1. Evolution of floral morphology
A primary research interest is in the
evolution of functional integration among traits, that is, how do groups
of traits such as an organ or organ system evolve to work together to
perform a single function? The flower is an excellent model system to
address this question. Flowers are made up of a number of different parts
(petals, pistils, stamens, etc.) that must work together to perform one
crucial function: sexual reproduction.
Since each of the floral parts is itself
a complex trait, affected by many gene loci and the environment, the tools
of quantitative genetics are appropriate for this research. Genetic
integration among any two complex traits can be quantified as the genetic
correlation, which measures the degree to which the two traits are
affected by the same genes (pleiotropy) or the same groups of genes
(linkage disequilibrium). If there is selection for increased functional
integration between two traits, then an increased genetic correlation can
evolve, so that the two traits are then inherited together.
This evolution of increased correlation
for functional integration may have occurred between the lengths of the
filaments and corolla tube in several species of Brassicaceae, including
my main study species, wild radish (Raphanus raphanistrum).
The correlation between filament and
corolla tube determines the position of the anthers relative to the
opening of the corolla tube, often referred to as anther exsertion (Fig.
1). Since the insect pollinators remain in the open part of the corolla,
flowers with anthers that are placed too high or two low may not be as
successful at placing their pollen on insects as flowers with intermediate
anther placement. If so, this would create selection for an increased
correlation between filament and corolla tube lengths to maintain this
intermediate anther position regardless of flower size. |
Measurements of selection on
correlations: To see whether plants with intermediate anther position do
have higher fitness as hypothesized, we measured selection on anther
position and several other floral traits in three field seasons in
Illinois. Since anther position should mainly affect male fitness, that
is, the ability to sire seeds on other plants, measurements of male
success were necessary. Indeed, theory predicts that most selection on
floral traits should be through differences in male fitness rather than
female fitness (seed production), but since male fitness is so difficult
to measure this prediction has gone largely untested. Lifetime male
fitness was determined by molecular genetic paternity analysis of over
6500 offspring. Plants with intermediate anther position did have the
highest fitness in at least one of the three years (Morgan and Conner
2001), suggesting that selection could have caused the increased
filament-corolla tube correlation in wild radish. More selection on
floral morphology occurred through differences in male fitness than female
fitness, in agreement with the theory mentioned above (Conner et al.
1996a; Morgan and Conner 2001).
Artificial selection on anther position:
The selection on anther position in only one of three field seasons
provides only modest support for my adaptive hypothesis. However, the
high correlation between filament and corolla tube means that there is
little variability in anther position, so the fitness consequences of
having extremely high or low anthers are difficult to observe. This is an
example of a central problem in evolutionary biology: attempting to
reconstruct what may have happened in the past by studying present-day
populations.
In collaboration with Keith Karoly at
Reed College, we performed six generations of artificial selection
designed to create more variability in anther position. This is an
attempt to "turn back the evolutionary clock", by recreating the ancestral
condition, which is a lower filament-corolla tube correlation. We
selected for increased anther exsertion in two lines, decreased anther
exsertion in another two lines, and had two randomly-mated lines as
controls. Results indicate responses to selection in the predicted
directions: anther exsertion increased in the two lines selected to
increase, and decreased in the two lines selected for a decrease. When
these lines are combined together with the two random-mated control lines,
the result is a composite population with the ancestral lower correlation
(Fig. 3). In the summer of 2001, the plants resulting from these
selection experiments were used in studies of selection through male
fitness similar to those described above, to determine the fitness
consequences of more extreme anther positions than exist in present-day
populations. We are currently performing paternity analysis with
microsatellite markers on the offspring to determine selection through
male fitness. |
Evolution of dimorphic anther height:
The studies outlined above focus on the position of the long filament
anthers relative to the opening of the corolla tube, but like most of the
over 3,000 species in the family Brassicaceae, wild radish has four long
and two short stamens (Fig. 1). Therefore, each flower is dimorphic for
anther height. In a parallel series of studies, also in collaboration
with Keith Karoly, we are exploring the reasons for the evolutionary
stasis of dimorphic anther heights across this large plant family. One
possible explanation is that there is no additive genetic variation for
this dimorphism, so it cannot evolve. We have shown that this is not the
case in wild radish, both using sibling analysis (Karoly and Conner 2000)
and artificial selection for decreased dimorphism. Another possibility is
that there is selection for dimorphism, i.e., it is an adaptation. In
work with an REU in my lab, Amber Rice, and Martin Morgan of Washington
State, we showed that dimorphism in anther height led to higher male
fitness (seed siring success) in one of three years (Conner et al.
