Control of the Drosophila Body Pattern

CONTROL OF THE DROSOPHILA BODY PATTERN:

THE HOMEOTIC GENES.

The adult fly body pattern is segmented. The establishment of segmental identity is determined in the embryo by the homeotic gene complexes. They are called complexes because several of the genes are clustered together on the DNA. Drosophila has two homeotic gene clusters. The ANT-C (Antennapedia complex) is largely responsible for segmental identity in the head and anterior thorax, whereas the BX-C complex (Bithorax complex) is responsible for segmental identity in the posterior thorax and abdomen.

Homeosis or homeotic transformation is the development of one body part with the phenotype of another. Three examples of body-part conversion phenotypes due to homeotic gene mutations are:

1. The loss of function bithorax class of mutations that cause the entire third thoracic segment (T3) to be transformed into the second thoracic segmen (T2), giving rise to flies with four wings instead of the normal two.

2. The gain of function dominant Tab mutation that transforms part of the adult T2 segment into the sixth abdominal segment (A6).

3. The gain of function dominant Antennapedia (Antp) mutation that transforms antenna into leg.

Note that, in all these cases the number of segments in the animal remains the same. The only change is in the identity of the segments. The results of the studies of these homeotic mutations have revealed much about how segment identity is established. The cloning of the Antp gene led to the discovery of the homeobox, an 180bp DNA fragment characteristic of homeotic genes. Homeodomain genes are transcriptional regulators, which specify the body plan by controlling the transcription of their target genes. They interact with the regulatory elements of other genes in a specific combination so that the expression pattern in each segment is unique. By inserting the Antp cDNA into a heat shock inducible vector, the body plan can be altered in a predictable way.

It is important to note that the expression patterns of Hox proteins, is matched by the linear arrangement of the corresponding genes along chromosome 3 (figure below; this is a common theme for Hox genes in many different species as we will see).

Using the homeobox as a probe, genes homologous to Hox from many species including vertebrates have been isolated. Study of these genes gave a spectacular demonstration for the universality of developmental principles. In the mouse, dominant gain and loss of function mutations result in segmental transformations of opposite direction, as in Drosophila. Also the mouse Hox genes can partially substitute the homologous Drosophila genes in transgenic flies. Mammals have their Hox genes clustered in a similar way as in flies. The major difference between flies and mammals is that there is only one Hox cluster (HOM-C) in the insect genome as opposed to four Hox clusters each located on a different chromosome in mammals. The four clusters are paralogous meaning that the order of genes in each cluster is very similar, as if the entire cluster has been quadruplicated in the course of vertebrate evolution. Each of the genes near the left end of each cluster is quite similar not only to the others but also to the insect genes at the left end of the HOM-C. Similar relations hold throughout the clusters. Finally, and most notably the Hox genes are expressed so as to define the segments in the developing somites (the segmental units of the developing spinal column) and central nervous system of the developing mouse embryo and presumably of the human embryo. Each Hox gene is expressed in a continuous block beginning at a specific anterior limit and running posteriorly to the end of the vertebral column. The anterior limit differs for different Hox genes. Within each Hox cluster, the leftmost genes have the most anterior limits. These limits proceed more and more posteriorly in the rightward direction in each Hox cluster. Overall the Hox gene clusters appear to be arranged and expressed in an order that is striking similar to that of the insect HOM-C genes. The correlations between cluster structure and expression pattern are further strengthened by consideration of mutant phenotypes. In vitro mutagenesis techniques permit efficient knockouts in the mouse. Many of the Hox genes have now been knocked out and the striking results are that all phenotypes are thematically parallel to the phenotypes of the homozygous null HOM-C flies. For example when the Hox-C8 gene (normally expressed in the first lumbar vertebrae L1; the first non ribbed vertebra behind vertebrae-bearing ribs) is knocked out then L1 is transformed into the segmental identity of a more anterior vertebra. This means that a fate shift has been caused to the anterior. The images below compare the loss of function bx mutation when T3 is transformed into the more anterior T2 and the Hox-C8- condition where L1 is transformed in more anterior vertebrae with ribs.

How can such disparate organisms like flies mice humans (and worms) have such similar gene sequences? The simplest interpretation is that the Hox and HOM-C genes are the vertebrate and insect descendants of a homeobox gene cluster present in a common ancestor some 600 million years ago. The evolutionary conservation of the HOM-C and HOX genes is not an unusual occurrence. In fact changes in Hox gene regulation and function can be used as ways to change the body patterns of animals and create a new form that evolution can then test. For implications of this see reference d below. As we are beginning to compare whole genomes, we are finding that Hox genes must have played an important role in the evolution of body patterns.

Control of the Drosophila Body Pattern

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