3. The genetics of outcrossing

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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

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Darwins pollination experiment with the primrose
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Heterostyly is controlled by a gene S with simple
inheritance
Ss yields a short-styled flower.
ss yields a long-styled flower.
Ss ? ss -gt Ss and ss in a 11 ratio
S
s
s
Ss
ss
ss
s
Ss
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Illegitimate pollinations provide a test of the
proposed genetic system
Ss (short-styled) flower ?ss (long-styled)
flower. 11shortlong
Ss (short-styled) flower ?Ss (short-styled)
flower. 31 shortlong
ss (long-styled) flower ?ss (long-styled)
flower. all are long
s
s
S
s
S
s
s
ss
ss
s
Ss
ss
S
SS
Ss
ss
ss
ss
s
ss
s
Ss
s
Ss
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short-style
long-style
Pollen and Stigma papilla size go with style
length
pollen
papillae
http//www.biologie.uni-hamburg.de/b-online/e10/19
.htm
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short-style
long-style
Pollen and Stigma papilla size go with style
length
pollen
papillae
legitimate unions in red
pollen on stigma, legitimate unions
http//www.biologie.uni-hamburg.de/b-online/e10/19
.htm
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Two kinds of self-incompatibility Gametophytic
S1S2 female ?S1S3 male yields inhibition of S1
pollen, not of S3 pollen --- happens in the
style
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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.
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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
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• The Primula supergene
• five genes linked together that
• structurally improve chances of legitimate
  pollination and
• inhibit illegitimate pollen

G - style length
Sp - stigma papilla length
I - self incompatibility (sporophytic)
P - pollen grain size
A - anther height
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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

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Selfing plants have little flowers, often without
petals, and grow in disturbed terrain
chickweed
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Violets have two kinds of flowers, one for
outcrossing, one for selfing.
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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

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aob.oupjournals.org/cgi/reprint/78/6/759.pdf
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The control of pollen growth by stigma and style

• Maintaining heterostyly
• 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)