August 5, 2006

Why Spiders Aren't Insects V: Hox Genes and the Evolutionary Ties that Bind

This is the fifth post in a series dealing with the evolution of spiders and insects,
while exploring modern genetics and evolutionary theory.
Part I, Part II, Part III and Part IV have led up to this point:

Last time we delved into some of the smallest components of spiders and insects, exploring their differences based on deviations in their genetic code through molecular homology.

But there is one particular unifying element to these creatures and their overall make up. They share a series of genes - sequences of DNA that code for an organism's traits - that determine the exact body plan of an arthropod. These genes, called Hox genes,* have become helpful in describing just how evolution by natural selection constructs each different organism from detailed blueprints within our DNA.

Researchers figured out the function of Hox genes when studying - what else - mutations in Drosophila, the fruit fly (Drosophila is considered the perfect organism for genetic studies due to the species' rapid reproduction rate and ease of care). When these mutations were rearranged in the genome (library of genes), the developing flies would grow legs where antennae were supposed to be or four pairs of wings instead of the normal two.

From these types of experiments, geneticists were able to deduce that these Hox genes are not responsible for the assemblage of the structure itself, but for the direction of the genes that assemble said structure.

In essence, Hox genes would direct chelicerae to be grown on the first segment of the spider, but would not actually code the production of the spider's mouth parts.

This has opened doors for evolutionary biologists. Here is a set of genes that has everything to do with the differentiation of species by changes in body plan. Hox genes are widely conserved, meaning that every animal possesses a set, and many of these ancestral genes have not changed much in the past 500 million years or so, even between vertebrate and invertebrate.

It has been suggested that the Cambrian explosion was merely a revolution in Hox gene expression. Since these genes set the rules for where every piece of the spider will end up, the modifications of those individual pieces - chelicerae, legs, eyes, etc. - are relatively simple mutations in the expression of specific genes.

This, however, has not exactly cleared up the origins and lineage of these creatures.

As we have discussed, much of the phylogeny of arthropods is a murky study in paleontology and molecular homology. But according to several new studies, Hox gene expression may help scientists tease out the finer points of their natural history.

New data are emerging that suggest that the first segment of the spider, which includes its mouth parts, pedipalps and all four pairs of walking legs, may in fact be the head of subsequent millipedes, centipedes and insects. This may seem like an obvious conclusion, first segment equated to first segment, but what did Hox gene expression through natural selection do with the ancestral spider's walking legs?

Insects might be eating with them.

Remember when we talked about one of the main morphological difference between spiders and insects? Spiders and insects have different mouth parts, and are ordered into separate groups because of it - chelicerate and mandibulate. The insects' mandibles do not seem to have arisen from the spiders' own mouth parts, but from a common ancestor's modification of the spiders' mode of transportation, its legs.

Only through the knowledge of the form and function of Hox genes was this possible.

Next time we'll get into more about Hox genes and evo-devo, talking about Ernst Haeckel's awkward theory of evolutionary recapitulation though embryonic development, and what that has to do with Hox genes, Darwin being wrong in science.

*Scientists sometimes specifically call invertebrate Hox genes HOM genes.

1 comment:

  1. Anonymous7:41 PM

    Could you explain how Spiders and Insects are closely related? Like their homology and how Natural Selection could have separated them..
    This is all very interesting stuff.