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Current Biology, Vol. 12, R424–R426, June 25, 2002, ©2002 Elsevier Science Ltd. All rights reserved.
Self-incompatibility: How to Stay
Incompatible
Deborah Charlesworth
Plant self-incompatibility is controlled by different
genes for the recognition reactions of pollen and
stigmas, yet correct association of the two genes
have been maintained in two Brassica species.
The self-incompatibility genes of flowering plants
control recognition reactions that allow self-incompatible plants to reject their own pollen, ensuring that
their ovules are available for outcrossing. The Brassica
system (Figure 1) involves a pollen surface protein,
known as SCR or SP11 [1,2], which is recognised by
a a receptor kinase, SRK, present on the surface of
stigmatic papillae [3] where pollen grains must germinate to grow down to the ovary and fertilise the
ovules. Incompatible pollen grains are generally
blocked before germination in Brassica, but the stigma
remains available to compatible pollen from other individuals. Pollen carrying rare alleles of self-incompatibility genes can pollinate a higher fraction of the
population than those carrying common alleles, which
will often arrive on a recipient plant whose stigma
expresses the same incompatibility type and consequently be rejected.
There is thus an advantage for new specificities to
arise, and once present, alleles are only rarely eliminated from a species. As a result, the self-incompatibility (S) loci are highly polymorphic, with tens of
alleles whose sequences are extremely diverged from
one another. The most similar SP11 alleles within
Brassica species rarely differ in fewer than 10% of
their amino acids, and usually they are much more
diverged than this [4–6] (Figure 2), showing that they
have been maintained for extremely long times, probably longer than the age of the species in which they
are now studied. One would predict that, during this
time, the correct association must be maintained
between each gene for a pollen SCR ‘key’ and the
cognate gene for the respective stigma SRK ‘lock’.
This prediction has now been investigated for the first
time in Brassica [5,6].
It has long been assumed that suppressed recombination in S-locus regions maintains correct relationships between the two components of the system. A
recombinant haplotype, with the pollen allele of one
incompatibility type and the stigma allele of a different
one, would be a self-compatibility mutation, which
would probably quickly be eliminated from self-incompatible populations. There is some circumstantial evidence for suppressed recombination in the S-locus
region. Sequence divergence is high between different
Institute of Cell, Animal and Population Biology, University of
Edinburgh, Ashworth Laboratory, King’s Buildings, West
Mains Road, Edinburgh EH9 3JT, UK.
PII S0960-9822(02)00915-6
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haplotypes, particularly in the regions outside the Slocus [7,8] and the haplotypes have different gene
arrangements (Figure 1). On the other hand, analyses
of patterns of diversity in S-locus sequence data, particularly the observation that linkage disequilibrium
between polymorphic sites decays with distance
between the sites, suggest that some recombination
occurs in the region of the S-locus [9–11].
It is now becoming possible to test the prediction
that the pollen and stigma genes are maintained in
particular co-adapted combinations, and that recombinant haplotypes are absent from populations. Plants
with the same incompatibility type should have similar
sequences at both the SP11 and SRK loci, while haplotypes from different incompatibility types should
differ at both loci. Conversely, the presence of very
similar SRK or SP11 sequences would imply the same
specificity. This is true for fungal incompatibility types
[12], but because incompatibility testing is laborious in
plants — especially in sporophytic self-incompatibility
systems such as that in Brassica, with recessive
alleles — sequence similarity is often simply assumed
to imply functional identity.
SRK and SP11 sequences are available from several
Brassicas, including B. oleracea (cabbages and kale)
and B. rapa (or campestris, the turnip and rape). Some
sequences are very similar in both species. SRK allele
7 of B. oleracea differs by only 5% of amino acids
from B. rapa allele 46. Now their SP11 sequences
have also been compared and, as predicted, they are
also unusually similar (Figure 2). Testing incompatibility between species is difficult because, in addition to
self-incompatibility, stigmas reject foreign species’
pollen, but the specificities of these alleles were
shown to be the same [6]. Thus, not only has this Sallele been preserved since before these species
separated, but the two component genes, SRK and
SP11, have maintained their association. Other alleles
probably also pre-date the split of these species.
