2 allele, with the expected features of the Sp gene was identified. AhSLF-S2 is located 9 kb downstream of S2 RNase gene and encodes a polypeptide of 376 amino acids with a conserved F-box domain in its amino-terminal part. Hypothetical genes homologous to AhSLF-S2 are apparent in the sequenced genomic DNA of Arabidopsis and rice. Together, they define a large gene family, named SLF (S locus F-box) family. AhSLF-S2 is highly polymorphic and is specifically expressed in tapetum, microspores and pollen grains in an allele-specific manner. The possibility that Sp encodes an F-box protein and the implications of this for the operation of self-incompatibility are discussed.

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