Andrea L. Case
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Plant
Evolutionary Ecology
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Biological
Sciences |
Office:
330-672-3699 |
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Box
5190 |
Lab:
330-672-3821 |
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Kent
State University |
Fax:
330-672-3713 |
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Kent,
OH, 44242 |
acase-at-kent.edu |
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2005-present Assistant Professor of Biological Sciences, Kent State University Project:
Evolutionary dynamics of gynodioecy in Lobelia, and how it relates to geographic
variation in population size and sex ratio |
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2003-05 Postdoc in Molecular Evolution and Comparative Genomics, Duke University Project: Genomic coevolution in plants: An investigation of cytoplasmic male sterility and nuclear fertility restoration in Mimulus guttatus Advisor: John Willis |
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2000-02 Postdoc, Biological Sciences, University of Pittsburgh Project: The ecological context of selection for dioecy in the Virginian wild strawberry (Fragaria virginiana) Advisor: Tia-Lynn Ashman |
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2000 |
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1994 B.A. Biology, University of North Carolina at Greensboro Honors Thesis: Parental effects in Plantago lanceolata L.: Manipulation of grandparental temperature and parental flowering time. Advisor: Elizabeth P. Lacey |
Summary of Research Interests: Reproductive Biology of Flowering Plants
My research is focused on the evolution of
reproductive systems in flowering plants. I am particularly interested in
understanding sexual diversity – for example, why some groups of organisms are
hermaphrodites while others are predominantly composed of males and females.
Most flowering plant species exhibit some form of hermaphroditism, where
individuals function as both male and female, while only about 6% of flowering
plants consist of separate females and males. These strategies are completely
reversed in animals, where majority of species have separate sexes. I'm
interested in finding genetic, life history, and ecological factors that
contribute to the evolutionary dynamics of sexual systems. My main approach is
to study evolution in a “transitional” sexual system, known as gynodioecy,
where females and hermaphrodites co-exist within populations. Gynodioecy is
thought to be the most common pathway through which completely separate sexes
has evolved from hermaphroditism. Intermediate stages provide us with important
clues as to how and why sexual systems change over time.
Why study breeding systems & sexual strategies?
One answer to this question was eloquently
phrased by D. Lewis and L.K. Crowe in 1956 in the journal Evolution (vol. 10 pg. 115):
“To know the breeding system is to know
the genetic architecture of a species;
to
know the evolution of a breeding system is to know how evolution works.”
Patterns of genetic diversity within
populations are determined by who mates with whom. And of course, sex is an
integral part of the mating process. Given that males and females can only mate
with the opposite sex, it would seem that they have a distinct disadvantage
relative to hermaphrodites, who can mate with any other sex as a male AND as a
female—or even mate with themselves!
Nevertheless, separate sexed plants have successfully invaded
hermaphroditic populations dozens and dozens of times. In order for them to
have done this, females and males must be able to produce either more or
better-quality offspring. I am interested understanding how this entire process
works. This research will contribute to our fundamental understanding of how
evolution by natural selection works.
Why study plants?
Plants play a critical role in the
maintenance of all ecological systems, providing the energetic basis for food
webs and continuous regulation of atmospheric carbon dioxide and oxygen. We use
them for food, habitat, clothing, fossil fuels, medicines, cosmetics, and many
other day-to-day necessities. We cannot possibly exist without plants.
Understanding their general reproductive biology will be very important in
preserving plant diversity and ecosystem stability.
The diversity of sexual systems in flowering
plants is simply astonishing, evidenced by the vast array of floral forms in
nature. Because of this variability, plants provide myriad opportunities for
testing hypotheses about the patterns and process of reproductive evolution. Their sexual strategies are readily
observed in the form and design of floral displays, allowing plant biologists
to gather detailed information relatively easily compared to animals. Being
immobile, they must be highly adaptable to their environments, and rely on
others (i.e., pollinators) to carry out the mating process. Plants are often much more amenable to
manipulative experiments compared to animals, plus they don’t run away, bite,
or leave their droppings everywhere!
Perhaps a better question is “why NOT study
plants?”
Current Research Projects: Evolutionary Ecology of Female Plants
Project 1: What kinds of genes cause plants to be
female, and how do mitochondrial male sterility genes evolve?
