Although RNase-based self-incompatibility (SI)
is suspected to operate in a wide group of plant families,
it has been characterized as the molecular genetic basis
of SI in only three distantly related families, Solanaceae,
Plantaginaceae, and Rosaceae, all described over a decade
ago. Previous studies found that gametophytic SI, controlled
by a multi-allelic S-locus, operates in the coffee family
(Rubiaceae). The molecular genetic basis of this mechanism
remains unknown, despite the immense importance of
coffee as an agricultural commodity. Here, we isolated
ten sequences with features of T2-S-type RNases from
two Coffea species. While three of the sequences were
identified in both species and clearly do not appear to be
S-locus products, our data suggest that six sequences may
be S-alleles in the self-incompatible C. canephora, and
one may be a relict in the self-compatible C. arabica. We
demonstrate that these sequences show style-specific
expression, display polymorphism in C. canephora, and
cluster with S-locus products in a phylogenetic analysis
that includes other plant families with RNase-based SI.
Although our results are not definitive, in part because
the available plant materials were limited and data
patterns relatively complex, our results strongly hint that
RNase-based SI mechanism operates in the Rubiaceae
family.
