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We use both molecular and experimental
tools to
study plant mating systems and life-history evolution. Current
work focuses on two issues: 1) The molecular evolution of the
self-incompatibility locus, and 2) The evolution of mating system
diversity in higher plants.
I. SELF-INCOMPATIBILITY IN THE SOLANACEAE
Many plants reject their own pollen to
avoid the detrimental
effects of inbreeding. Self pollen rejection is often controlled
by a single locus (S). If the allele carried by the pollen matches
either allele in the female parent, pollen tube growth is arrested.
Rare alleles at this locus have a selective advantage, being
compatible with more potential mates. This frequency-dependent
selection leads to some of the highest levels of polymorphism
known for any locus with 30-50 alleles often segregating within
single populations. The number of alleles maintained in a population
provides a genetic means of estimating effective population size.
At the molecular level allelic lineages are often tens of millions
of years old, older than the species in which they currently
reside. This is reflected in the fact that an allele found in
one species is often more closely related to an allele found
in another species than it is to other alleles from the same
species. Because of this property, the S-locus provides a tool
for historical inference that extends much deeper in time than
neutral variation.
Using RT-PCR to amplify S-alleles from
stylar tissue, we
can rapidly survey S-allele diversity within and between natural
populations and simultaneously gain sequence information that
can be used to study evolutionary processes above the species
level. At the ecological level, we use this locus to study the
relationship between the ecological characteristics of species
and their effective population size, a parameter of fundamental
importance to evolution and conservation. The locus is also useful
for detecting the frequency of population restrictions that occurred
millions of years in the past. Such restrictions are required
for founder event speciation models, so examination of the S-locus
allows the frequency of this mode of speciation to be assessed.
Finally, we uncover closely related sequences which may lead
to an understanding of the relationship between sequence differences
and rejection specificity.
II. THE EVOLUTION OF PLANT MATING SYSTEMS
Flowering plants exhibit far more
breeding system diversity
than do vertebrate animals. We use both experimental and phylogenetic
approaches to test evolutionary hypotheses concerning mating
system diversity. Collaborating work and work originating in
our lab involved the evolution of separate sexes vs. hermaphroditism
and self-fertilization vs. outcrossing.
Currently, we are trying to discern the
process(es) involved
in creating a strong locally structured geographic pattern of corolla
color variation in the species complex, Mimulus aurantiacus (bush
monkeyflower). Because mating traits have strong effects on fitness,
polymorphic systems such as these provide unusually good opportunities
to study the mechanics of natural selection.
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