PhD Student, University of California, Berkeley
Dissertation Research at the University of California, Berkeley
My first encounter with a carnivorous plant was at the Huntington
Botanical Gardens in San Marino, California. I was only about
4 feet tall and stood as high as I could to see the deadliest
plant to all known insects: the tropical pitcher Nepenthes.
A peek inside this plant’s traps revealed a pool dotted
with bugs. After experiencing Nepenthes at the botanical
garden, I began to wonder how such a plant could have come to
exist. Fifteen years later, I continue to question how organisms
diversify. I am interested in mutations that occur at the nucleotide
level, and I hope to use evolutionary biology as a method to promote
preservation by showing what contributes to the uniqueness of
a species. For my dissertation project in Chelsea D. Specht’s
lab at the University of California, Berkeley, my dissertation
research includes a study of relationships between genera within
the carnivorous plants of the Caryophyllales and a molecular evolutionary
analysis of their digestive enzymes.
The Carnivorous Caryophyllales
The carnivorous plants of the angiosperm order Caryophyllales
(core eudicot) have a unique evolutionary history, encompassing
taxa that have diverse morphologies and include genera that have
lost the ability to digest prey. One project that is part of my
dissertation research at the University of California, Berkeley
is to create a more robust phylogeny for the carnivorous Caryophyllales
that includes greater taxon sampling from each genera and focuses
on the evolution of gland morphology within this group.
In 1960, Léon Croizat proposed that carnivorous plants
comprised a single group at the base of the Angiosperms based
upon similarities in trap type . More recently, however, carnivory
has been hypothesized to have arisen independently at least five
times in the plant kingdom, within orders Ericales, Lamiales,
Oxalidales, Poales, and Caryophyllales as a result of convergent
evolution [2, 3]. Based upon recent phylogenetic studies of angiosperms,
four families, comprising in total more than 300 species, have
been placed as a monophyletic group within the order Caryophyllales:
Droseraceae, Drosophyllaceae, Nepenthaceae and Dioncophyllaceae.
Included within these families are the carnivorous genera Drosera
(sundews), Dionaea (Venus flytrap), Nepenthes
(tropical pitcher plants), Aldrovanda (aquatic flytrap),
Drosophyllum (Portuguese sundew) and Triphyophyllum,
along with non-carnivorous genera Ancistrocladus, Habropetalum,
Outside of this monophyletic group are the families Polygonaceae
and Plumbaginaceae, which together share a number of synapomorphies
with the carnivores including vascularized, multicellar glands
. In Polygonaceae and Plumbaginaceae, these glands function
in the secretion of mucilage or salt for protection in halophytic
conditions, epizoochory and to deter herbivory . In the carnivorous
plants, homologous glands have evolved to function in the secretion
of digestive enzymes as well as to absorb amino acids and other
organic nutrients .
Within the carnivorous Caryophyllales, a variety of glands that
can both exude and absorb exist. In the genera Drosera,
Drosophyllum and Triphyophyllum, two types of
glands are present: (1) vascularized, stalked, multicellular glands
like those of Polygonaceae and Plumbaginaceae, and (2) non-vascularized,
sessile glands. In Dionaea, marginal multicelluar glands
have been reduced to “teeth” and trigger hairs held
perpendicular to the lamina, although sessile glands that secrete
enzymes are still present. Within Nepenthaceae, glandular pits
partially covered by epidermis are positioned at the base of the
pitcher, while vascularized, multicellular stalked glands are
absent. In the non-carnivorous genera sister to Nepenthaceae (Ancistrocladus,
Habropetalum, Dioncophyllum), glandular pits
are present on the abaxial surface of the leaf, but function in
the secretion of epicuticular waxes rather than digestive enzymes.
The current phylogeny supports the hypothesis that embedded glands
in Nepenthaceae, Ancistrocladaceae and Dioncophyllaceae are derived
from glands found in Droseraceae .
Chitinolytic Enzymes in the Carnivorous Caryophyllales
My second project in the Specht lab is to determine the degree
of homology among chitinases in carnivorous plants and to elucidate
the method by which these enzymes have become specifically adapted
for the carnivory.
