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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2011-present Associate Professor of Biological Sciences, Kent State
University Curator of the Kent State
University Herbarium (KE) 2005-2011 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
femaleâ 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.
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. 2011)
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 have analyzed 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 (see Mower et al. 2012).
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. 2011) investigated 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), Maia
Bailey (Providence College), Chris Blackwood (Kent State Univ.), Eric Knox
(Indiana Univ.)
Current graduate students: Binaya Adhikari (Ph.D. candidate)
Former graduate Students: Stephanie
Hovatter (M.Sc. 2008), Julie Proell
(M.Sc. 2009), Hannah
Madson (M.Sc. 2012)
Honors Thesis Student: Alicia Durewicz
Current and former Undergraduate research assistants: Jacob Dennis, Nick Baron, Scott Vernon, Kelly Barriball, Chad Skutle, Chris McKenna, Ashley Hill, Chris Dejelo, Tony Vitale, Peter Euclide, Christa McCarthy, Nicole Moscolic, Jennifer Knott, Danny Himes, Victoria Ellis. Mauri Hickin
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; Case & Caruso 2010, Caruso et al. 2012, Caruso & Case 2013) and my graduate students, Julie Proell and Hannah Madson 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 mating patterns and population dynamics. Binaya Adhikari is investigating the genetic basis of sex determination in L. siphliitica, including searching for CMS genes in the mitochondrial genome.
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 (Hovatter et al., in review). We are investigating both abiotic
soil characteristics, such as nutrients and texture, as well as characterizing
the biotic soil community using T-RFLP analysis (see Hovatter et al. 2011).
This project is funded by the
National Science Foundation.
Understanding the causes of geographic
variation in sex ratio of a gynodioecious plant (August 2009-July 2014)
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.
Caruso, C.M. & A.L.Case. 2013. Testing models of sex-ratio evolution in a gynodioecious plant: female frequency co-varies with the cost of male fertility restoration. Evolution, in press.
Caruso, C.M., A.L. Case & M.F.Bailey. 2012. The evolutionary ecology of cytonuclear interactions in angiosperms. Trends in Plant Science 17:638-643.
Mower, J.D., A.L. Case, E.R. Floro & J.H. Willis. 2012. Evidence against equimolarity of large repeat arrangements and a predominant master circle structure of the mitochondrial genome from a monkeyflower (Mimulus guttatus) with cryptic CMS. Genome Biology and Evolution 4:670-686.
Karron, J.D., C.T. Ivey, R.J. Mitchell,
M. Whitehead, R. Peakall & A.L. Case. 2012. New
perspectives on the evolution of plant mating systems. Annals of Botany 109:493-503.
Hovatter, S.R., C. Dejelo, A.L. Case & C.B.
Blackwood. 2011. Metacommunity organization of soil
microorganisms depends on habitat defined by presence of Lobelia siphilitica plants. Ecology 92: 57-65.
Macfarlane, T.D. & A.L. Case. 2011. Wurmbea fluviatilis (Colchicaceae), a new riverine species from the Gascoyne region of Western Australia. Nuytsia 21:25-30.
Case, A.L. & C.M. Caruso. 2010. A novel approach to
estimating the cost of male fertility restoration in gynodioecious
plants. New Phytologist 186:549-557. [pdf]
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. [pdf]
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. [pdf]
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] [pdf]
Macfarlane, T.D. & A.L.
Case. 2007. Wurmbea inflata (Colchicaceae), a new species from the Gascoyne
region of Western Australia. Nuytsia 17:223-228. [pdf]
Caruso, C.M. & A.L. Case. 2007. Sex ratio variation in gynodioecious Lobelia siphilitica: effects of population size and geographic location. J. Evolutionary Biology 20:1396-1405. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]
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. [pdf]