Transcript Fast identification and statistical evaluation

Mimulus as a model for
speciation
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Long history of ecological genetic studies
Small genome (comparable to rice)
Short generation time (~2 months)
Easy to cultivate
Highly speciose genus
Diversity in mating system, floral form and habitat
Frontiers in Biological Research:
Integrated Ecological and Genomic
Analysis of Speciation in Mimulus
Michigan State Univ. Univ. of Washington
Toby Bradshaw
Doug Schemske
Duke Univ.
John Willis
Andrea Sweigart
Univ. of Montana Andrea Case
CUGI
Fred Dietrich Jeffery Tomkins
Lila Fishman
UNC Chapel Hill
Todd Vision
Selfing has evolved several times
in the Mimulus guttatus complex
Mimulus guttatus
Mimulus platycalyx
Mimulus laciniatus
Mimulus nasutus
Reproductive isolation between
M. guttatus and M. nasutus
• Largely allopatric
• Sympatry in some
ephemeral habitats
• Natural hybrids are
observed in the
field
M. guttatus
M. nasutus
M. guttatus
M. guttatus + M. nasutus
Isolating mechanisms
• Premating barriers
– Microhabitat
– Flowering time
– Cleistogamy
– Pollen tube competition
• Postmating barriers
– Partial male and female sterility of F1 and F2
hybrids
Quantitative trait loci (QTL)
P1 (+)
P2 (-)
F1 (0)
F2
+
+
0
LOD
M. guttatus x nasutus map
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600 F2 plants
174 markers over 1780 cM
~85% coverage
14 linkage groups
Fishman, Kelly and Willis (2002) Evolution 56, 2138-2155.
QTL for species differentiation
• Prezygotic isolation
– Many minor QTL underly floral morphological
differences
• Postzygotic isolation
– Hybrid sterility due to two interacting nuclear loci
– Cytonuclear incompatibility
– Meiotic drive
Dobzhansky-Muller loci and
hybrid sterility/inviability
AABB
aaBB
AAbb
X
AaBb
DM factors in M. guttatus x nasutus?
N
+
G
G
Male sterile:
N
F1
Bg
F2
Bn
0
3/16
1/4
Model: sterility requires aaB- genotype
Genotype
AABB
AABb
AAbb
AaBB
AaBb
Aabb
aaBB
aaBb
aabb
F1
0
0
0
1
0
0
0
0
0
Total Steriles
F2
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
Bg
1/4
1/4
0
1/4
1/4
0
0
0
0
Bn
0
0
0
1/4
1/4
0
0
1/4
1/4
ABn*
0
0
0
0
0
0
0
1/2
1/2
Male_sterile
no
no
no
no
no
no
yes
yes
no
0
3/16
0
1/4
1/2
* Using male steriles as dams
Fine mapping the A locus
• Analysis of linkage to A in aaBB x AaBB
• ~1000 plants screened with markers
from all over the genome
• Mapped to a ~15 cM interval
MgSTS45 MgSTS11
A
MgSTS104
Fine mapping the B locus
• Analysis of linkage to B in aaBb x aabb
• 2900 plants screened with markers from all
over genome
• Only 4 recombinants with MgSTS28
– If region is typical, this implies ~30 kb distance
0.14 cM
B
MgSTS28
0.7 cM
MgSTS606
What happens in the opposite
cross?
N
G
+
F1
F2
Male sterile:
1/4
Cytoplasmic male sterility
• Common in hermaphroditic plants and
animals
• The mechanism is typically a mitochondrial
fusion protein
• Evolutionary dynamics
– Maternally transmitted organelles that increase
female fitness at the expense of male fitness
spread rapidly
– Nuclear restorers arise that suppress CMS
– CMS can be uncovered when wide crosses
separate CMS cytoplasm from its restorer
Evolutionary dynamics of
CMS
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CMS
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CMS
R
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CMS
F2
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Isolating CMS factors
• Model: M. guttatus has male sterilizing
cytoplasm and a dominant restorer (R)
• Restorer being mapped in Rr x rr
– On linkage group 4, 20 cM from MgSTS34
• Mitochondrial factor being isolated by
looking for fusion proteins
Segregation distortion
• In F2, 50% of alleles should be from each
parent
• Eleven regions of severe segregation
distortion in guttatus x nasutus cross
– Nine in favor of guttatus
• Occurs during gamete formation in females
Female gametogenesis in
angiosperms
• Only one of the four meiotic products
leads to an egg
• Potential for conflict between alleles
Evolutionary dynamics of meiotic drive
• Distorter locus spreads due to its
transmission advantage
– Linked suppressor locus can arise to prevent drive
against itself
• Once both are fixed, the system is quiescent
until revealed by a wide cross
• May contribute to pericentromeric
heterochromatin tracts in mammals, plants
• Question: do distorters map to centromeres in
guttatus?
Are speciation genes fixed by
genetic drift or natural selection?
x
Neutrality
Population
bottleneck
Selection
B
y
Some closing thoughts
• Genomic conflicts should be stronger in
outcrossers than selfers
– CMS in guttatus
– Meiotic drive in guttatus
• Genetic incompatibilities may be accelerated
by a shift in mating system
• Not something that can be studied in
Drosophila, the predominant model for the
genetics of speciation