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For example, the same gene that’s responsible for the tail of the mouse, as well is responsible for the rear extremities of the grasshopper. Sermonti
It is not the genes that elicit nascent form, but the nascent form that selects the genes and recruits them for its program. Sermonti
It is tempting to extrapolate that gain of hox genes in a cluster increases the complexity of an organism by allowing additional segments to be specified. Initially these would be just like adjacent segments, but there would be opportunity to evolve into more specialized functions. For example, if there are three sets of legs in insects, could another set of legs be added just by duplicating a hox gene that specified a leg segment of the body? What do the hox gene clusters of spiders, centipedes and millipedes look like? Are there dozens of duplicated hox genes that specify many identical segments? This provides the possibility of macroevolution. Duplication of hox genes, or whole hox gene clusters, followed by deletion and mutation might alter a species very dramatically in a short time period.
The notion that additional body segments might arise from duplicated hox genes was disproven by analyzing ther hox gene clusters of centipedes and onychophorans. The International Society of Developmental Biologists and the Society for Developmental Biology met in July 1997 at Alta, Utah. Researchers reported that centipedes and onychophorans, primitive, wormlike creatures believed to be the closest living relatives of the organisms that gave rise to the arthropods, including insects, have the same eight homeobox (Hox) genes as insects themselves. This indicates that the diverse body segments of insects did not evolve as a result of Hox gene duplication as previously thought, but may instead have arisen as a result of changes in Hox gene regulation. (Science 277, 639 1997).
Mutations in the 8 genes of the HOM complex cause large scale mutations in flies. A mutation in bithorax causes a fly to have an extra set of wings. Mutation in antennapedia causes a leg to grow where an antenna should be. These genes are not master switches for making wings or legs, but they specify position in the fly's body. The order of the genes on the chromosome is the same as the order of segments in the fly's body where they are expressed. The left most gene is expressed in the head, the right most gene is expressed in the abdomen. When a gene is deleted or mutated, the segment where it is normally expressed cannot tell where it is because its position clue is gone, so it behaves like the closest segment to it. That is why a bithorax mutation causes an extra set of wings. The segments adjacent to the bithorax segment dictated what should be made.