HOMEOTIC GENE ORGANIZATION IS CONSERVED THROUGH EVOLUTION

The extent to which gene sequence and organization are conserved between organisms gives us clues about the amount of evolutionary time that has passed since the two organisms diverged from one another. For example, the homeotic gene clusters in the flour beetle (Tribolium) and the fruit fly (Drosophila) are very similar. In contrast, the homeotic gene clusters in these two insects differ greatly from those in the mouse. The presence of homeotic gene sequences in animals as different as insects and mammals suggests that this type of gene has a crucial function in many, and perhaps all, animals.

This figure shows the organization of homeotic gene clusters in the flour beetle, the fruit fly and the mouse. The colored boxes represent different homeotic genes as they are laid out on a chromosome (signified by the blue line).

The squiggle in the middle of the Drosophila chromosome represents the space on the chromosome between the two homeotic gene clusters, ANT-C and BX-C. Each of the four homeotic gene clusters in the mouse (inside the pink box) resides on a different chromosome.

The mouse, fruit fly and flour beetle have similar homeobox genes. What can gene organization tell us about evolutionary relationships between these organisms?

Flour Beetle

One way scientists study this type of problem is to compare the chromosomal arrangement of homologous genes in different organisms. The homeotic genes of the flour beetles cluster tightly on one chromosome - scientists refer to this clustering as tight linkage. Genes that are closer together on a chromosome are more likely to be inherited by the next generation as a linked group.

Fruit Fly

Homeotic genes in the fruit fly are broken into two clusters on the same chromosome and separated by a long stretch of DNA (blue line). The five genes shown at left (blue and pink boxes) represent the Antennapedia complex (ANT-C). The three genes shown at right (yellow boxes) are part of the Bithorax complex (BX-C).

Mouse

The mouse has four homeotic gene clusters. All four are similar, but they are located on different chromosomes. Scientists believe that this sort of gene arrangement results from the duplication of a chromosomal region. The homeotic gene cluster in mammals has apparently been duplicated four times. Each cluster is somewhat different. Most are incomplete, missing some fraction of the genes that are present in the ancestral gene cluster.