The isolation and characterization of carnivorous plant digestive
enzymes secreted from glands began in the nineteenth century,
when Joseph Hooker discovered the first protease in Nepenthes
sp. trap fluid cir. 1874 . In 1875, Charles Darwin published
his accounts on Drosera rotundifolia and the ability
of Drosera to digest nitrogenous and phosphate-containing
compounds . However, it was not until the 1970s that the basic
enzyme composition in carnivorous plant mucilage was characterized.
Among the various enzymes identified to be important in plant
carnivory, chitinases have been one of the most thoroughly studied
Active production of chitinolytic enzymes were first demonstrated
in Drosera and Nepenthes by Amagase et al.
. Several years later, Robins and Juniper found Dionaea
muscipula traps to exhibit chitinase activity . Twenty
years later, two types of Nepenthes khasiana chitinases
(class Ia and Ib) involved in digestion were sequenced and described,
only one of which was found to be upregulated in response to the
colloidal chitin . During the same year, Matusikova et
al. demonstrated the localized expression of chitinase mRNA
within the tentacles of Drosera rotundifolia via in
situ hybridization .
Based on their on the structure of their domains and percent
sequence homology, class I chitinases involved in plant carnivory
may have descended from class I chitinases that play a role in
pathogen defense. Changes in selectional pressure may be evident
if protein function has shifted from defense to digestion. My
goal is to compare selectional pressure between chitinases that
are induced in response to chitin and those that are constitutively
expressed. Preliminary studies of class Ib chitinases in Nepenthes
have shown a noticeable decrease in the overall number of amino
acids under positive selection, which may be an indicator of a
shift from positive to purifying selection during the evolution
of chitinase enzymes in carnivorous plants.
Acknowledgements: This project was funded in
part by the UC Berkeley College of Natural Resources & the
NSF Graduate Research Fellowship Program. Thank you to the entire
Specht Lab for their insight & suggestions, especially my
advisor Dr. Chelsea D. Specht and undergraduate researcher Sarah
Starkey. In addition, the following sources for specimen donation
are integral to this project: The following sources for specimen
donation are integral to this project: UC Berkeley Botanical Garden,
Missouri Botanical Garden, Bogor Botanical Garden, University
of Würzburg (Dr. Gerhard Bringmann and Andreas Irmer) &
California Carnivores in Sebastopol, CA (Peter D’Amato).
Many of the carnivorous plant photos for this page are used with
thanks and by permission of www.sarracenia.com
(Dr. Barry Rice, UC Davis).
Visit the Specht lab website at http://pmb.berkeley.edu/~specht/.
Figure 1: An introduction to the carnivorous
plants and the five origins of carnivory. The Caryophyllales holds
the largest number of genera and is a unique in that there has
been a gain and loss of carnivory. Figure adapted from the Angiosperm
Phylogeny Group II, 2003 and pictures from Dr. Barry Rice (UC
Figure 2: Bayesian inference of phylogeny (MrBayes
v3.1.2, Huelsenbeck, Larget, can der Mark, Ronquist) constructed
using a partitioned analysis of two concatenated nuclear and chloroplast
molecular markers (GTR+gamma, ITS and GTR+invgamma, psbA-trnH)
ran for 10 million generations. In this phylogenetic reconstruction,
solid lines represent genera that are not carnivorous, whereas
dashed lines represent carnivorous and part-time carnivorous genera.
Throughout the evolution of the carnivorous Caryophyllales and
their gland types, a general trend exists: a gain and loss of
multicellular stalked glands that can exude and absorb. Micrograph
drawings of glands: , , .]
Figure 3: Class I chitinases have been shown
to be under strong positive selection in Arabis, with
adaptive replacements occurring disproportionately in the active
cleft. This is thought to be due to an arms-race between chitinases
and chitinolytic inhibitors produced by pathogenic fungi. However,
in carnivorous plants, class Ib chitinases that induced in response
to chitin might be instead undergoing purifying selection, as
the main constituents in plant traps are insects, which do not
produce any known chitinolytic inhibitors. Class Ia chitinases
that are constitutively expressed in carnivorous plants might
be undergoing positive selection as in non-carnivorous plants.
Image adapted from the following images: crystal structure of
barley chitinase, mushroom (Hygrocybe sp., and tenebrionid
beetle (Nhu Nguyen, UC Berkeley).
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