Revisiting and Revising the Self-Incompatibility Genetics of <i>Phalaris coerulescens</i>

Abstract

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.