CBNS 108 lectures 6-7

How were genetic screens used to determine developmental genes?

Mutagenesis screens. studying mutations that cause birth defects.

How are the A-P and D-V axes are established in the Drosophila egg?

Maternal determinants

What are the stages of embryo development in drosophila?

Cleavage –> Cellular blastoderm –> Gastrulation and segmentation –> Organogenesis

What is a syncytium?

a single mass of cytoplasm containing multiple nuclei that is not divided into separate cells

After 9-10 division cycles, what happens to most of the drosophila nuclei?

most nuclei move outward to lie just beneath the plasma membrane, forming a syncytial blastoderm.

What is unique about Drosophila cleavage divisions?

They occur without cellular division, meaning all the dividing nuclei share a cytoplasm.

In drosophila, which cells are first to form? Where do they form?

Pole cells (Primordial Germ Cells); posterior pole

How is the the cellular blastoderm formed in drosophila?

Plasma membrane invaginates between nuclei, extending to separate each nucleus from the other. The membranes around each nucleus extend towards each other, and the gaps are closed to cellularize all nuclei simultaneously

During gastrulation of the drosophila, cells of which germ layer(s) are internalized and which one(s) remain(s) outside?

The mesoderm and endoderm are internalized; the ectoderm remains outside

What occurs during segmentation?

The body is defined into segments: head, thorax, abdomen

The process by which cells in a developing embryo acquire identities in a well-ordered spatial pattern is called:

Pattern formation

What conclusion did Nusslein-volhard and Weischaus make about genetic mechanisms controlling early development of multicellular organisms?

They are highly conserved

What are the two broad classes of pattern formation mutations identified in drosophila?

Maternal and zygotic

What is the general function of maternal effect genes?

establishing initial polarity and positional information along the primary body axes (A-P and D-V)

Describe the effect of maternal effect genes.

Embryos from a mutant mother will develop a patterning defect, even if the embryo has a normal copy of that gene

What is the general function of zygotic genes?

pattern formation along the A-P and D-V axes that were established by maternal genes

What happens to embryos that are homozygous for mutant zygotic genes?

They will develop a patterning defect

Which drosophila segment is considered anterior?

Head and thorax

Which drosophila segment is considered posterior?

abdomen

What are the function of terminal genes?

required for patterning anterior and posterior extremities

Bicoid is an __________ determinant.

Anterior

Nanos is an __________ determinant

Posterior

Transcriptional activator of a gene called hunchback

Bicoid

Translational repressor of hunchback mRNA

Nanos

What happens to embryos from bicoid mutant mothers (no functional bicoid in the embryo)?

develop with defects in anterior segments

What happens to embryos from nanos mutant mothers (no functional nanos in the embryo)?

develop with defects in posterior segments

Where in the drosophila larvae are bicoid mRNAs localized?

Anterior side

Where in the drosophila larvae are nanos mRNAs localized?

Posterior side

How do we know Bicoid is necessary for anterior development?

Bicoid mutant larva don’t have bicoid in the anterior(break it experiment)

The _______ ___________ experiments led to the idea that Bicoid and Nanos act as anterior and posterior determinants, respectively

Protein localization

What would you expect if you place Bicoid in a different region of a mutant embryo?

The region with the highest amount of Bicoid adopts anterior-most fate

What happens if you increase the maternal dosage(# of gene copies) of bicoid in a larva?

stronger gradient of Bicoid protein, the location of the cephalic furrow moves posteriorly

True or false: bicoid is a morphogen.

True

The A-P cytoplasmic gradient of Bicoid protein becomes an A-P _______ gradient of Bicoid protein

Nuclear(meaning in the nucleus)

How does Bicoid instruct different fates at different concentrations in the nucleus?

Bicoid regulates the transcription of many target genes, and different target genes are activated by different threshold concentrations of Bicoid

What is the effect of bicoid on the hunchback gene?

Hunchback is transcriptionally activated by Bicoid in the anterior of the embryo

What happens to hunchback in the absence of nanos?

Hunchback accumulates in both anterior and posterior regions, leading to no posterior fates

How is maternal hunchback mRNA distributed in the egg?

Uniformly

How do Bicoid and Nanos generate an A-P gradient of Hunchback protein?

Bicoid, which is concentrated in the anterior region, transcriptionally activates Hunchback. Nanos, which is concentrated in the posterior region, translationally represses hunchback. As a result, Hunchback is activated in a gradient that in concentrated in the anterior region.

How many segments is the fly body plan divided into?

14

Identify the AP and DV axis, thorax, and abdomen on a fly larva.

Each segment of the drosophila larvae has polarity along which axis?

A-P

How does drosophila larva divide in response to zygotic gene expression?

Progressive division of the embryo, first into broad zones and then into 14 parasegments (each with a posterior and an anterior half)

What is a transcription factor cascade? What initiates it?

activation of transcription factors in series; maternal determinants

What are zygotic patterning mutations?

mutations in zygotic patterning genes of the embryo that affect its development

Name the 4 classes of zygotic genes are involved in segmentation along the A-P axis

GAP genes

PAIR-RULE genes

SEGMENT POLARITY genes

SEGMENT IDENTITY genes

Mutations in these genes affect the number or polarity of segments:

GAP genes

PAIR-RULE genes

SEGMENT POLARITY genes

Mutations in these genes affect the identity of segments

SEGMENT IDENTITY genes

What defect occurs in Kruppel mutant embryos?

Missing segments T1-A5

Define gap gene mutants.

Mutant embryos are missing one or more Continuous segments

Name 3 types of gap genes.

Hunchback, knirps, kruppel

What was the first gap gene to be expressed in the Drosophila embryo?

Hunchback