CAS BI 569
Major Animal Characters
Part 1 — Developmental Characters by J. R. Finnerty
CHARACTERS
A character is any measurable trait possessed by an organism.
“A character can be viewed as a set of alternate conditions, called character states, that are able to evolve one to another.” Maddison & Maddision, 1992.
A character can be though of as a category, e.g.,
“type of appendage” or “number of digits.”
The character state is the value for that category in a particular “taxon” under consideration.
For example, the character “type of appendage” could have the character states “fins,” “limbs,” or
“none.”

CHARACTERS are Hypotheses of Homology
While states of one character may evolve one to another, states of two different characters may NOT evolve one to another.
For example, number of appendages = four may not evolve into eye color = blue.
In other words, character states for the same character are homologous. Character states of different characters are not homologous.
Wings in birds should not be coded as the same character as wings in insects because we know they are not homologous. (That is to say, the wings of birds and the wings of insects are not derived from wings in their common ancestor.)
HOMOLOGY versus HOMOPLASY
HOMOLOGY is similarity that is directly attributable to common descent .
The wings of a robin are homologous to the wings of an owl because both species inherited this feature from their common ancestor.
HOMOPLASY is similarity that is not attributable to common descent .
Both birds and bats possess wings. Their common ancestor was wingless. This similarity is a case of homoplasy.
Homoplasy results from CONVERGENT
EVOLUTION , where unrelated taxa evolve similar traits.

Three Classes of Characters Useful in
Inferring Relationships Among Animal Phyla
Developmental stages & processes
Major features of adult anatomy (the“bodyplan”)
Gene sequences
Developmental Characters
Since development produces morphology, all differences between the morphology of two taxa must be due to differences in their development.
Closely related taxa with similar morphologies will exhibit similar “ontogenies.”
However, distantly related taxa with distinct body plans may also exhibit developmental similarities, particularly in early developmental stages.
Thus, development may provide evidence of relatedness that is not evident from the adult morphology.

Darwin & Developmental Characters
In Darwin’s opinion, “embryology is by far the strongest class of facts in favor of the change of forms.”
In other words, he regarded development as the strongest evidence for evolution.
-Charles Darwin
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The Barnacles–Darwin’s Favorite Example
Until the 1830’s, barnacles were classified as molluscs due to their superficial resemblance to bivalves.
However, the nauplius larva suggests otherwise.
The nauplius is a planktonic larva with 3 sets of segmented appendages (think Arthropod).
Nauplius larvae are characteristic of crustaceans
(phylum Arthropoda).
The ciliated trochophore larva is characteristic of the phylum Mollusca.
The nauplius reveals “community in embryology” and therefore “community in descent” between barnacles and other crustaceans.

1st antenna
(Ruppert & Barnes, Invertebrate Zoology )
2nd antenna labrum mandible
(Ruppert & Barnes, Invertebrate Zoology ) apical organ prototroch mouth metatroch telotroch
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Barnacle
Lippson, A.J. & Lippson, R.L. 1984. Life in the Chesapeake Bay . Johns Hopkins University Press.
Nauplius and Adult Copepod

GAMETE
Fertilization
ZYGOTE Cleavage
BLASTULA
Gastrulation
GASTRULA Morphogenesis LARVA
Metamorphosis
JUVENILE Maturation ADULT
loss of a pre-adult stage
addition of a pre-adult stage

Developmental Process — CLEAVAGE
Definition —The early cell divisions whereby the embryo increases greatly in cell number though very little in overall size .
Characters
Depth of cleavage plane—Holoblastic versus
Meroblastic
Affected by amount of yolk in egg.
Prone to homoplasy
Holoblastic
Cleavage plane passes through entire egg
Meroblastic Cleavage plane passes partially through egg

Developmental Process — CLEAVAGE
Definition —The early cell divisions whereby the embryo increases greatly in cell number though very little in overall size .
Characters
Depth of cleavage plane—Holoblastic versus
Meroblastic
Affected by amount of yolk in egg.
Prone to homoplasy
Relative size of daughter blastomeres—Equal versus Unequal (micromeres, macromeres)
Equal
Unequal
Animal pole micromeres macromeres
Vegetal pole

Developmental Process — CLEAVAGE
Definition —The early cell divisions whereby the embryo increases greatly in cell number though very little in overall size .
Characters
Depth of cleavage plane—Holoblastic versus
Meroblastic
Affected by amount of yolk in egg.
Prone to homoplasy
Relative size of daughter blastomeres—Equal versus Unequal (micromeres, macromeres)
Orientation of cleavage plane—Radial versus
Sprial
Radial
Daughter blastomeres situated orthogonally
90°
Spiral
Daughter blastomeres situated at 45° 45°

2B
1d
1B
1b
1b
1d
1D
2D

Developmental Stage — BLASTULA type
Definition —The early multicellular embryo that is produced by cleavage, that precedes the formation of distinct tissue layers (or “germ layers”) .
Character States
Stereoblastula
solid cell mass
commonly produced by spiral cleavers
Coeloblastula
hollow ball of cells
commonly produced by radial cleavers
the fluid-filled cavity is called the blastocoel
Discoblastula
Disc of cells sitting atop a large amount of yolk (e.g., chicken)
Periblastula
Cell layer surrounding central yolk mass (e.g., fruitfly)
Formed from a “centrolecithal” egg.
Coeloblastula Stereoblastula
Discoblastula Periblastula

Developmental Process — GASTRULATION
Definition—cell movements that create a multi-layered embryo with distinct tissue layers (“germ layers”)
Can occur in many different ways
Invagination—a portion of the continuous blastoderm becomes indented, impinging on the existing blastocoel and creating a new cavity that will become the primitive gut or
“archenteron.”
Ingression—movement of individual cells to the interior of the embryo
Epiboly—movement of a cell sheet to the interior of the embryo
Delamination—inner layer of cells forms through cleavage tangential to the surface of the embryo.
Involution— “a tissue sheet rolls inward to form an underlying layer via bulk movement of tissue.” http://worms.zoology.wisc.edu/frogs/gast/gast_morph.html#vagition
Invagination
Ingression
Delamination
Epiboly
Involution

http://worms.zoology.wisc.edu/frogs/gast/gast_morph.html#vagition
Bodyplan Feature — GERM LAYERS
Definition—The primordial tissue layers formed during development by the process of gastrulation .
Germ layer theory was formalized by embryologists in the late
19th century (e.g., Haeckel, Huxley)
Three germ layers
endoderm
usually derives from lower vegetal pole of the embryo
forms lining of the gut
ectoderm
usually derives from upper animal pole of the embryo
forms outer epidermis and central nervous system
mesoderm
called secondary because it derives from one or both of the primary germ layers (endoderm & ectoderm)
gives rise to connective tissues (e.g., muscle, vascular tissue, skeletal elements, gonads)
These germ layers are thought to be homologous across the animal kingdom.

Bodyplan Feature — GERM LAYERS
Character States
Triploblastic—possessing all three germ layers
endoderm
ectoderm
mesoderm
Diploblastic—possessing the two primary germ layers
endoderm
ectoderm
NOTE on neural crest
a “4th germ layer” unique to vertebrates
derives from ectoderm adjacent to the developing neural tube
migrates ventrally in streams
contributes to a wide range of structures and differentiated cell types including elements of the facial skeleton, peripheral nervous system, and melanocytes.
Blastula
Gastrula (diploblast)
Archenteron
Blastocoel
Archenteron
Triploblast
Blastocoel
Endoderm
Ectoderm
Endoderm
Mesoderm
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