2003a). As we did for anther exsertion in 2001, the composite population
formed from the selected and control lines was placed in the field in the
summer of 2002 to measure selection with this increased variation in
anther exsertion; results await completion of microsatellite paternity
analysis. A final possible explanation for maintenance of anther
dimorphism is that it is constrained by genetic correlations with other
traits under selection. To test this, we measured many traits on a sample
of about 630 plants from the selected and control lines to look for
evidence of correlated responses to our artificial selection. This large
dataset is still being analyzed.
Mechanisms of genetic correlations: A
critical piece of information for understanding the evolution of
correlations is the genetic mechanism causing the correlation. However,
little is known empirically about the mechanisms that generate genetic
correlations. To determine the mechanisms of correlations, we subjected
two replicates of 300 plants each to nine generations of random mating
using hand-pollination in the greenhouse. Means, correlations, variances,
and covariances were all remarkably stable over the nine generations,
despite considerable statistical power to detect changes (Conner 2002).
This strongly suggests that the correlations are caused by pleiotropy
rather than linkage disequilibrium, despite evidence for correlational
selection acting on the filaments and corolla tube.
QTL mapping: Our random-mating
experiment shows that the strong correlation between filament and corolla
tube is due to pleiotropy. Quantitative genetic theory suggests that a
strong pleiotropic correlation like this should constrain the independent
evolution of the two correlated traits. However, our artificial selection
experiment clearly demonstrates that independent evolution of filament and
corolla tube can occur, because this is the only way to get increases and
decreases in anther exsertion. Thus our two sets of traditional,
statistical quantitative genetic results disagree. Despite the great
power and utility of statistical quantitative genetics, it treats the
genome as a black box; it does not provide information on how many gene
loci are affecting the traits, where those loci are located in the genome,
what the allele frequencies are at those loci, and what proteins the loci
code for. The newer technique of QTL mapping is a first step at breaking
open the black box of the genetics of quantitative traits, and should
provide the means of resolving the discrepancy between our two sets of
results in wild radish.
My
long-term goal is to use QTL mapping and other molecular techniques to
provide a comprehensive view of the evolution of genetic correlations at
three hierarchical levels. At the microevolutionary level, the genetic
mechanism underlying floral correlations within a natural population will
be determined. QTL mapping has rarely been used within a natural
population, which is the unit within which evolution by natural selection
occurs. At the interface between micro- and macroevolution, genes causing
the difference in anther exsertion between our divergent selected lines
are currently being mapped. These lines are experimental analogues of
natural populations undergoing speciation by disruptive selection. This
work is in collaboration with my MSU colleagues Alan Prather and Jim
Hancock, and is funded by USDA, MSU, and the Michigan State Agricultural
Experiment Station We have measured floral traits on over 500 F2 progeny
resulting from a cross between our selection lines. We are currently
genotyping these F2 plants at 15 microsatellite markers that differ
between our parental plants and will use approximately 60 additional AFLP
markers. Finally, at the macroevolutionary level, the genes that cause
differences in correlation patterns between closely related species will
be mapped. Based on current information, I plan to cross Barbarea vulgaris
and B. verna (we have successfully crossed these species). The
former shows the high filament - corolla tube correlation, while the
latter does not. This will allow me to identify gene regions that have
caused the evolution of high correlation from the lower ancestral
correlation.
Field measurements of genetic variances,
covariances, and selection on breeding values: A quantitative-genetic
half-sibling analysis of seven floral traits and lifetime female fitness
was conducted in the field at Kellogg Biological Station. Four major
conclusions can be reached from the results: (1) There was significant
additive genetic variance for lifetime female fitness (seed production);
this is an important evolutionary parameter that has rarely been estimated
in the field. (2) Phenotypic selection analysis corroborated female
fitness results from Illinois; that is, selection for increased flower
size and number through female fitness differences. Selection analysis
based on breeding values showed that this selection was real, not just an
artifact of environmental correlations. (3) Flower number should respond
to this strong selection, since there was significant genetic variance for
flower number in the field and no strong genetic correlations with other
measured traits. (4) Heritabilities of the floral traits were much lower
in the field compared to earlier greenhouse estimates (Conner and Via
1993); this was due to both increased environmental variance and decreased
additive genetic variance expressed in the field (Conner et al 2003b).
