Evolution of Vertebrate Animal Design
Assignment Notes
Many introductory developmental biology courses examine how changes in Hox gene
expression can explain differences in vertebrate body plan. However, the majority of
undergraduate students are not familiar with the axial skeletal anatomy of various
vertebrates, and are not readily able to visualize how changes in Hox gene expression or
Hox gene targets result in different vertebral structure. This laboratory exercise asks
students to work backwards from the adult anatomy to deduce embryonic gene
expression patterns.
Assignment. We use articulated skeletons that are available on campus (and some
borrowed from other colleges), including alligator, cat, chicken, dog, eagle , frog, human,
Necturus, perch, rat, snake, Squalus and turtle.
Students are also provided with resources to identify anatomical features in different
organisms, information about a model organism (Mus musculus) in which Hox gene
expression patterns and vertebral identity are correlated and a rubric detailing
expectations for the assignment.
Student work – Part I
Determine vertebral axial identity. Students determine the number and axial identity of
each vertebra (i.e. region: cervical, thoracic, lumbar, sacral, or coccygeal/caudal) in three
articulated skeletons, and present this information in a figure.
Student work – Part II
Hypothesize the genetic differences that result in anterior-posterior body plan changes.
After analyzing their observations (e.g. are there equal numbers of lumbar vertebrae? do
the lumbar vertebrae from each organism look the same?), students generate hypotheses
about anterior/posterior patterning for each skeleton. As support for their model, students
are asked to include a figure with relevant hypothetical in situ hybridizations of embryos.
Conclusions. Student self-reports and final exam performance indicates that this
assignment promotes the integration of the understanding of the developing embryo, the
adult animal and the regulation of gene expression.
Ideas for future use of the assignment. While I have not had the opportunity to try these
options, it would be interesting to use this as a collaborative rather than an independent
assignment.
Evolution of Vertebrate Animal Design
Introduction
Homeotic changes involve the replacement of one normal body part for another. Genetic screens
identified linked genes that affect segment identity in Drosophila, known now as Hox genes. All of
these Hox genes encode transcription factors. Each Hox gene has a similar 180 base pair stretch of
DNA (homeobox) that encodes for a 60 amino acid domain (homeodomain) within each Hox protein
that binds DNA. Since that time researchers have identified Hox-related genes from many other
animals. When these genes were physically mapped, it was discovered that in vertebrates there were
four, large linked Hox complexes, in mice 39 genes in total.
With such genetic conservation between animals, yet such differences in morphology, how did new and
different forms evolve from the common bilaterian ancestor?
What are the genetic differences that underlie the diversity of the animal body plans?
Changes in the common body plan are most often a result of (1) regulatory changes downstream of
Hox gene function ((think of the difference between the chick and yourself in how the protein
produced by the Tbx5 gene (activated by Hox genes) activates different target genes (producing a wing
for the chick and a hand in yourself)) or (2) the modification of Hox expression patterns within
developmental fields (changing its temporal or spatial distribution).
During this laboratory you will examine the relationship between evolution of the body plan and
regulatory evolution.
Procedure
For each of your three articulated skeletons, determine the number and axial identity of each vertebra
(i.e. region: cervical, thoracic, lumbar, sacral, or coccygeal/caudal). You need to record enough
information (sketch, photograph) to justify your observations. You are welcome to discuss these
observations with your classmates during the laboratory period.
Figures & Results
Prepare a figure (not a table) with the data collected above, compared to the mouse (Figure 8-10 from
Schoenwolf et al. (2009)). Your figure may be done with the computer or by hand (whichever you
prefer). Include an appropriate figure legend. Use the figure and a critical analysis of your
observations to write up a results section. Some questions to think about - Is the total number of
vertebrae identical in each? Are there equal numbers of lumbar vertebrae? Do the lumbar vertebrae in
each organism look the same?
Discussion
Based upon your results generate a hypothesis about anterior/posterior patterning for each of your three
organisms. Create a hypothetical* model of how these patterns could have been created (per the
introduction). Please include relevant hypothetical* in situ hybridizations of embryos to support your
hypothesis.
References
Carroll, S.B. et al., (2005), From DNA to Diversity: Molecular Genetics and the Evolution of Animal
Design, 2nd edition, Blackwell Press.
Schoenwolf et al. (2009), Larsen’s Human Embryology, 4th edition, Elsevier Press, p230.
*
or actual
Readability
Date(s)
Title/Purpose
Introduction
Procedures
Observations
Hypotheses
Discussion
Entry is made
in Table of
Contents
 Information to
find digital
images
 Uses References to pertinent
resources
 Scientific names
 Source for skeletons
 Attempt to determine vertebral
identity and number
 Description of how results obtained
with observations
 Figure displaying overview of
anterior/posterior vertebral
patterning in each animal
 Three distinct hypotheses (1 for
each skeleton) to account for
differences
 Embryological “evidence” to
support your hypotheses (in figure
form)
 Contains standard
grammar.
