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exploring links between evolution and development: the Hox genes

exploring links between evolution and development: the Hox genes. overall motivation: When a new form evolves, the developmental processes of the ancestor must be altered. Development is really complicated. How does this work?

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exploring links between evolution and development: the Hox genes

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  1. exploring links between evolution and development: the Hox genes overall motivation: When a new form evolves, the developmental processes of the ancestor must be altered. Development is really complicated. How does this work? Today’s case study: evolution of anterior-posterior anatomy

  2. egg blastula to gastrula (larva) adult Q: how do adult segments “know” which one they are?

  3. from mutant flies come a big clue

  4. Homeotic mutants affect genes. How do we figure out which genes? • use recombination mapping to narrow down region of genome where mutation resides. • compare DNA of mutants and wild-type flies to identify mutation • confirm mutation causes the phenotype: • get independent mutations in same gene have same phenotype • add back a wild-type transgene, rescue mutant defects

  5. Homeotic mutants affect cluster of Hox (“homeobox”) transcription factors • homeobox sequence found in all Hox proteins (and others) • forms a DNA-binding protein domain Antennapedia (Antp) Ultrabithorax (Ubx)

  6. colinearity of Hox gene order and expression • expression of each Hox gene restricted to segments transformed in loss-of-fuction mutants • expressed in same order in embryo as they lie on chromosome:

  7. from mutation to mutant This is a loss-of-function mutation: loss of Ubx expression in thorax turns haltere-bearing segment (T2) into wing-bearing segment (T3). This is a gain-of-function mutation: ectopic expression of Antp in head (where it normally isn’t active) turns antennae into legs. Antennapedia (Antp) Ultrabithorax (Ubx)

  8. also Hox genes also pattern vertebrates

  9. even acoels do it Hox genes are “an animal thing”

  10. even acoels do it Hox number, A-P differentiation. Coincidence or causation?

  11. Are Hox genes really involved in the evolution of new body forms? • Ideal test case: Relatively recent change in body plan with obvious alteration of A-P axis. • Snakes! • Lizard ancestors have ~15 ribs. • Snakes have hundreds. • Are snake ribs really homologous to ribs of lizard ancestors? • How did they get so darned long?

  12. ribs and Hox go together chick ----------------------------------- python ----------------------------- h Cohn & Tickle, Nature 1998

  13. reduced expression of “tail”Hox genes allowed elongation? Di Po et al., Nature 2010

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