Lecture 06: EvoDevo

Question 1

Three Enigmas

Answer 1

Enigma 1: Common ancestry of very different organisms
* macroevolution
* “major transitions”

Enigma 2: Phenotypic differences with almost identical genomes
* Chimpanzee sequencing shows more then 80% (paper: 95%) of the base pairs are exactly shared between chimpanzee and human DNA

Enigma 3: Evolutionary reversals of complex characters
* Example: second Molar (M2) lost in evolution of Felidae, but comes back in 10% of Lynx population.

(90% of Lynx and all Ocelot -> M1
Common ancestor of Felidae and Civet -> M2)

Question 2

Universality of developmental systems: Hox gene Evolution

Answer 2

Hox gene

• responsible for the pattern of the body -> regionalization genes
• set of genes patterning the antero-posterior body axis
• organization in chromosomal
  complexes, (as four clusters in mammals)
  in spatial and temporal colinearity
  (5’ equals posterior, 3’ equals anterior)
• first discovered in Drosophila melanogaster, (where they are secondarily split into 2 clusters, the Antennapedia complex and the Bithorax complex.)
• Vertebrates (e.g. mice, humans), on the other hand, usually have 4 Hox clusters, which are probably the result of a double duplication of the genome
• The identity of structures is determined by the Hox code,
  -> in Drosophila for example the identity of the segments (e.g. the expression of Ubx determines the identity of the 3rd thorax segment [homeotic mutant]. )
  -> In vertebrates e.g.the identity of the vertebrae along the spinal column (homeosis).

Question 3

Ubx and wing morphology

Answer 3

• Hox gene expression in Drosophila -> in Diptera: normally only forewings & halterer
• In larvae Ubx expression shows that third hypothesis is right, because Ubx is in the same region (between T2/T3) -> Ubx determines the identity of the 3rd thorax segment
• Ubx expression in adult insect
  Control: normal pattern (fore/hind wing diff)
  Ubx suppressed: forewing pattern in hindwing
• Ubx regulates different target genes -> from ancestral insect with hindwing, to Dipteran halterer and Lepidopteran hindwing

Question 4

Example Enigma 3

Answer 4

Salamanders with direct development vs. biphasic life cycle

• Plethodontidae (lungless salamanders): Direct development evolved independently at least twice
  In Desmognathines: direct development was lost and the aquatic larval stage re-appeared

Neoteny (juvenilization): is the delaying or slowing down of the physiological, or somatic development of an organism e.g. Axolotl (verlbleiben im Larvenstadium)

Enigma 3: Reversal evolution

Question 5

EvoDevo def.

Answer 5

The field of Evo Devo seeks to understand the mechanisms by which development has evolved.

Evo-Devo stands for evolutionary developmental biology
- it compares the developmental processes of different organisms to understand how developmental processes evolved
- this may help to understand how the broad diversity of organisms we know today could evolved and explain the common ancestry of very different organisms

e.g evolution of Hox- genes cluster in different organisms etc.