Summary: The studies outlined above
integrate genetics and ecology into studies of the evolution of complex
traits. They also span the range from microevolutionary studies within
populations to macroevolutionary studies of genetic differences between
related species. The results will be critical to our understanding of
floral evolution in the model plant family Brassicaceae. More
importantly, these results should improve our understanding of how genetic
correlations affect the integration and evolution of complex traits in any
species, using floral morphology as a model system. Finally, because wild
radish is an economically important weed worldwide, our work may have
agricultural applications as it addresses mechanisms of rapid adaptation
of a weed to novel environments.
2. Evolutionary ecology of plant-animal interactions
With
several different collaborators, I am working on a variety of projects
addressing the evolution of both plant-pollinator and plant-herbivore
interactions. Some of this work addresses how these interactions are
being altered by anthropogenic environmental change.
Genetic variation in induced resistance
to herbivory: Using quantitative genetic techniques, Anurag Agrawal
(University of Toronto) and I measured genetic variation in induced
resistance to herbivory. We found evidence for genetic variation in
induction of gluconsinolates, a defensive chemical, and corresponding
evidence for genetic variance in induced resistance to cabbage butterfly
herbivory (Agrawal et al. 2002). This plasticity (induced defense) was
costly to the plants, however, as families with higher plasticity had
lower fitness in the absence of herbivory.
Evolution of tolerance to herbivory: Plant responses to herbivory can be divided into two components, plant
defenses and tolerance. Tolerance refers to the ability of plants to grow
and reproduce following herbivore damage; i.e., the ability to
compensate. Most studies to date addressing the evolution of plant
resistance have focused on defense, while much less is known about how
tolerance evolves. My colleague, Ken Paige (University of Illinois), has
studied a special case of tolerance in scarlet gilia (Ipomopsis aggregata).
He has found several populations of scarlet gilia that overcompensate,
that is, have higher lifetime male and female fitness when eaten by deer
and elk. However, other populations of scarlet gilia studied by Paige and
others have lower fitness after ungulate herbivory. In our collaborative
project, funded by NSF, we are using reciprocal transplants among two
pairs of populations (one that does and one that does not overcompensate
in each pair) to determine the relative importance of genetics versus the
environment in determining differences in tolerance. In addition, we will
test for differences among the four populations in natural selection on
traits associated with tolerance.
Effects of insect herbivory on
plant-pollinator interactions: Sharon Strauss (UC Davis) and I studied
three-way herbivore-plant-pollinator interactions in wild radish (funded
by NSF). In early work, we have found that the consumption of leaves by
herbivores reduces flower size, attractiveness to insect pollinators, and
pollen production (Strauss et al. 1996). We subsequently estimated the
effect of herbivory on seed-siring ability and total male fitness in two
large field experiments, using the same molecular genetic paternity
analyses that we have used in our studies of selection on floral traits.
We found no effect of herbivory on male fitness when plants were in potted
arrays in the field, and an increase in male fitness when plants were
grown from seed in the field (Strauss et al. 2001). A greenhouse
experiment showed no effect of herbivory on pollen competitive ability.
These results are surprising given the reduction in floral traits due to
herbivory cited above, and may reflect differences in allocation to
different components of male and female fitness under different
conditions. The results of this work should be of interest to a broad
spectrum of ecologists and evolutionary biologists, because the vast
majority of herbivory studies to date have measured only effects on female
fitness.
Effects of enhanced ultraviolet radiation
on plant-pollinator interactions: My lab group, in collaboration with Gene
Robinson of the University of Illinois and Jim Cane of Auburn University,
studied the effects of increased UV-B radiation on plant-pollinator
interactions and total population fitness (funded by USDA). Our goal was
to understand how destruction of the ozone layer will affect the crucial mutualistic relationship between plants and their insect pollinators. We
exposed populations of field mustard (Brassica rapa) and black mustard (B.
nigra) to control and enhanced doses of ultraviolet radiation, at levels
designed to mimic current and possible future conditions on the earth. In
studies of plants that were grown under greenhouse conditions, lifetime
seed production actually increased under increased UV-B in three out of
four cases, with little effect on seed quality (Feldheim and Conner
1996). In a parallel field study, however, increased UV-B caused
significant declines in seed production in one of the two species, and
declines in seed quality in both (Conner and Zangori 1997). Therefore,
under more stressful field conditions, Brassica is apparently less able to
handle UV-B than in a more benign greenhouse environment. However, in
subsequent greenhouse studies we found no evidence for interactions
between UV-B and either water and nutrient stress in Brassica (Conner and
Zangori 1998) or intraspecific competition in Phacelia (Conner and
Neumeier 2001). These studies also found that the other stresses had a
much greater effect on plant fitness than did UV-B. Future work could
include applying the techniques that we use in our floral evolution work
to determine the potential for evolutionary adaptation to UV-B.Fig |
