Drosophila melanogaster• Each segment in the adult fly is anatomically
distinct– Characteristic appendages
Drosophila embryonic development• Subsequent embryonic events create clearly
visible segments– Initially look very similar
• Some cells move to new positions– Organs form
• Wormlike larva hatches– Eats, grows, & molts
Drosophila early gradients• Bicoid gene product is concentrated at anterior
end of fly embryo– Gradient of gene product– Essential for setting
up anterior end of fly
• Gradients of other proteins determine the posterior end and the dorsal-ventral axis
Drosophila segmentation genes
• Segmentation genes– Genes of embryo– Expression regulated by products of egg-
polarity genes– Direct the actual formation of segments after
the embryo’s major axes are defined
Three sets of segmentation genes
• Three sets of segmentation genes are activated sequentially– Gap genes
– Pair-rule genes
– Segment polarity genes
• The activation of these sets of genes defines the animal’s body plan– Each sequential set regulates increasingly fine
details
Gap genes• Gap genes
– Map out basic subdivisions along the embryo’s anterior-posterior axis
– Mutations cause “gaps” in the animal’s segmentation
Pair-rule genes• Pair-rule genes
– Define pattern in terms of pairs of segments– Mutations result in embryos having half the normal
number of segments
Segment polarity genes• Segment polarity genes
– Set the anterior-posterior axis of each segment
– Mutations produce segments where part of the segment mirrors another part of the same segment
Recombination favours negative epistasis favours sex
• Only without strong directional selection on a particular gene expression pattern
• Mutational load is lower with recombination AND negative epistasis
• What are the possible predictions?
The structure of the genetic code
• Amino acids in the same column of the genetic code are more related to each other physico-chemically
• „The genetic code is one in a million” (Freeland & Hurst)
Connectivity
• The average connectivity of the neighbours of the black node with k = 3 links is < kn > = 4.
Secondary extinctions resulting from primary species loss in 16 food webs ordered by
increasing connectance (C ).