Lecture 26

Question 1

Changes in Body Plans in Evolution

Answer 1

• i.e. loss of legs
• snakes, lizards and whales
• same causes or different?–>snakes front limbs lost first; in whales hind limbs lost and fore limbs retained

Question 2

Hox Genes Involved in Loss of Limbs

Answer 2

• control of regions along anterior-posterior axis
• identities of regions can change in evolution
• in insects this happens by Hox gene expression remaining unchanged, but downstream responses change
• in vertebrates the expression of Hox genes in the regions change

Question 3

Evolution of Snake Body Form and Limb Loss

Answer 3

• change in Hox pattern produces a body that has mostly thorax identity
• fore limbs lost first-lost by change in axial identity and loss of expression pattern needed for limb bud outgrowth
• hind limb bud present but slowly losing genetic controls and interactions needed for bud to develop into a limb
• pythons make limb bud but it doesn’t form into limb
• more advanced snakes have lost limb buds all together

Question 4

Whales Limb Loss

Answer 4

• involves only hindlimb
• unlike snakes, body axis and its Hox gene regulation is not so drastically reorganized
• overall a number of other kinds of novel features have evolved in whales, including sense organs, locomotion and reproduction
• 55 million year fossil record and DNA gene sequence phylogeny now allow us to reconstruct their evolution from a land mammal in great detail

Question 5

Failure of Whale Hind Limb

Answer 5

• bud forms in proper location, but fails in way similar to hind limb reduction in python
• breakdown of the signaling systems that sustain limb bud growth and differentiation
• shh signal lost, FGF8 and AER not maintained

Question 6

Details of Convergence

Answer 6

• fish have vertical tail fin that moves side to side–undulation of tail; primitive vertebrate design that’s a stiff rod with paired bands of muscle; contraction on alternate side leads to lateral undulation
• whales have horizontal tail fins–undulation in vertical plane is a power stroke by muscle contraction in up epaxial muscles; dorsal side of body; had to evolve de novo as a novel feature which can now be traced

Question 7

Two Limb Buds Not Identical

Answer 7

• Forelimb identity happens via Tbx5
• hind limb identity happens via Tbx4
• positional differences
• differences mean it is possible for selection to act on genes that will change one limb bud but not the other. One can be reduced and the other can keep normal function

Question 8

Mermaids

Answer 8

• not possible to ever exist because there is no way a mammal will converge on a vertical oscillating fish tail that moves side to side
• our bodies aren’t built to support that kind of motion

Question 9

Whale Origins

Answer 9

• ancestor ca. 55 mya a land mammal that spend considerable time in water-the primitive artiodactyl indohyus
• artiodactyla ancestry of whales is confirmed by DNA phylogeny-closest living relative is hippo
• rapid evolution of new aquatic body plan took about three myr and is well recorded in fossil record
• invention of novel propulsive system as well as novel means of hearing underwater

Question 10

Stages in Evolution of Whale Locomotion (least to most primitive)

Answer 10

• primitive whales-greatly reduced hind leg used for copulatory guide; fully aquatic intermediate ear
• rodhocetus-reduced legs tail fluke
• ambulocetus-able to walk awkwardly on land, hand and feet with small hooves; swimming by dorsoventral oscillation of hind limbs
• pakicetus-terrestrial; more likely a primitive artiodactyl; semiaquatic?

Question 11

Indohyus

Answer 11

• first whale

- artiodactyla

Question 12

What are the closest relatives of whales?

Answer 12

• genes show a surprising answer
• may have to rethink the artiodactyla classification because genes show that hippos (artiodactyla) are the closest relative to whales

Question 13

Are whales own independent order?

Answer 13

• or peculiar artiodactyls even though they have evolved so far from them?
• purely on morphology people have always put whales as own order
• bring in molecular data says whales and hippos are very closely related making them artiodactyls