Marty Shankland
Professor
Section of Molecular Cell and Developmental Biology
Graduate Program
in Cell and Molecular Biology &
Graduate Program in Ecology, Evolution and Behavior
Research
interests: developmental biology; evolution of animal
body plans
Education: Ph.D, University of California, Berkeley,
1982
Contact me by EMail.
Class webpages:
BIOLOGY 214 - Fall 2002
BIOLOGY 349 - Spring 2002
Our lab investigates the
developmental mechanisms that establish the body plan of the
embryo and direct the spatial patterning of its cellular
differentiation. We take a particular interest in the way that
such patterning mechanisms have evolved. For example,
segmentation of the anteroposterior body axis is a characteristic
of three major animal taxa -- the arthropods, the annelids, and
the vertebrate chordates. There is a longstanding debate as to
whether body axis segmentation has evolved once, twice or three
separate times. We have shown that the phylogenetic relationship
of these taxa in and of itself is not a sufficient basis for
drawing a statistically reliable conclusion. Rather, our
laboratory approaches this problem by characterizing the sequence
of cellular and molecular events that underlie segmentation in
different taxa, and comparing those mechanisms between taxa in
order to learn which aspects of their segmentation are homologous
(similar due to a derivation from a common segmented ancestor),
homoplastic (similar due to convergent evolution), or divergent.
The cellular and molecular basis
of segmentation are already known in great detail in one
arthropod - the fruitfly Drosophila - and to a lesser
extent in a number of other arthropod species. Our primary goal
has therefore been to analyze the segmentation of annelid worms,
particularly the leech Helobdella. The leech embryo
develops via a stereotyped cell lineage, and is amenable to a
wide variety of experimental manipulations. One avenue of
research involves the isolation and characterization of Helobdella
genes that are homologous to developmental regulatory genes
(e.g. engrailed; hedgehog; Hox genes) already known to
play a defined role in the segmentation of Drosophila. A
second avenue involves the use of cell ablation and
transplantation techniques to determine how the individual,
lineally identified segment founder cells of the leech embryo
acquire the positional information that determines their
developmental fate. We find that the leech's segment founder
cells acquire a segment identity (i.e. commitment to
produce descendant tissues appropriate to one specific segment)
at or shortly after the time of their birth. We have also found
that - in contrast to what occurs in the Drosophila embryo
- the engrailed expressing cells of the leech segment
primordium do not function as a signalling center that patterns
segment polarity throughout the remainder of the segmental
repeat. This latter finding suggests that there are fundamental
differences in the developmental mechanisms used by annelid and
arthropod embryos to generate segment polarity, and suggests that
these two phyla may in fact have evolved body axis segmentation
independently.
The lab has two other on-going projects
relating to both development and evolution. First, the
specification of dorsoventral polarity in the ectoderm of the
leech embryo involves a well-characterized sequence of cell
interactions, and we are currently testing whether those cell
interactions employ the Notch-delta signalling pathway. Second,
we are interested in the genetic basis of the distinction between
head and body trunk. Nearly all bilaterian animals have
morphologically discrete head and trunk regions. It has been
shown that there are distinct genetic pathways responsible for
the developmental patterning of these two regions, and those two
genetic pathways appear to be highly conserved among phyletically
diverse bilaterian species. Based on our studies of head gene
expression in the leech embryo, we recently put forward a
"Radial Head" hypothesis which proposes that the modern
bilaterian body plan evolved from a radially organized ancestor
(> 600 million years ago) by remodeling the radial body plan
into the head domain and adding a genetically distinct trunk
domain that has subsequently experienced a large degree of
allometric expansion and lineage-specific specialization. To
further test the Radial Head hypothesis, we will have to isolate
and characterize both head and trunk regulatory genes from a much
wider variety of animal taxa than that which has been examined to
date.
Selected publications:
Nardelli-Haefliger, D., Bruce, A.E.E., and Shankland, M. (1994). An axial domain of HOM/Hox gene expression is formed by morphogenetic alignment of independently specified cell lineages in the leech Helobdella. Development 120:1839-1849.
Kourakis, M.J., Master, V.A., Lokhorst, D.K., Nardelli-Haefliger, D., Wedeen, C.J., Martindale, M.Q., and Shankland, M. (1997). Conserved anterior boundaries of Hox gene expression in the central nervous system of the leech Helobdella. Dev. Biol. 190:284-300.
Wedeen, C.J., and Shankland, M. (1997). Mesoderm is required for the formation of a segmented endodermal cell layer in the leech Helobdella. Dev. Biol. 191:202-214.
Bruce, A.E.E., and Shankland, M. (1998). Expression of the head gene Lox22-Otx in the leech Helobdella and the origin of the bilaterian body plan. Dev. Biol. 201:101-112.
Shankland, M., and Seaver, E.C. (2000). Evolution of the bilaterian body plan: what have we learned from annelids? Proc. Nat. Acad. Sci. USA. 97:4434-4437.
Seaver, E.C., and Shankland, M. (2001). Establishment of segment polarity in the ectoderm of the leech Helobdella. Development 128:1629-1641.
Section of Molecular Cell and Developmental Biology
Graduate Program in Cell and Molecular Biology
Graduate Program in Ecology, Evolution and Behavior