LETTER TO THE EDITOR

Revisiting and Revising the Self-Incompatibility Genetics of Phalaris coerulescens

In the grasses, self-incompatibility is controlled by two unlinked loci, S and Z. Previously, Li et al. 1994 presented a differential screening technique to identify a putative S gene clone from Phalaris coerulescens. In this approach, a cDNA library made from mature pollen of P. coerulescens was screened to identify sequences showing pollen-specific expression and differences in hybridization intensity when probed with cDNA made from pollen RNA isolated from plants with different S or Z genotypes. The putative S gene clone was named Bm2. No Z candidate was identified (Li et al. 1994 ). Several lines of evidence seemed to support Bm2 as representing S: (1) cosegregation of a restriction fragment length polymorphism detected with Bm2 correlated with the S genotype in over 120 plants; (2) gene expression predominantly in mature pollen; and (3) identification of a highly conserved C-terminal catalytic domain and a variable, potentially allelic, N-terminal domain in the deduced protein sequence.

A series of recent experiments, however, has shown that Bm2 in fact does not represent S. The bases for this revised conclusion are: (1) recombination between Bm2 and S. It now appears that Bm2 may be as much as 2 centimorgans away from S. Several recombinant plants have been identified that demonstrate linkage between the Bm2 and S loci; (2) sequencing of S homologs from other grasses, including members of the Phalaris genus, along with additional alleles of Phalaris coerulescens. Most Bm2-like sequences encode only a thioredoxin protein and no allelic domain. All mRNAs, moreover, show several in-frame stop codons and an ATG start directly before the thioredoxin domain. Thus, Bm2 sequences encode a novel thioredoxin, but do not contain a region that could determine allele specificity, as was previously suggested (Li et al. 1994 ).

In summary, it is now clear that Bm2 cannot represent S. Bm2 is a gene closely linked to S that may be involved in some aspect of the self-incompatibility reaction but is unlikely to be involved in the recognition process per se.

REFERENCES

Li, X., Nield, J., Hayman, D., and Langridge, P. (1994) Cloning a putative self-incompatibility gene from the pollen of the grass Phalaris coerulescens.. Plant Cell 6:1923-1932[Abstract/Free Full Text].

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