 Contains standard
spelling, particularly
discipline-specific
terms.
 Contains enough
details to reproduce
work at a later date.
 Demonstrates an
understanding of all of
the procedures
demonstrated in the
laboratory
 3 or more
format
requirements
were not
satisfied.
Comments:
 3 or more content requirements were
omitted.
2 points
 2 or 3 content requirements were
omitted/missing details.
0 points
 1 or 2 format
requirements
were not
satisfied.
7 points








5 points
Content includes…
25points
Format
0 points
0 points
2 points
5 points
Laboratory Rubric
Evolution of Vertebrate Animal Design
 1 readability
requirement was
deficient.
 Two or more of the
items above are
missing.
References & Resources
!
Sources for information about Hox gene expression
Developmental Biology/Embryology Textbooks!
From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, 2nd ed.,
S.B. Carroll, J.K. Grenier, & S.D. Weatherbee, Blackwell Press (2005)
See pp. 46-48, 85-87, 140-144
Developmental Biology, 10th ed., S.F. Gilbert, Sinauer Associates Inc. (2014)
See pp. 311-314
Larsen’s Human Embryology, 4th ed., G.C. Schoenwolf, S.B. Bleyl, P.R. Brauer & P.H.
Francis-West, Elsevier Press (2009)
See pp. 227-232
Essential Developmental Biology, 3rd ed., J.M.W. Slack, Wiley-Blackwell Press (2013)!
See pp. 157-158, 262-265, 426-428!
Open Access On-Line!
Myers, P. (2008) Hox genes in development: The Hox code. Nature Education 1(1):2.
http://scienceblogs.com/pharyngula/2007/09/the_hox_code.php !
Burke, A.C., Nelson, C.E., Morgan, B. A & C. Tabin (1995) Hox genes and the evolution of
vertebrate axial morphology. Development 121: 333-346.!
http://dev.biologists.org/content/121/2/333.long
Vertebral Morphology & Identity
A Laboratory Text for Comparative Vertebrate Anatomy: Structure and Evolution of
Vertebrates, Alan Feducca, W.W. Norton & Co. (1975)
Articulated chicken skeleton (Figure 5-13, p. 80)
A Photographic Atlas for the Anatomy & Physiology Laboratory, K.M. VanDeGraaff, D.A.
Morton, & J.L. Crawley, Morton Press (2007)
Sacrum & coccyx, rib cage, and rib (Figures 5.33-37, p. 49)
Vertebral column and representative vertebrae (Figures 5.26-28, p. 47)
Comparative Anatomy of the Vertebrates, G.C. Keat & R.K. Carr, McGraw Hill Publishers
(2001)
Vertebrae and ribs of an alligator (Figure 8.14, p. 151)
Vertebral column and pelvic girdle of an anuran (Figure 8.13, p. 151)
Sacral vertebrae of selected vertebrates (Figure 8.17, p. 153)
Vertebral column of pigeon (Figure 8.18, p. 153)
Comparative Vertebrate Anatomy, K.V. Vardong & E.J. Zalisho, McGraw Hill Publishers
(2009)
References & Resources
!
Squalus
Axial column of a cat (Figure 5.14, p. 52)
Handbuch der Anatomie der Tiere für Künstler, Herman Dittrich (Illustrator), Wilhelm
Ellenberger & Hermann Baum, University of Wisconsin (Wikimedia Commons)
Engraving of canine skeleton
Larsen's Human Embryology, 4th ed., Gary Schoenwolf et al. Elsevier Press (2009)
Hox code patterns the vertebrae (Figure8-10, p. 230)
Manual of ornithology, Avian Structure and Function, Noble S. Proctor and Patrick J. Lynch,
Yale University Press (1993)
Golden Eagle skeleton (p. 139)
Vertebrate Biology, Donald Linzey, McGraw Hill Publishers (2001)
Lateral view of the skeleton of a boy fish (Teleostei) (Figure 5.12, p.99)
Structure of two kinds of vertebrae found in teleosts (Figure 5.14, p.100)
Salamandar skeleton and trunk vertebrae (Figure 6.12a & b, p.141)
Skeleton of a bullfrog (Figure 6.12c, p.141)
Ward's Natural Science Establishment, Inc. (1963)
Turtle (ventral)
Wikipedia Commons, SmithTestudoSkeleton.png
Side view of skeleton of turtle
Examples of student work