Homeotic Gene Clusters

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The biggest similarity found in genetic comparisons between insects and vertebrates is in the homeotic gene clusters.

Insect HOM-C

Drosophila has two homeotic gene clusters: the ANT-C (Antennapedia complex) is responsible for segmental identity in the head and anterior thorax and the BX-C (Bithorax complex) which is responsible for segmental identity in the posterior thorax and abdomen.

These two gene clusters found on chromosome 3 are found in one cluster in more primitive insects, called the HOM-C (homeotic gene complex). The general case is that there is only one homeotic gene cluster in insects and in the evolution of Drosophila it was separated into two clusters.

Mammalian Hox genes

Mammals have four homeotic gene clusters called Hox clusters, each located on a different chromosome. The four Hox clusters are very similar. The genes near the left end of each Hox cluster are similar to each other and to one of the insect HOM-C genes at the left end of the cluster.

Each Hox gene has a different anterior limit from the developing vertebral column. Within each Hox cluster, the leftmost genes have the most anterior limits.

The Hox genes are arranged and expressed in an order that is very similar to the insect HOM-C genes. Both the HOM-C and the Hox clusters are arranged on the chromosome in order of their expression. The genes at the left-hand side of the chromosome are transcribed near the anterior end of the embryo and those at the right-hand side are transcribed more posteriorly.

Correlations between structure and expression patterns are strengthed by mutation experiments using knockout mice. The phenotypes of the homozygous knockout mice parallel the homozygous null HOM-C flies.

The Hoxc-C8 knockout causes ribs to be produced on the first lumbar vertebra which is normally the first nonribbed vertebra. When Hox C8 is knowcked out, the L1 vertebra is homeotically transformed to the segmental identity of a more anterior vertebra.

The absence of the Drosophila Ubx gene also causes a fate shift toward anterior in which T3 and A1 are transformed to T2.