Among the twenty two B. rapa SP11 alleles sequenced,
four are highly similar to sequences from B. oleracea
(Figure 2) [4–6].
There are no data from other genes whose divergence might suggest how long these species have
been separated, but alleles with similar sequences
must descend from a common ancestral allele, and
differences between them probably arose since the
species diverged. Despite their striking overall similarity, the B. oleracea allele 7 and B. rapa allele 46 haplotypes differ greatly in some regions, probably
because of insertions in B. oleracea (Figure 1B). Even
excluding rearranged regions, nucleotide divergence
is about 2% to 9% in different regions, and amino acid
site differences in the exons range from 4% to 8%
[5,6] implying a separation of roughly 2–3 million years
(though there is no reliable molecular clock for plants).
A similar estimate comes from camparison of the most
similar SP11 pair, B. rapa allele 40 and B. oleracea
Figure 1.
(A) Various Brassica oleracea and B. rapa
S-gene haplotypes, showing the stigmaexpressed S-domain genes, SRK and
SLG, the anther/pollen expressed
SCR/SP11 gene, and other genes not
labelled individually. Genes are shown as
arrows, with the arrowhead indicating the
directions of transcription (different in different haplotypes). Grey lines indicate
genes not detected in the B. rapa S46 haplotype. (B) Regions of the SRK and
SCR/SP11 genes whose sequences have
been compared [6]. Green triangles indicate insertions of sequence in B. oleracea.
SLG9
SRK9
A
SCR
SP11
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oleracea S9
oleracea S8
13kb
rapa-S46
Regions of sequence compared
S-domain
SRK
promoter
SRK
SP11
SP11
promoter
1kb
B
allele 5, whose protein products have just five amino
acid differences and no synonymous differences.
Within B. rapa, recessive alleles are moderately
diverged from one another, but there is a large excess
of DNA sequence changes that cause amino acid
differences over synonymous ones, clear evidence
that selection has favoured variants with amino acid
replacements, presumably new SP11 specificities [5].
How new specificities can arise in a two-locus system
remains a mystery, as a change in just one component
of the haplotype will not produce a new functional
specificity, but merely self-compatibility [13–15]. The
sequence similarity among the recessive alleles —
and lesser, but still appreciable, similarities among
dominant alleles — compared with the gulf between
dominant and recessive sets, suggests that new
Brassica alleles arise from pre-existing ones with the
same dominance.
This implies that new specificities do not require
major sequence changes, such as exchanges between
very different sequences (which certainly seem unlikely
to occur between such different haplotypes). It also
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implies that dominance and specificity are determined
by separate sequence features. Indeed, control of
expression of the pollen gene is turning out to have
surprising features. In Brassica and Arabidopsis lyrata,
a distantly related self-incompatible species in the
Brassica family, homozygous dominant alleles are
expressed sporophytically in tapetal cells of the
anthers and gametophytically in pollen, while recessive
alleles have only sporophytic expression [16]. In heterozygotes with dominant alleles, however, expression
of the recessive allele was undetectable [5,16].
It is remarkable that the dominance of different
alleles is apparently the result of suppressed mRNA
expression of recessive pollen alleles in the presence
of dominant ones. This extraordinary feature can explain
the old puzzle of how dominance can differ between
the pollen and stigmatic incompatibility reactions in
many self-incompatible plants. In Brassica, it may
reflect the ancient origins of these two classes of Salleles, in which their entire haplotypes, including presumably the regions that control expression, differ
between alleles, though such differences would never
have been predicted. As so often in the history of selfincompatibility, a new result has led to more new
interesting questions. It will be fascinating to discover
the mechanism, and test whether systems with more
than two levels of dominance behave in this manner.
70
60 Amino
acid
50
identity
40 (%)
30
20
10
7 12
32
40
Within
B. oleracea
46
47
46 47
8
5
7
0
8 5
Within B. rapa
40
12 32 Between species
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Figure 2.
Results of comparing sequences of SCR/SP11 alleles from
haplotypes with different specificities — indicated on the horizontal axes — within B. oleracea and B. rapa (blue columns)
contrasted with the similar sequences of some alleles between
the two species (pink columns).
References
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