Evolutionary Bio Chapter 13

Evolutionary Development

• Study of how changes in embryonic development can lead to the evolution of new traits and how developmental processes themselves evolve over time

• Bridges the gap between an organism's genetic code and its final form through the mechanisms that shape an organism from embryo to adult

Development

The progressive changes in size, shape, and function during the life of an organism by which its genetic potentials (genotype) are translated into functioning mature systems (phenotype)

Parallelism

Pattern from simple to complex seen everywhere

Aristotle: Scala naturae

Great Chain of Being

Biogenic law by Ernst Haeckel

“Ontogeny recapitulates phylogeny” – development of “higher” animals repeats phylogenetic ancestry

Modern Understanding by Karl von Baer

During development, general, totipotent traits develop before specialized traits that distinguish different species

Prediction implicit

Structures that appear early very resistant to evolutionary change

Heterochrony

Some genes change rate or sequence of development, and thus timing at which developmental stages occur

Sequence heterochrony

The order of events has flipped in development

Flour beetles

• Acceleration relative to flies

• Differences in rates of developmental in homologous nerve clusters tied to locomotion, vision, spatial orientation

Paedomorphosis - Heterochrony

Slower somatic development

Progenesis - Paedomorphosis

Advanced sexual maturation and unchanged appearance of traits

Neoteny - Paedomorphosis

Slower developmental rates and retarded appearance of traits

Post-Displacement - Paedomorphosis

Retarded onset of organ growth

Peramorphosis - Heterochrony

Acceleration of somatic development

Peramorphosis - Heterochrony

Acceleration of somatic development

Hypermorphosis - Peramorphosis

• Unchanged appearance of somatic traits

• Retarded appearance of reproductive traits

Acceleration - Peramorphosis

• Accelerated appearance of somatic traits

• Unchanged appearance of reproductive traits

Pre-Displacement - Peramorphosis

Advanced onset of organ growth

Homeotic genes (Master regulators)

• Determine identity and positioning of anatomical structures during development

• Critical for construction of phenotype (which is where selection acts)

• Gene products from combinations of homeotic genes act as activation signatures that generate instructional map for where structures should develop

Hox genes

• Set of homeotic genes that affect anterior to posterior positioning of structures on the embryo’s body by encoding transcription factors

• Recognize DNA motifs (in combination with cofactors) to activate or repress sets of target genes.

What are regulated by spatial and temporal pattern of HOX genes?

• Cell migration

• Differentiation programs

• Identity

• Structural formation

Regulatory enhancers and Regulatory silencers

Turn the expression of their targets on or off

Homeobox

Same 180-base pair sequence in homeotic genes in a wide array of animal species – allowed for identification of many other homeotic genes

Collinearity

As more Hox genes discovered, position of Hox genes on chromosomes corresponds to place on body that Hox gene regulates

cis-regulatory elements

Non-coding stretch of DNA that lies outside of gene but is involved in the timing and level of that gene’s expression

Three-spine Stickleback

• Cis-regulatory enhancer inserted into genome of freshwater fish

• Developed same pelvic morphology as marine fish

Heliconius

• Reddish color associated with transcription factor optix

• Recombinational rearrangements of cis-regulatory domains across lineages drive color patterns

Gene duplication

Establishment of multiple copies of same gene (paralogs) - generate new developmental pathways

Hox duplications contribute to …

body plan complexity in vertebrates

Duplicate genes influence gene expression by

• Increasing histone production

• Increasing rRNA

• Increasing transcription/translation products

Subfunctionalization

Duplicate genes can divide work done by single gene

Neofunctionalization

Duplicated genes can diverge with others taking on new function

Ancestral chordates have…

neural crest cell progenitors

Neural crest cells

• Migrate to ultimate destination – implications for domestication

• Brain nccs associated with HPA -axis.

• Select for lower stress responses (calmer)

• Select for fewer/slower nccs migrating to head/brain