Study system:
Yellow monkeyflowers (Mimulus guttatus)
Collaborators: John Willis (Duke University) Lila Fishman
(Univ. of Montana); Jeff Mower (Univ. of Nebraska,
Lincoln)
Graduate students: Eric Floro (M.Sc. candidate)
Female plants evolve when hermaphroditic
plants lose their male function—that is, they become male sterile. Genes that
cause male sterility in plants are often cytoplasmic (called CMS genes for
‘cytoplasmic male sterility’), and specifically they are mitochondrial. It
makes sense that these genes should spread through populations because they are
feminizing factors that are almost always maternally inherited. Research by
plant molecular biologists,
primarily working in agricultural systems, has told us much about the structure of these genes;
theoretical models predict what should happen to them when they arise. But we
have very little empirical data to test model assumptions in wild plant
species.
My research on Mimulus guttatus began as a
collaborative effort with John Willis, Lila Fishman, and Camille Barr to
understand the origin and evolution of a CMS gene in this species, and
particularly why the male-sterile phenotype is completely suppressed in nature.
We are able to take advantage of the increasingly available genetic
tools for this system, including a full genome sequencing project by JGI.
This research is aimed at understanding molecular and population genetics of
CMS, how it spreads within and among populations, and whether or not CMS in
this hermaphroditic species may contribute to reproductive isolation, either
among populations of M. guttatus or between species of Mimulus (see Case & Willis 2008).
With the full genome sequence now available,
Jeff Mower and I are analyzing the Mimulus mitochondrial genome for structural and
sequence variation that may be related to CMS. Novel CMS genes are likely to be
created by structural rearrangements within the mt genome, and these are often
facilitated by recombination between repeat sequences. By identifying and
characterizing repeats, we are beginning to see patterns that may help us
understand how the M. guttatus CMS was created.
In screening for mt variation among natural
populations, we have found evidence of mitochondrial heteroplasmy in M. guttatus.
Like CMS, heteroplasmy appears to be geographically restricted. Eric Floro
(M.Sc. candidate) is investigating how widespread heteroplasmy is within the
species, and the mechanisms that generate and maintain heteroplasmy. We are particularly
interested in the potential for occasional biparental inheritance (i.e.,
paternal leakage) to contribute to heteroplasmy. If this phenomenon is indeed
limited to only a few populations, it will be interested to investigate other
genetic factors that may regulate the occurrence of paternal leakage.
Project 2: What factors regulate the frequency of
females in natural populations?
Study system:
Great Blue Lobelia
(L.
siphilitica)
Collaborators:
Christina Caruso (Univ. of Guelph), Chris Blackwood (Kent State Univ.)
Current graduate
students: David Nogas (Ph.D. candidate)
Former graduate
Students: Stephanie Hovatter (M.Sc. 2008), Julie Proell (M.Sc. 2009)
Current and former Undergraduate researchers: Scott Vernon, Kelly Barriball, Chad Skutle, Chris McKenna, Ashley Hill, Chris Dejelo
My research on Lobelia siphilitica aims to understand
why the frequency of females is so variable among populations, and why we tend
to find more females in southern populations and in small populations (Caruso
& Case 2007). I'm currently collaborating with Chris Caruso (Univ. of
Guelph) and my graduate student, Julie Proell, to explore the connections
between population size and sex ratio by assessing relative fitness of females
and hermaphrodites in the field, monitoring pollinator behavior, and using
microsatellite markers to determine the influence of mating patterns
(especially inbreeding) on population sex ratio dynamics.
Another part of this problem involves the
causes of geographic variation in population size. Individuals of this species
can produce up to 1000 seeds per fruit and up to 200 fruits per plant! With
that amount of seed production, it is difficult to understand why any
population of L.
siphilitica should be small. With Chris Blackwood and our graduate student,
Stephanie Hovatter, we are exploring the extent to which soil properties may be
affecting population size by influencing seed germination, seedling
establishment, growth, or survival. We are investigating both abiotic soil
characteristics, such as nutrients and texture, as well as characterizing the
biotic soil community using T-RFLP analysis.
This project has recently been funded by the National
Science Foundation.
Understanding the causes
of geographic variation in sex ratio of a gynodioecious plant (August 2009-July 2012)
PI: Andrea Case (Kent State University); Co-PI: Christina Caruso (University of Guelph)
Evolutionary biologists have long been interested in understanding how populations of organisms become different from one another, because it is a necessary precursor to speciation. Variation in population sex ratio (for example, the number of females vs. males, or females vs. hermaphrodites) provides an excellent model for investigating mechanisms of population differentiation. Because the population sex ratio determines the identity and number of available mates, it affects how genetic variation is distributed within and between populations. This project will investigate why the proportion of females vs. hermaphrodites in a flowering plant (Lobelia siphilitica) are higher in small populations and at warmer sites. We will test whether female frequencies vary because of natural selection, or if other evolutionary forces, such as genetic drift and gene flow, are preventing many populations from reaching an equilibrium sex ratio. Distinguishing the effects of these different evolutionary mechanisms is important because they produce distinct patterns of population genetic structure.
