Fish to Tetrapod - Hox Genes

Australia: The Land Where Time Began

Fish to Tetrapod - Hox Genes

Developmental genetics studies have produced some of the insights into the changes that occurred in the transition from fish to tetrapods, and these can now be integrated with those gained through more traditional methods, such as studies of phylogeny and palaeontology, leading to new ideas and interpretations.

The discovery of the existence and functioning of homeobox genes has been one of the far-reaching discoveries of the past 20 years. It is these genes that coordinate some of the most basic processes occurring during the development of nearly all organisms, determining the axes front to-back and top-to-bottom in embryos, and the order and position in which certain organs and structures appear. These genes are present on a particular chromosome or chromosomes, some of them being known as Hox genes. It appears these genes are biochemically related to one another, though Clack¹ says it is more surprising that the order of their occurrence corresponds to the order in which they are expressed in the animal. An example is the genes occurring at the front end the chromosome, the 3-prime or 3’ end, mainly affect the front end of the animal, and those at the back, the 5’ end, mainly affect the more posterior end of the animal.

It has been found that in the genomes of most animals there are 1 or more clusters of such genes, and plants also possess Hox genes that are comparable, though not necessarily homologous. These genes have been found in a wide variety of invertebrates, as well as vertebrates, though not many animals have been studied in detail, and the genes comprising the sequences can be quite closely equated between different species of animal. Groups with a similar structure and function have been given names or numbers. About 13 groups comprise Hox genes of vertebrates, with each group being labelled with a number, an example being Hox6 in a vertebrate can be compared with its equivalent in another species from a different type of animal. These are paralogue groups. It has been found that paralogue groups that are positioned similarly often have comparable functions in different animals, including those they are not closely related to. An example given by Clack¹ is the instruction “make legs”, that has been found to have a paralogue group in Drosophila, a fruit fly, and in tetrapods. It is implied by this that such genetic instructions arose early in the evolutionary history of animals, at least in the most common ancestor of insects and vertebrates, which cannot have occurred any later than 600 Ma.

There is only a single cluster of Hox genes in most organisms, though some extraordinary events occurred in vertebrate evolution that have resulted in the duplication of the Hox string. In all modern jawed vertebrates there are at least 4 copies of the string, e.g., the mouse, and some, such as the zebra fish, have up to 7. There are only 3 sets in the lamprey, a primitive jawless fish (Sharman & Holland, 1998), and it is possible there were some duplication events that increased the number of duplication events in the history of jawed vertebrates (Holland & Garcia-Fernàndez, 1996; Cohen, 1992). The hagfish have recently been found to have up to 7 duplications of a particular paralogous group, but fewer of others, as well as some that appear to be missing (Stadler et al., 2004).

Duplications of individual genes have also occurred. Clack¹ suggests that it appears the duplications have meant that as one set is carrying out the original function, the new one can go on to acquire new functions while not disrupting the animal’s development. Evolutionary events that are more rapid and profound might have been allowed to have occurred than would be possible otherwise. The Hox genes of animals and birds have been give the letters HoxA, HoxB, HoxC and HoxD to distinguish them because of the additional duplications in each set, the 4 sets differing very slightly from one another. An example is there are subtly different paralogue groups in each set, the structure and function of Hoxa13 differs from the structure and function of Hoxb13. Sometimes genes were deleted during the process of evolution, alongside gene or cluster duplication.

It is made possible to work out the order in which genes are expressed by the detection of the proteins produced by the instructions of a particular gene. Whether the products of the Hox gene are produced at any particular point depends on the interplay of several other chemicals, as there are many subtleties to the system. These may be other proteins, such as transcription factors, or morphogens that are simple chemicals. According to Clack¹ it appears changes to the Hox genes have been largely influential in evolution events such as the acquisition of limbs with digits.

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