Genetics of Axis Specification in Drosophila: Segment Polarity Genes & Hox Genes Gilbert - Chapter 9

Today’s Goals Describe the use of a Genetic screen to identify functionality of individual gene products Identify Eric Weischaus and Christianne Nusslein-Volhard Identify key molecular players in establishing polarity and segments in various types of embryos Define the term “positional information” Describe how evolutionarily conserved genes play similar roles in development of a wide variety of species Define transcription factor

Pair-rule genes: Summary Tight control of expression by enhancer/repressor interactions in the promoter regions results in 7 stripes of each gene This divides the embryo into the 14 parasegments along the axis Each gene is expressed in different parasegments Each parasegment has a different combo of pair-rule genes

Segment polarity Genes Pair-rule genes control expression of the next set of genes - Segment Polarity genes Rather than being set up in the embryonic syncitium, these genes and proteins interactions occur between cells Complete the segmentation of the embryo Establish cell fates in the parasegments Have expression patterns and mechanisms that work the same way in each parasegment

Segment polarity genes Ex. Engrailed (en), wingless (wg), armadillo (arm), hedgehog (hh) –Components of the Wingless and Hedgehog cell signaling pathways Mutations in segment polarity genes result in missing 1/2 of each parasegment

Segment Polarity genes Upon cloning these genes and examining functions and sequences –Wg = involved in Wnt signaling pathway Wg is actually the homolog of the vertebrate Wnt protein Arm = ß-catenin homolog –HH = hedgehog signaling pathway

Basics of segment Polarity Each segment created by the Segmentation genes & pair-rule genes consists of only a few rows of cells Cells within each segment communicate with each other to provide each cell in the segment with an address –Some with denticle bands, some bare cuticle

Segment Polarity Genes Within each segment, gene expression is controlled down to the SINGLE cell level. EXAMPLE: Only 1 cell in each row is capable of expressing Wingless (wg) Same is true of Engrailed (en)

Segment polarity - mechanisms High levels of Eve (even-skipped) and ftz (fushi tarazu) (pair-rule genes) activate expression of engrailed (en) –en expression allows only 1-2 rows of cells in each parasegment to begin to express hedgehog Low levels of Eve and ftz in cells next to the en expressing cells activate wg expression –This is the band of cells directly anterior to the en expressing cells

Maintenance of Wg, HH expression Pair-rule genes set up the expression of these genes but not for long! Cells expressing Wg and those expressing HH interact with one another via these signaling pathways to maintain expression patterns and downstream effects

Interaction between Wg and HH Wingless expressing cells secrete wg protein Only cells expressing en can make the receptor for wg (frizzled) and receive the wg signal Via the wnt signaling pathway the transcription of downstream targets begins - including hh HH is then secreted and receptors on wg expressing cell specific for HH (smoothened) transmit signal to maintain wg expression

Morphological result Due to the interactions of segment polarity genes and the genes they activate, outward differences in morphology of the cells can be observed, even at the embryonic stage The pattern of the cuticle is different in each row of cells –Wg expressing cells have fine hairs –HH have long, spiked denticles –The next row of cells are smooth

Segment polarity genes: summary Expression domains are initially set-up by Pair-rule genes Segment polarity genes then interact with one another by the Wnt and HH cell signaling pathways to maintain expression Result is determination of the fates of the cells within each segment

The genes we’ve examined so far –give the embryo an A-P polarity –Set up the segmentation of the drosophila body plan –Define cells within in each parasegment Each segment is similar How can the embryo instruct the cells to know which segment they will be along the axis?

Homeotic Genes Control the Developmental Fate of Segments along the Anterior-Posterior Axis Create a global embryonic pattern – Segments are established – How do they know if they will be T1 or T3 or A4 or A6??? What structures will they form in the adult? In homeotic mutants, the structure formed by one segment is transformed so it is the same as that of a neighboring segment

Ed Lewis was far ahead of his time

Homeotic Selector genes Discovered by Ed Lewis in 1978 Found mutants in which segmental identities were altered. Shared the nobel prize w/ Weischaus & Neusslien-Volhard

Head from wild-type Drosophila Figure 23-15a Copyright © 2006 Pearson Prentice Hall, Inc.

Head from an Antp mutant, showing the replacement of normal antenna structures with legs. This is caused by activation of the Antp gene in the head region. Figure 23-15b Copyright © 2006 Pearson Prentice Hall, Inc.

1 Cluster - Split into 2 Complexes Table 23-3 Copyright © 2006 Pearson Prentice Hall, Inc.

The Antennapedia complex contains five genes that specify structures in the head and the first two thoracic segments

Figure Copyright © 2006 Pearson Prentice Hall, Inc.

The bithorax complex contains three genes that specify structures in the posterior portion of the second thoracic segment, the entire third thoracic segment, and the abdominal segments

Figure Copyright © 2006 Pearson Prentice Hall, Inc.

Hox Genes Hox genes encode transcription factors that include a 60-amino-acid DNA-binding homeodomain. – Often referred to as the Homeobox Expression of the Hox genes is colinear with position in the embryo

Figure Copyright © 2006 Pearson Prentice Hall, Inc.

Hox genes are found in the genomes of most eukaryotes with a segmented body plan, and some without segments – C. elegans – Cnidarians

Figure Copyright © 2006 Pearson Prentice Hall, Inc.

Humans and most vertebrates have four clusters of Hox genes containing 39 genes. – Paralog groups – Homologous to each other These genes control the pattern of structures along the anterior-posterior axis

Figure Copyright © 2006 Pearson Prentice Hall, Inc.