Homeotic genes in Drosophila body patterning

Department of Biochemistrypetros.ligoxygakis@bioch.ox.ac.uk

Developmental biology: Drosophila segmentation and repeated units

1

* egg: generate the system

* larva: eat and grow

* pupa: structures inlarvae grow out to form adult fly: metamorphosis

(Drosophila is a holometabolous insect)

Homeotic gene complexes in Drosophila

• ANT-C (Antennapedia complex) is largely responsible for segmental identity in the head and anterior thorax.

• BX-C (Bithorax complex) is responsible for segmental identity in the posterior thorax and abdomen.

HOMEOSIS

• Homeosis or homeotic transformation, is the development of one body part with the phenotype of another.

The bithorax mutations

• This class of loss of functions mutations cause the entire third thoracic segment to be transformed into a second thoracic segment giving rise to flies with four wings instead for the normal two.

The Tab dominant mutations

• These gain of function mutations transform part of the second thoracic segment into the sixth abdominal segment.

The Antennapedia mutations

• These gain of function mutations transform antenna into leg.

The Homeodomain

• The homeotic genes encode transcription factors of a class called homeodomain proteins. The homeodomain is a 60aa protein domain, which binds DNA. Hox genes bind DNA regulatory elements of their target genes in a specific combination so that the expression pattern in each of the different segments is unique.

The co-linearity principle: Homeotic gene expression in Drosophila

• The anterior boundary of homeotic gene expression is ordered from SCR (most anterior to ANTP, UBX and ABD-B (most posterior). This order is matched by the linear arrangement of the corresponding genes along chromosome 3.

Mechanisms underlying functional diversity of Hox

proteins

Understanding how function is encoded within Hox protein structure

The mystery of the homeodomain specificity

• In vitro, homeodomains have a very broad binding specificity, which does not explain the refined specific regulation of target genes observed in vivo. So how can this be explained?

The co-factor hypothesis

• There is the possibility of specific co-factors, which are expressed in the domain of expression of the Hox-gene. Until now very few were found, the most prominent example being Extradenticle and Homothorax (EXD, HTH; Ryoo et al, Development 126, pp 5137-48, 1999).

A2

A1

T2

T3

T1 Gebelein et al, Dev. Cell, 2002

Dll repression: a paradigm for the study of Hox/Exd interaction

DME-lacZ / Ubx

UbxAbdA DllExd +

The DNA sequence motif hypothesis

• Different combination of DNA modules would give different combination of co-factors bound on the promoter and thus a different array of transcriptional interactions with each Hox protein (Li et al, Development 126, 5581-5589, 1999).

Insect vs. mammalian Hox genes

Expression patterns of mouse Hox genes

Phenotype of a homeotic mutant mouse

• Mice mutant for a targeted knockout of the HoxC8 gene reveal ribs duplication and a clenched-fingers phenotype.

Developmental strategies in animals are ancient and highly conserved. In essence, a mammal, a worm and a fly-three very different organisms-are put together with the same basic building blocks and regulatory devices.

Wild type

Hox mutant

Lewis et al. 2000

Is there a “ground” state?What could constitute a “ground”

state? Where all HOX genes are expressed in all segments

Averof and Patel 1997

Averof and Patel 1997 Nature 388, 682-686Averof 2002 Curr Op Genetics and Development 1386-392

Insects

Branchiopoda (Artemia, the brine-shrimp)

Malacostracans (Lobsters, hermit crabs)

Expression of Hox genes in arthropods

crustaceans

Changes in Hox gene expression can help explain the evolution of arthropod

body plans

species 1 species 2

Evolution of crustacean maxillipeds

Triops (no mxp): Ubx expression in all thoracic segments

T1

T2

T3

T1

T2

T3

Mysid (1 mxp): Ubx expression from T2 to the posterior

Changes in Hox genes• Duplication of genes or whole clusters

(mammals) by unequal crossing over• Following duplication there is

diversification of both coding and regulatory sequences

• Changes (rare) in coding sequences (Ubx in Diptera vs. Ubx in Lepidoptera)

• Changes in the expression of Hox targets

Reading ListTextbooks: 1). Scott F Gilbert (2003). Developmental Biology 7th edition, chapter 9, pp285-290; 2). Wolpert Evolution and development chapter in Principles of developmentGeneral review: McGinnis W and Krumlauf R (1992). Homeobox genes and axial patterning Cell, 68, pp283-302.Evolution of body pattern review: Averof M (2002) Curr Op Genetics and Development 1386-392