12 and 15 November, 2004 Chapter 19 Molecular Evolution and Phylogenetics

Overview Comparing genome sequences has offered unprecedented insight into the nature and evolution of species. Most animals have essentially the same genes. Duplication may be followed by structural or regulatory diversification. Regulatory divergence may involve a change in location of expression, in the structure of a regulator, or in the arrangement of enhancers that respond to a signal. Manipulation of pattern formation genes can lead to striking changes in morphology. Hox gene cluster organization is conserved from Drosophila to mammals. Morphological differences between Arthropod groups are explicable in molecular terms. Conservation of pattern formation mechanisms is common across phyla. Humans have only thousand genes, and develop complexity from combinatoric factors.

Metazoan Phyla

Genomic Phylogeny

FGF genes in Ciona and Vertebrates

Duplication may be followed by divergence in both sequence and regulation.

Changes in Pattern-determining Gene Expression

Misexpression of Pax6 causes abnormal eye formation.

Phenotypes of changes in pattern-expression genes

Duplication of ancestral gene leading to Antp and ftz

Two, ten, and fourteen base pair changes in the enhancer have large consequences.

Consequences of Ubx deletion and misexpression in the mesothorax

Swapping a repressor for an activator domain makes overexpressed Ubx produce a phenotype like that of overexpressed Antp.

Functional change in the Ubx binding site

Hox genes in Drosophila and mouse

Organization of Hox gene clusters in Drosophila and mouse

Homeotic nature of the Hoxb8 deletion mutant

A mutation in the Ubx enhancer led to altered expression of Ubx at the branchiopod - isopod split, and accounts for the presence of a maxilliped instead of a swimming limb in isopods.

Drosophila Dll is repressed by Drosophila Ubx, but not by shrimp Ubx.

Dll expression in several phyla

Expression ob butterfly hindwings probably results from loss of Ubx binding sites.

Nonconservative FOX2p changes may account for some of the speech abilities of humans.

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