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On the Function of Flowers
3. The genetics of outcrossing
Maintaining heterostyly
Self-incompatibility
4. Selfing as a way of life
Darwin’s pollination experiment
with the primrose:
Heterostyly is controlled by a gene S with simple inheritance:
Ss yields a short-styled flower.
ss yields a long-styled flower.
Ss  ss -> Ss and ss in a 1:1 ratio
S
s
s
Ss
ss
s
Ss
ss
Illegitimate pollinations provide a test of the proposed genetic
system…
Ss (short-styled)
flower ss (longstyled) flower.
1:1short:long
S
Ss (short-styled)
flower Ss
(short-styled)
flower.
3:1 short:long
s
S
s
ss (long-styled)
flower ss (longstyled) flower.
all are long
s
s
s
Ss
ss
S
SS
Ss
s
ss
ss
s
Ss
ss
s
Ss
ss
s
ss
ss
Pollen and Stigma
papilla size go with
style length:
pollen
short-style
long-style
papillae
http://www.biologie.uni-hamburg.de/b-online/e10/19.htm
Pollen and Stigma
papilla size go with
style length:
pollen
short-style
long-style
legitimate unions papillae
in red
pollen on stigma,
legitimate unions
http://www.biologie.uni-hamburg.de/b-online/e10/19.htm
Two kinds of self-incompatibility:
Gametophytic:
S1S2 female S1S3 male yields inhibition of S1 pollen,
not of S3 pollen
--- happens in the style
Gametophytic self-incompatibility continued:
Inhibition is through enzymes moving into pollen tube from
style.
Evidence lies in a protein found in styles of petunias with
incompatible pollen tubes, not with compatible tubes Multiple alleles are found in species to allow a diverse
definition of self---at least 75 in whiter clover and 150 in red
clover.
Two kinds of self-incompatibility:
Sporophytic:
S1S2 S1S3 does not yield inhibition of pollen
Only the same two alleles yield inhibition
--- happens on the stigma
inhibition is through preventing hydration of pollen on stigma
The Primula supergene:
five genes linked together
that
1. structurally improve
chances of legitimate
pollination and
2. inhibit illegitimate pollen
G - style length
Sp - stigma papilla length
I - self incompatibility
(sporophytic)
P - pollen grain size
A - anther height
On the Function of Flowers
3. The genetics of outcrossing
Maintaining heterostyly
Self-incompatibility
4. Selfing as a way of life
Selfing plants have little flowers, often without petals,
and grow in disturbed terrain
chickweed
Violets have two kinds of flowers, one for outcrossing, one for
selfing.
On the Function of Flowers
3. The genetics of outcrossing
Maintaining heterostyly
Self-incompatibility
4. Selfing as a way of life
aob.oupjournals.org/cgi/reprint/78/6/759.pdf
The control of pollen growth by stigma and
style
1. Maintaining heterostyly
2. Self-incompatibility-Campbell p. 788
matching alleles at the s locus prevent pollen success
(the haploid allele matches either of the diploid
style alleles)
--gametophytic (RNAses from style invade pollen
tube and shut it down - Ros, Leg, Sol)
--sporophytic (signal transduction prevents
germination of pollen on stigma, perhaps by
preventing hydration - Bra)