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Comparative Vertebrate Anatomy: Hox Genes

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    Homeotic complex (Hox) genes are defined as a group of related genes that specify the anterior-posterior axis and segment identity of metazoan organisms during early embryonic development. These are situated very close to one another on the chromosome in groups of clusters. The orders of these genes located on the chromosomes are the same as the expression of the genes in the development of an embryo, whereas the first gene is expressed in the anterior end of the developing organism. These groups of genes are short sections of DNA that occur in clusters called homeoboxes.

    A homeobox contains 180 nucleotides that code for a protein domain known as the homeodomain. Hox genes produce protein products belonging to a class known as transcription factors. Transcription factors are capable of binding to DNA, in which they regulate the transcription of genes. This homeobox sequence codes for a 61 amino acid helix-turn-helix protein, resulting in the homeodomain. The homeodomain acts as a switch that turns the gene transcription on and off by binding to specific sequence enhancers of a gene that either activates of represses the gene.

    These same HOX proteins can act as an enhancer for one gene and a repressor for another. Hox genes have an historical importance in evolution as well as in their relationship to human development, and its specific regulation and their mutations. Animals have very different body plans. Hox genes play a huge role in the diversification of the segments that develop different animal segments within closely related groups. This diversity of the Hox genes are produced by evolutionary changes in the timing of the gene expression and the different combinations of Hox genes are used together.

    This is where homologous structures are formed. Homologous structures are things that are similar in evolution, position, or shape but differ in function. Biologists are able to isolate Hox genes from many animal groups. Looking at the different types of Hox genes present in different animals shows the amount of evolution that has occurred. Another way to look at it is the more the Hox genes, the more the species has evolved from its common ancestor. Homeosis was discovered by

    Bateson in 1894 in order to describe the natural variants of a species where certain parts of the body plan displayed morphological features typical of other regions. Christiane Nusslein-Volhard, Eric F. Wieschaus, and Edward B. Lewis went on to do more research and found that in the fruit fly (Drosophila melanogaster) Hox genes govern the development of a larval segment into a specific body segment. Lewis found that the time and space between the order of the genes in the bithorax complex and their affected regions in the segments were collinear to each other.

    In 1995, they went on to win a Nobel Peace Prize in Physiology/Medicine for their research. In 1989, Hox genes were named according to the order in which they were discovered. The homeotic complex governs the occurrence of both overt and non-overt segmentation in vertebrates. In vertebrates, the main evidence of segmentation is in the vertebral column. Hox genes are arranged in four clusters in deuterosomes: Hox A, Hox B, Hox C, and Hox D (see Appendix A). These four clusters are a consequence of the ancestral vertebrate genome being twice duplicated in it’s entirely.

    The duplication occurred before the Cnaidaria-Bilateria split and the second during the evolution of the fishes. According to Lewis’ research, Hox genes all derived from Abd-A through a gene duplication, while similarly two other Hox genes are derived from proboscapaedia. In the Amphioxus, there is only one Hox gene that resembles a complete Hox A cluster. That means that a Hox A-8 is present in this animal and has since then disappeared. It is also concluded that the homeotic cluster first quadruplicated itself four additional times, after which individual genes were lost in evolution.

    The head, the fore- and hindlimbs are later adaptations onto the early vertebrate skeleton. The earliest expression studies focused on the expression of Hox genes in the prevertebra of mid-gestation embryos. In these experiments, it was realized that specific paralogue groups had anterior boundaries within prevertebra and that each vertebra had its own Hox address or code. Whereas the anterior boundaries are well defined, the posterior boundaries of Hox genes are not; suffice it to say that many more homeotic genes are expressed within posterior segments than in anterior segments.

    Hox genes are not expressed anterior to the hindbrain. The forebrain and midbrain are more advanced structures in vertebrate and although there is evidence that these structures are also segmented, the homeotic gene family had nothing to do with the elaboration of these structures. An important feature of the rhombencephalon is that cranial neural crest cells delaminate from this structure as the neural tube is closing. The rhombencephalon is the CNS precursor of the hindbrain.

    These specialized neuroectodermal cells are a more recent adaptation of jawed vertebrates and will give rise to, or contribute to, the formation of the jaw, the cranial nerves and soft tissues of the neck and throat, including the thyroid and thymus. Anterior Hox genes that are expressed in rhombencephalon are also expressed in their derivative neural crest cells. These neural crest cells will migrate to the brachial arches and give rise to these specialized structures. Each brachial arch therefore has its own Hox code, and – as we shall see – mutations within the anterior genes can affect the emergence of these specialized tissues.

    Whereas Hox gene expression has been closely studied in cranial neural crest, little is known about the role of Hox genes in trunk neural crest. Although various Hox genes have been found to be expressed in soft tissues such as the lung, gut, etc. , little is known about whether this expression is the result of segmentation (i. e. , being derived from trunk neural crest migration) or whether this expression is de novo, this is the reason why specific homeotic genes in evolution are not drastically affecting the body plan. Some Hox genes are regulated and have mutations.

    One example is the function of Pax 6. It was first identified in humans because it can cause a disease called aniridia when it carries a mutation. Aniridia is a blindness disease caused from the underdevelopment of retina in mice and fruit flies. This states that Pax 6 is involved in the eye development in all three species. Having two copies of a gene can relax the strength of natural selection acting to preserve the gene, freeing it up to potentially evolve new functions. Another example is the disruption of Hox A3 and the phenotype of soft tissue defects in the neck and upper chest.

    The glottis and epiglottis were aberrant, the thymus and thyroid were reduced and there were significant alterations of the aortic arteries. Because all of these structures were derived from the neural crest, which invaded the fourth brachial arch, these results were the first to demonstrate that Hox gene expression was important for specifying the fate of neural crest cells. In conclusion, Hox genes are very essential in the development process of duetersomes. Hox genes are responsible for the evolutionary change of the ancestors versus the modern day species.

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