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Homeobox Genes and Evolution Lecture 3 Hox Gene Function 3’ Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of 3’ Gene D function? Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of Gene D function? A B C D Hox Gene Function 3’ Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of 3’ Gene D function? Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of 3’ Gene B function? Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of Gene B function? A B C D Hox Gene Function 3’ Gene A Gene B Gene C Gene D 5’ Which phenotype would you predict from loss of 3’ Gene B function? Gene A Gene B Gene C Gene D 5’ What order would you expect the Hox genes to be in on the chromosome? • • • • 3’-A-B-C-D-E-5’ 3’-C-A-E-B-D-5’ 3’-B-D-E-A-C-5’ 3’-D-B-E-A-C-5’ Gene D Gene B Gene E Gene A Gene C Mutations in Hox genes can lead to what type of phenotype? a. The anterior portion of the embryo does not develop b. Several adjacent segments will be missing in an otherwise intact embryo c. The affected segment will develop like its posterior neighbour d. Duplication of a segment Which statements describe the phenotype of the Hox mutant? mutant a. b. c. d. Anterior segments have been transformed into posterior ones Abdominal segments develop as thoracic segments Posterior segments have been transformed into anterior ones An example of a homeotic transformation Which gene is predicted to control the development of the most anterior structures? 5’ A B C D 3’ The Antennapedia Mutation Wild-type Antennapedia mutation Why do Antennapedia (Antp) mutants have legs where their antennae should be? • Absence of Antp gene function in the head transforms that segment’s appendage into one normally found in the thorax • Antp is needed for normal antennae development and is missing in these mutants • The mutants misexpress Antp in the head, transforming that segment’s appendage into one normally found in the thorax • They have no head How to get legless The vertebrae of snakes show homeosis Pythons have >300 vertebrae Very few cervical (no ribs) vertebrae: lost to form thoracic (rib bearing) vertebrae Whole body resembles thorax No forelimbs Greatly reduced hindlimbs Evolution of modern snakes Hox gene expression boundaries correlate with morphological boundaries Changes in body plan correlate with changes in Hox expression Chicken HEAD limb HoxC6 HoxC8 TAIL limb Python HEAD TAIL limb What do you predict the pattern of HoxC6 and HoxC8 look like in python embryo? What do you predict the pattern of HoxC6 and HoxC8 look like in python embryo? a) Same as the chicken b) HoxC6/C8 are not expressed in python c) HoxC6/C8 expression is expanded anteriorly and posteriorly d) HoxC6/C8 expression is expanded anteriorly Changes in body plan correlate with changes in Hox expression Chicken HEAD limb HoxC6 HoxC8 TAIL limb Python HEAD HoxC6 HoxC8 Expansion of Hox expression domains creates thoracic, ribbearing vertebrae along almost entire body length TAIL limb Also results in loss of forelimb, through expansion of expression into anterior somites Role of Hox genes in evolution 1. Most, if not all, bilaterally symmetric animals, possess one or more Hox clusters that are arranged co-linear with their head to tail expression domains 2. The Hox cluster functions during development to determine head to tail organisation by controlling region specific gene expression 3. Changes in Hox gene expression can be correlated with changes in head to tail organisation 4. New body designs DO NOT require new genes, rather the modification of the function of existing ones Role of Hox genes in evolution 1. Most, if not all, bilaterally symmetric animals, possess one or more Hox clusters that are arranged co-linear with their head to tail expression domains 2. The Hox cluster functions during development to determine head to tail organisation by controlling region specific gene expression 3. Changes in Hox gene expression can be correlated with changes in head to tail organisation 4. New body designs DO NOT require new genes, rather the modification of the function of existing ones Role of Hox genes in evolution 1. Most, if not all, bilaterally symmetric animals, possess one or more Hox clusters that are arranged co-linear with their head to tail expression domains 2. The Hox cluster functions during development to determine head to tail organisation by controlling region specific gene expression 3. Changes in Hox gene expression can be correlated with changes in head to tail organisation 4. New body designs DO NOT require new genes, rather the modification of the function of existing ones Role of Hox genes in evolution 1. Most, if not all, bilaterally symmetric animals, possess one or more Hox clusters that are arranged co-linear with their head to tail expression domains 2. The Hox cluster functions during development to determine head to tail organisation by controlling region specific gene expression 3. Changes in Hox gene expression can be correlated with changes in head to tail organisation 4. New body designs DO NOT require new genes, rather the modification of the function of existing ones