Zygotic genes and pattern formation

Question 1

What do the bcd, hb, and cad transcription factor gradients do in the syncytial blastoderm?

Answer 1

• directly activate the transcription of several zygotic embryo lethal patterning genes (gap genes) in nuclei of the syncytial blastoderm
• these genes are expressed in specific regions of the embryo such that the gap gene proteins form several localized gradients

Question 2

How is the pattern of gap gene transcript established?

Answer 2

specific concentrations of maternal bcd+, hb+, and cad+ protein activates or represses the expression of specific gap genes

Question 3

Where does the bicoid protein bind?

Answer 3

to the specific cis-acting sequence in the enhancer region of the hb gene

Question 4

What does deleting some of the bcd binding sites do to the zygotic expression?

Answer 4

decreases the size of the expression domain and the level of expression

Question 5

What does deleting all of the bcd binding sites do to zygotic expression?

Answer 5

eliminated expression entirely

Question 6

How is the pattern of Kr+ gene expression established?

Answer 6

• high levels of bcd+ and hb+ protein inhibit Kr+ transcription
• low levels of bcd+ or hb+ protein activate kr+ transcription

Question 7

How is the pattern of Kni+ gene expression established?

Answer 7

• bcd+ protein inhibits kni+ expression

- low levels of cad+ and hb+ activates kni+ expression

Question 8

What is the pair rule genes?

Answer 8

1. an embryo homozygous for a loss of function mutation in a pair rule gene lacks every other segment. The role of the gene is to promote segment formation
2. Several pair rule genes have been cloned and all encode transcription factors

Question 9

How is the pair rule genes expressed?

Answer 9

• seven stripes
• 3-4 cells wides
• perpendicular to the anterior posterior axis
• stripes of expression are separated by a 3-4 cell wide strip of no-expression

Question 10

What are the two types of patterning?

Answer 10

• even skipped (the lines are on the even number 2,4,6..)

- fushi tarazu (the lines are on the odd number 1,3,5..)

Question 11

What activates pair rule and homeotic selector genes?

Answer 11

GAP proteins

Question 12

When are gap genes first expressed?

Answer 12

11-13th cycles of nuclear division - cellular blastoderm

Question 13

Where is the gap proteins?

Answer 13

the membranes form around the nucleus and gap proteins are trapped inside

Question 14

How is the expression patter of the primary pair rule genes established by the gap gene transcription factors?

Answer 14

1. Pair rule genes are activated by a combination of the gap gene morphogens
   a) pair rule genes are expressed after blastoderm cellularization
   b) in gap gene mutants the pair rule genes are not expressed correctly - some of the stripes of expression do not form
2. the pair rule enhancer regions are very complex involving multiple binding sites for all of the gap gene transcription factors
   a) the binding of different amounts and combinations of gap gene protein to the pair rule promoter result in its activation or repression
   b) deletions of parts of the enhancer region of pair rule genes can result in the loss of only one or two of the stripes

Question 15

What results in the loss of a single stripe?

Answer 15

deletion of one enhancer element (L16/17 slide 41)

Question 16

What regulates the identify of body parts?

Answer 16

Hox genes

Question 17

What are gap gene proteins?

Answer 17

• transcription factors
• act as morphogens, activating and/or repressing expression of the pair-rule and homeotic selector genes in different cells of the cellular blastoderm

Question 18

What is the purpose of the pair-rule gene?

Answer 18

to determine the position and number of segments in the drosophila embryo

Question 19

Where is the homeotic selector genes first transcribe?

Answer 19

contiguous cells in the cellular blastoderm

Question 20

How is the homeotic selector genes first transcribed?

Answer 20

broad overlapping domains

Question 21

What have we learned concerning pattern formation?

Answer 21

1. Asymmetry in the female is used to generate asymmetry in the oocyte
2. Asymmetry in the oocyte is used to generate maternal morphogens in the early embryo
3. Maternal morphogens established a specific pattern of zygotically-expressed morphogens
4. Zygotic morphogens activate a complex set of genes required to determine the differential fates of cells in the blastoderm thus establishing a spatial pattern of morphological structures

Question 22

What are the principles of pattern formation?

Answer 22

1. must establish positional information within the developing embryo
2. direct simple patterns early and use to elaborate more complex pattern
3. use a cascade of determination events to coordinate timing
4. use master regulatory proteins each of which will be expressed in specific groups of cells marking them for a particular fate

Question 23

What things are involved in pattern formation of dorsal-ventral axis?

Answer 23

• SPZ ligand
• TOLL
• DL protein
• CACT

Question 24

What is SPZ ligand?

Answer 24

-a peptide derived from Spatzle protein

Question 25

What is TOLL?

Answer 25

a transmembrane receptor for SPZ ligand

Question 26

What is DL protein?

Answer 26

dorsal transcription factor

Question 27

What is CACT?

Answer 27

cytoplasmic anchor protein for dorsal

Question 28

Where is spatzle ligand produced?

Answer 28

only in the ventral follicular cells

Question 29

What happens on the ventral side of the zygote?

Answer 29

SPZ ligand induces translocation of DL to nucleus

Question 30

What happens on the dorsal side of zygote?

Answer 30

DL remains in cytoplasm

Question 31

Why is drosophila embryogenesis so important?

Answer 31

• drosophila and vertebrate Hox protein show striking similarities
• model organism!!!