Agriculture Reference
In-Depth Information
10
Lessons on signalling in plant
self-incompatibility systems
Andrew G. McCubbin
10.1
Introduction
One of the key adaptations that have led to the success of the angiosperms is the
flower, which functions to improve reproductive efficiency in a number of ways. One
vital function performed by the flowers is screening of the genetic relatedness of
gametes prior to fertilisation. Pollen from other species is generally prevented from
germinating on the stigma or growing down the style, providing an effective inter-
specific, pre-zygotic breeding barrier. The majority of flowers are 'perfect', bearing
both male and female reproductive organs in close proximity. Left unchecked, this
results in a tendency to self-fertilise and inbreed. In response to this problem, a
variety of reproductive strategies have arisen, one of the most widespread being
termed self-incompatibility (SI). SI allows the pistil of a flower to discriminate
between pollen from the same species that is genetically related and that which
is unrelated. This self-/non-self-recognition results in the inhibition of self-pollen
germination at the stigma surface or tube growth within the style, so promoting
outbreeding. Whilst the effects of SI are consistent, i.e. rejection of genetically re-
lated pollen, both the phenomenology associated with rejection and molecular data
suggest that a wide variety of signalling mechanisms have been recruited to achieve
this goal.
SI appears to have evolved at least 21 different times during the evolution of flow-
ering plants (Steinbachs & Holsinger, 2002) and historically has been split using two
criteria. The first is whether mating types differ morphologically (heteromorphic SI)
or are anatomically indistinguishable (homomorphic SI). The second factor is the
genetic regulation of the system, the number of loci involved in encoding specificity
and whether pollen phenotype is related to its own haploid genotype (termed game-
tophytic) or the diploid genotype of its parent plant (sporophytic) (see Fig. 10.1). For
single-locus SI systems, the locus has in each case been termed
S
,but importantly
it is evident that multiple genes reside within each
S
-locus, and the allelic complex
of genes has been termed the
S
-haplotype.
Extensive molecular information is available for only three types of SI at present,
two distinct forms of single-locus gametophytic SI and the single-locus sporophytic
system of the Brassicaceae. In the most phylogenetically widespread form of single-
locus gametophytic SI found in the Solanaceae, Rosaceae and Scrophulariaceae
(Steinbachs & Holsinger, 2002), the pistil
S
-gene product is a glycoprotein (Kehyr-
Pour & Pernes, 1985) with ribonuclease activity (McClure
et al.
, 1989) and the
