BIO 203 Lecture Notes - Lecture 38: Treehopper, Cervical Vertebrae, Leafhopper
14 Dec 2016
School
Department
Course
Professor
Principles of animal evolutionary development (evo-devo)
One of the first and most important discoveries of evo-devo is the developmental
toolkit.
• The gene regulatory networks that are common to development of all
animals are very old.
Example of developmental toolkit genes.
• One gene, called Distal-less, is responsible for limb formation in organisms
ranging from marine worms to mice (pictured here at 17 days) to humans.
A small number of mutations can make a large evolutionary difference
• Mutations to Hox genes can put an entire leg where an antenna should
sprout out and produce other equally grotesque transformations.
• Expression of Ubx prevents limb growth, one difference resulting in insect
vs. shrimp leg types
Evolution of vertebrate appendages – fins to limbs
Tetrapod limbs evolved from the fins of fishes
In a recent paper, Renata Freitas engineered a limb in a modern zebrafish by
tinkering with Hox expression
Development can bias evolutionary outcomes
• Birds have necks of very different lenths. The long-necked birds have more
neck vertebrae. Number of neck vertebrae ranges from 13 – 25
• All mammals on the other hand, have only 7 neck vertebrae (even giraffes!).
What is the most plausible explanation for this lack of variation in mammals?
Mechanistic explanations for this specific developmental bias
• Hox genes for axial patterning (cervical vertebrae) are pleiotropically linked
with genes that cause major developmental abnormalities.
• Evidence: Among human stillborn babies, a very high percentage have
abnormal cervical vertebrae development.
Evolution of insect wings: Membracidae
Almost all modern insects have only two pair of wings.
• Is this 2-winged condition the result of selection against extra wings or
developmental constraint?
• Developmental constraint – morphological evolution is limited by
developmental pathways already in place.
• In 400 my of evolution, no insects have evolved >2 flight wings.
find more resources at oneclass.com
find more resources at oneclass.com
Fossil mayfly nymph with elaborated gill plates on T1-T3
• Early insects had elaborate gill plates on each thoracic and abdominal
segment
• Growth of these is repressed in modern insects by a gene called Scr (sex
combs reduced)
• If you turn this gene off in segments you can make extra wings develop
Thought that leafhopper helmets were outgrowths of thoracic cuticle.
• Extant insects all have wings on T2 and T3 the second and third thoracic
segments.
• Leafhopper helmets grow from T1
• Scr is reduced and genes that turn on wing-patterning, Distal-less and
Homothorax are active in growth of helmet
From examination of this tree, answer the following: What insect group are the
Membracidae related to… When did this occur?
• In the Membracidae a mutation that turned off Scr in T1 that resulted in wing
growth on T1
• This appendage did not contribute to flight and was free to vary.
• Evolved into an extremely morphologically diverse appendage
• Resulted in a new family of insects the Membracidae.
Most insects have two pairs of wings due to: a selection against extra wings
Simple Sensory Pathway
• Reception: information obtained from the environment
• Transduction: converts information to membrane potential
• Transmission: action potentials travel down axon
• Perception: Ganglia (brain) processes information
RECEPTION: Sitmuli
• Stimulation of the plasma membrane of a receptor cell (a neuron) results in
depolarization (change in the charge surrounding the membrane)
• Receptors activated depend on strength of signal
o 2 way signal is stronger
▪ A higher frequency of action potentials signaled
▪ More receptors activated
o These 2 can act in tandem to create a very strong signal.
o Single sensory receptor activated
▪ Gentle pressure; Low frequency of action potentials per
receptor
▪ More pressure; High frequency of action potentials per
receptor
o Multiple receptors activated
▪ Gentle pressure; Fewer receptors activated
find more resources at oneclass.com
find more resources at oneclass.com
