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Australia: The Land Where Time Began |
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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-Fernŕndez, 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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |