Research :
I am a plant evolutionary biologist with interests that
span the areas of population genetics, phylogenetics and
speciation. For me, questions surrounding the nature of
plant species are boundlessly interesting – How do
populations of a species, scattered across hundreds or thousands
of kilometers, maintain their genetic and evolutionary integrity?
How do factors such as changes in chromosome numbers and
breeding system influence the ecological and genetic interactions
of populations that differ in these traits? How does natural
selection influence the divergence of populations and groups
of populations when gene flow still occurs between diverging
units? These are just a few of the questions that fuel my
research interests.
Research in my lab takes a variety of approaches from comparative
to experimental, from the level of populations, through
the study of whole genera. I believe that taking a broad
approach and using multiple study systems is both satisfying
for my students and me, and allows for broader insights
into the evolutionary process. Finding common threads and
points of difference in the stories of diversification of
multiple groups reminds us that understanding evolution
involves both the search for generalities and an appreciation
of individual stories.
Polyploidy and Apomixis in Crepis and
Townsendia
My research group studies multiple systems in the sunflower
family (Asteraceae). Each system possesses some shared and
some unique characteristics that allow us to undertake complementary
studies of specific processes. In particular, the family
holds many examples of polyploid asexual (agamic) complexes,
and numerous edaphic specialists that are a key focus of
my research program.
Townsendia, a Rocky Mountain Agamic Complex
Townsendia is a Rocky Mountain genus of roughly
30 taxa in the Astereae. The 1957 monograph of the genus
by John Beaman described the group’s diversity, characterizing
ploidy and breeding system variation and laying the foundation
for further studies. The genus is highly charismatic (cutest
plants ever!), including a number of diminutive forms that
are easily overlooked, along with some more conspicuous
members. Two features of the complex are of special interest
to our research. First, Townsendia includes at
least 13 species with both diploid sexual and polyploidy
asexual populations. Our research has focused on understanding
the origins of asexual polyploids, so far focusing mostly
on one species, Townsendia hookeri.
Former graduate student Stacey Thompson focused on, Townsendia
hookeri. This species includes sexual diploid and asexual
polyploid populations. This species ranges from the foothills
of Colorado, northward to Jasper NP in Alberta. Disjunct
northern populations also occur in the Yukon and Alaska.
Sexual diploids are restricted to unglaciated regions of
Colorado and adjacent Wyoming, or the Yukon. Only polyploid
asexuals occupy previously glaciated regions. Stacey’s
work showed that polyploids originated at least four times
in this system, and also characterized ploidy levels and
reproductive mode in a large number of populations, from
Colorado to Yukon (Thompson and Whitton 2006). Stacey also
characterized breeding patterns and patterns of diversity
in Yukon populations, which include sexuality and apomixis
(Thompson et al. 2008).
A key focus of ongoing research on T. hookeri will be to
understand what governs this distribution. Are asexuals
successful in previously glaciated areas because they are
better colonizers (it only takes one seed to establish an
asexual population), or do diploids and polyploids differ
in ecological traits? Whatever we learn from T. hookeri
serve as a point of comparison for other species in the
genus. If we find that multiple sexual/ asexual groups are
governed by the same factors, this will suggest that these
factors are generally important.
Current graduate student Chris Lee is examining patterns
of relationship in the genus as a whole. We are especially
interested in using niche modeling approaches, together
with phylogenetic hypotheses, to test ideas about patterns
of speciation. We want to know what role ecological divergence
has played in the diversification of species in this group,
and also in governing the distribution of sexuals and asexuals,
as noted above.
The North American Crepis Agamic Complex
The situation in this second agamic complex is probably
even more complicated than that in Townsendia,
for two reasons: First, the polyploids of Townsendia
are triploid or tetraploid, while those in Crepis
can go higher, perhaps to decaploids (with 10 sets of chromosomes).
Second, many of the polyploids in Crepis look like
hybrids, combining features of at least two diploids. The
existence of so many hybrid types, propagated by asexual
reproduction, has produced a bewildering array of morphological
types in nature.
As with Townsendia, we are using previous studies
to help inform our approach. E.B. Babcock and G. L. Stebbins
first documented patterns of variation in this complex,
in their 1938 monograph. While that work was often cited,
this group of plants was largely untouched until the early
1990s. I studied the group as part of my Ph. D. thesis,
despite being warned against it by G. L. Stebbins himself,
in his response to a letter I wrote asking for suggestions
(interestingly, one of the groups he thought would be more
promising was Townsendia – it only took me
10 years to heed his advice). In most ways, his concerns
were well founded. While I learned that the group was at
least as challenging as Stebbins and Babcock had thought,
the tools available at the time of my Ph. D. limited sampling
depth, and did not provide any shortcuts to estimating ploidy.
So that chapter of my thesis sat on a shelf, but did not
gather dust. We have tried to ask some questions, for example
in a 2008 paper examining the relationships and breeding
patterns of two sets of populations (Whitton et al. 2008),
but the big breakthroughs in our understanding are coming
through the work of Ph. D. student Chris Sears. Chris has
been able to sample many populations (more than 80) and
characterize ploidy using flow cytometry, a technique that
estimates DNA content of cells. This means that we can now
examine complex relationships while knowing the ploidy of
each individual under study. For a group in which ploidy
variation is well known, this is an essential step in understanding
the role that ploidy and breeding system shifts have played
in these groups. Chris nearing the completion of his thesis,
and will have more to say about this fascinating system
soon.
Broader Context of Apomixis Research
Fewer than 1% of plant species are apomictic, but nonetheless,
what we learn studying these systems has potential relevance
for the other 99% of species. One interesting observation
is that Crepis populations very often include more
than one species. For the cases that we understand well,
it seem that these species can coexist in part because they
are asexual – they do not appear to share gene flow,
and thus distinct types can be maintained in one place.
This suggests that it is interference through breeding,
and not ecological interactions (such as competition) that
limit coexistence.
In addition, while apomixis is relatively uncommon, polyploidy
is a pervasive feature of flowering plants. Studying asexual
polyploids can provide broader insights into the role of
multiple polyploid origins on patterns of diversification.
Evolution and Ecology of Invasive Plants
In addition to working on these systems, I am part of a
research collaboration led by Loren Rieseberg, with Keith
Adams and Sally Otto that aims to look for the genetic traits
that may enhance a species’ ability to become invasive.
This project involves comparisons of invasive and native
populations of 5 species in the sunflower family.
M. Sc. Student Jamie Leathem is studying plant invasion
in Yukon. While more southern regions have a relatively
long history of plant invasions, the spread of invasive
in Yukon is more recent. Jamie is documenting this phenomenon
and looking for traits that correlate with high local densities.
Identification of such traits may aid in predicting invasions
and managing or mitigating the spread of invasives
