As organisms evolve, their existing structures or body parts are frequently modified to suit their needs. For example, an invertebrate with a working limb design may end up changing it and incorporating it somewhere else in its body plan . The practice of modifying a specific structure more than once and using it somewhere else is known as serial homology .
As evolution is necessarily a stepwise process, certain complex structures, such as legs or wings, cannot spring into being instantly. They must slowly evolve over time, and each new and slightly different version must be more useful to its owner than the last. Replicating previously existing parts and building on them is a common strategy in organismal evolution. As such, serial homology is a widespread evolutionary tactic that can be observed in a large number of animals.
Explaining Strange Mutations
Comparative biologists first had an inkling that copies of body parts were altered and used again when they started noticing odd mutants in their collections. At the end of the nineteenth century, comparative biologist William Bateson found some specimens in his collection that were odd-looking: he had arthropods with limbs in odd places, such as legs popping out of an animal's head. He also noticed certain ribs or vertebra swapped in other animals. However, these examples were rare and frequently unique. It wasn't until 1915 that Calvin Bridges, while breeding fruit flies (Drosophila), came across a mutant that he could consistently breed where the rear flight appendage, or the haltere, resembled a wing.
This wing resemblance was no accident. The Drosophila wing and haltere are serially homologous . They were both modified from the same basic structure, and it should come as little surprise that interrupting the proper development pathway of one of them might cause it to resemble another. In terms of genetics, the two appendages are quite closely related; indeed, nearly identical. But at one location on the body, only a haltere will grow. At another, only a wing will grow. What mechanism makes this decision possible, the decision to grow a certain appendage on a certain part of the body? In the case of the wing and the haltere, the answer is Hox genes .
Hox genes are extremely common and evolutionarily very, very old. They are first described as belonging to a common ancestor of bilateria and cnidaria (in the neighborhood of 700 million years old). It is the Hox genes' job to locate different structures inside the organism's body plan. The particular gene in charge of making sure the haltere develops properly, and not into a wing or something else, is called Ubx.
What Bridges found was a partial mutation in Ubx. The halteres in his flies had wing bristles. Ubx controls a variety of other genes integral to haltere formation as well as genes important in the suppression of wing formation. Ubx discourages spalt-related, a gene that makes veins for the wings. It also stops genes that control wing epithelium formation, and other mechanisms and structures.
If there is a certain problem with the Ubx gene, a fly will grow halteres that have wing-like characteristics. While complete removal of a Hox gene generally results in death during early development, a certain triple mutation in Ubx can cause a second pair of fully-formed wings to develop where the halteres are supposed to be. Different mutations in Hox genes have produced flies with legs where antennae are supposed to be, and other odd body modifications.
Hox Genes in Fruit Flies
In Drosophila, eight Hox genes, organized into two gene complexes, orient the body plan. By investigating where and when Hox genes were expressed, developmental biologists discovered that various genes were restricted to various body segments. Some of them overlap one another: abd-A and abd-B share a portion near the end of the abdomen, for example. The Hox genes in particular body segments affect the development of structures inside those segments. For example, the shape of the first pair of adult legs is influenced by the Scr gene, the second pair by the Antp gene, and the third pair by the Ubx gene.
While the Hox genes dictate the identity of a certain developing segment, they are not required for structures inside that segment to form. In Drosophila, the mouthparts and legs are serially homologous to the antennae. Thus, in the absence of these controlling Hox genes, the segments will still develop all their structures, but differently. Where the legs or mouth-parts should be, antennae will develop instead. Such substitution of one part for another that is serially homologous is known as homeotic substitution. As William Bateson discovered with his collection, such substitution happens occasionally in nature to serially homologous structures.
So, as it is a useful evolutionary tactic to copy existing structures and modify them, the Hox genes evolved to keep tabs on what body segment is where so that the proper structural modifications can take place during development. While Hox genes are integral to differentiating the different segments of Drosophila, it is important to remember that they also delineate different areas of early developmental tissue (ectoderm, endoderm, and mesoderm) and specify location in a variety of fields for a staggering number of organisms.
There is a remarkable similarity in Hox gene sequences between Drosophila and vertebrates, such as mice, frogs, zebrafish, and humans. Alterations of Hox gene expression in Drosophila can prevent expression or growth of certain limbs or organs, and similar results have been found in birds, amphibians, and fish. This demonstrates that Hox genes function similarly over a wide variety of organisms, and that serial homology is a true evolutionary tactic when it comes to generating novel structures on one's body plan.
Hox Genes and Body Designs
Different organisms use serial homology to different extents. Vertebrates have modified vertebrae and ribs in a large number of ways; the basic vertebra has been reiterated and altered into different backbone types: sacral, lumbar, thoracic , and cervical. However, perhaps one of the best examples of serial homology can be found in the body design of the common cray-fish.
All appendages of crayfish, with the possible exception of the first antennae, are called biramous, which is to say they are derived from double-branched structures. The three components that make up these branches are known as the protopod (the base), the exopod (lateral; or on the side) and the endopod (in the middle). Crayfish have quite a large number of appendages in their body plans. The biramous structure plan is incorporated in crayfish head appendages, legs, swimmerets, mandibles, and many others.
Evolutionarily, it is easy to see how the earliest design might have been copied and modified several times, producing a highly specialized and versatile body plan. And as all the new structures are based on one older one, variations in Hox (or other developmental) genes will affect only a small number of structures, rather than all of them. Serial homology is a method for creating new, more specialized body plans, and it is observed in species throughout the living world.
see also Morphology.
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Davidson, Eric H. Genomic Regulatory Systems. San Diego, CA: Academic Press, 2001.
Hickman, Cleveland P. Jr., Larry S. Roberts, and Allan Larson. Integrated Principles of Zoology, 10th ed. Dubuque, IA: Wm. C. Brown Publishers, 1997.