This research will
provide a framework for evaluating how breeding systems influence the ability
of species to modify their range in response to climate change, particularly
global warming. If female L. siphilitica
plants are more common in areas with higher annual mean temperatures because of
natural selection, then populations should become more female-biased in
response to global warming, which could affect migration rates and species
persistence.
Case, A.L. & C.M. Caruso. 2010. A novel approach to
estimating the cost of male fertility restoration in gynodioecious plants. New Phytologist,
in press.
Case, A.L. & T-L. Ashman. 2009. Resources and
pollinators contribute to population sex-ratio bias and pollen limitation in Fragaria
virginiana (Rosaceae). Oikos
118:1250-1260.
Case, A.L. & J.H. Willis 2008. Hybrid male sterility in Mimulus guttatus (Phrymaceae) is
associated with a geographically restricted mitochondrial rearrangement. Evolution 65:1026-1039.
Case, A.L.1, S.W. Graham1, T.D. Macfarlane,
& S.C.H. Barrett. 2008. A phylogenetic study of evolutionary transitions in
sexual systems in Australasian Wurmbea (Colchicaceae). International
Journal of Plant Sciences 169:141-156. [1equal
contribution by first two authors]
Macfarlane, T.D. & A.L. Case. 2007. Wurmbea inflata (Colchicaceae), a new
species from the Gascoyne region of Western Australia. Nuytsia 17:223-228.
Caruso, C.M. and A.L. Case. 2007. Sex ratio variation in gynodioecious Lobelia siphilitica: effects of population size and geographic location. J. Evolutionary Biology 20:1396-1405.
Case, A.L. & T-L. Ashman. 2007. An experimental test of
the effects of resources and sex ratio on maternal fitness and phenotypic
selection in gynodioecious Fragaria virginiana (Rosaceae). Evolution 61:1900-1911.
Barrett, S.C.H. & A.L. Case. 2006. The ecology and
evolution of gender strategies in plants: the example of Australian Wurmbea
(Colchicaceae). Aus.
J. Botany 54:1-17.
Case, A.L. & T-L. Ashman. 2005. Sex-specific physiology and its implications for reproductive cost. In The Allocation of Resources to Reproduction in Plants. E. Reekie and F. Bazzaz, eds. Springer-Verlag, New York
Case, A.L. & S.C.H.
Barrett. 2004. Floral biology and gender monomorphism and dimorphism in Wurmbea dioica
(Colchicaceae) in Western Australia. Int. J. Plant Sci. 165(2):289-301.
Case,
A.L. & S.C.H. Barrett. 2004. Environmental stress and the evolution of
dioecy: Wurmbea
dioica
(Colchicaceae) in Western Australia. Evolutionary Ecology 18:145-164.
Case,
A.L. & S.C.H. Barrett. 2001. Ecological differentiation of combined and
separate sexes of Wurmbea dioica (Colchicaceae) in sympatry. Ecology 82 (9): 2601-2616.
Barrett,
S.C.H., M.E. Dorken, & A.L. Case. 2000. A geographical context for the
evolution of plant reproductive systems. In Integrating Ecological and Evolutionary
Processes in a Spatial Context. (Eds. J. Silvertown & J. Antonovics).
Blackwell, Oxford. UK.
Barrett,
S.C.H., A. L. Case, & G.B. Peters. 1999. Gender modification and resource
allocation in subdioecious Wurmbea dioica (Colchicaceae). Journal of Ecology 87 (1): 123-137.
Case,
A.L., P.S. Curtis, & A.A. Snow. 1998. Intraspecific variation in stomatal
and growth response to elevated CO2 in wild radish, Raphanus
raphanistrum (Brassicaceae). American Journal of Botany 85(2):253-258.
Lacey, E.P., S.Smith, & A.L. Case. 1997. Parental effects on seed
mass: seed coat but not embryo/endosperm effects. American Journal of Botany
84(11):1617-1620.
Case,
A.L., E.P. Lacey, & R.G. Hopkins. 1996. Parental effects in Plantago
lanceolata L. II.:
Manipulation of grandparental temperature and parental flowering time. Heredity
76(3): 287-295.