HOX Gene Expression (Molecular Biology Intelligence Unit)

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New Password. Some biologists have also envisioned special mutations in regulatory homeobox or "Hox" genes, where simple mutations might be able to make large developmental changes in an organism which might case a radically different phenotype. However, manipulating "Hox" genes does little to solve the problem of generating novel functional biostructures, for making large changes in phenotype are rarely beneficial. Hox gene mutations may be a more simple mechanism for generating large change, but they also do not escape the problem of the "hopeful monster":"The drawback for scientists is that nature's shrewd economy conceals enormous complexity.

Researchers are finding evidence that the Hox genes and the non-Hox homeobox genes are not independent agents but members of vast genetic networks that connect hundreds, perhaps thousands, of other genes. Change one component, and myriad others will change as well--and not necessarily for the better. Thus dreams of tinkering with nature's toolbox to bring to life what scientists call a "hopeful monster"- such as a fish with feet--are likely to remain elusive.

But again and again, in every species and at every level of organization, we find that complex structures are made by repeating a few basic themes, with variations. Thus, a limited number of basic differentiated cell types, such as muscle cells or fibroblast, recur with subtle individual variations in different sites.

These cell types are organized into a limited variety of tissue types, such as muscle or tendon, which again are repeated with subtle variations in different regions of the body. From the various tissues, organs such as teeth or digits are built— molars and incisors, fingers and thumbs and toes—a few basic kinds of structure, repeated with variations.

1. Introduction

The segments of the insect body provide a good example. We have thus far sketched the way in which the rudiment of a single body segment is constructed and how cells within each segment become different from one another.

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We now consider how one segment becomes determined, or specified, to be different from another. The first glimpse of the answer to this problem came over 80 years ago, with the discovery of a set of mutations in Drosophila that cause bizarre disturbances in the organization of the adult fly.

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In the Antennapedia mutant, for example, legs sprout from the head in place of antennae, whereas in the Bithorax mutant, portions of an extra pair of wings appear where normally there should be the much smaller appendages called halteres. Homeotic Genes A set of 8 to 11 homeotic genes directs the formation of particular structures at specific locations in the body plan. The Hox genes are organized in genomic clusters. Drosophila has one such cluster and mammals have four Fig. The genes in these clusters are remarkably similar from nematodes to humans. In Drosophila, each of the Hox genes is expressed in a particular segment of the embryo and controls the development of the corresponding part of the mature fly.

The terminology used to describe Hox genes can be confusing. They have historical names in the fruit fly for example, ultrabithorax , whereas in mammals they are designated by two competing systems based on lettered A, B, C, D or numbered 1, 2, 3, 4 clusters.

Biological 'Rosetta Stone' brings scientists closer to deciphering how the body is built

Loss of Hox genes in fruit flies by mutation or deletion causes the appearance of a normal appendage or body structure at an inappropriate body position. An important example is the ultrabithorax ubx gene. When Ubx function is lost, the first abdominal segment develops incorrectly, having the structure of the third thoracic segment. Other known homeotic mutations cause the formation of an extra set of wings, or two legs at the position in the head where the antennae are normally found Fig. The Hox genes often span long regions of DNA. The ubx gene, for example, is 77, bp long.

More than 73, bp of this gene are in introns, one of which is more than 50, bp long. Transcription of the ubx gene takes nearly an hour.

A2 Biology: Gene Regulation

The delay this imposes on ubx gene expression is believed to be a timing mechanism involved in the temporal regulation of subsequent steps in development. Many Hox genes are further regulated by miRNAs encoded by intergenic regions of the Hox gene clusters. All of the Hox gene products are themselves transcription factors that regulate the expression of an array of downstream genes.

Identification of these downstream targets is ongoing. Many of the principles of development outlined above apply to other eukaryotes, from nematodes to humans. Some of the regulatory proteins are conserved. For example, the products of the homeoboxcontaining genes HOXA7 in mouse and antennapedia in fruit fly differ in only one amino acid residue. Of course, although the molecular regulatory mechanisms may be similar, many of the ultimate developmental events are not conserved humans do not have wings or antennae.

The different outcomes are brought about by differences in the downstream target genes controlled by the Hox genes. The discovery of structural determinants with identifiable molecular functions is the first step in understanding the molecular events underlying development. As more genes and their protein products are discovered, the biochemical side of this vast puzzle will be elucidated in increasingly rich detail. If evolution is to generate the kind of changes in an organism that we associate with a different species, it is the developmental program that must be affected.

Developmental and evolutionary processes are closely allied, each informing the other. The continuing study of biochemistry has everything to do with enriching the future of humanity and understanding our origins.

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South America has several species of seed-eating finches, commonly called grassquits. Perhaps driven by a storm, they lost sight of land and traveled nearly 1, km. Small birds such as these might easily have perished on such a journey, but the smallest of chances brought this group to a newly formed volcanic island in an archipelago later to be known as the Galapagos.

It was a virgin landscape with untapped plant and insect food sources, and the newly arrived finches survived. Over the years, new islands formed and were colonized by new plants and insects—and by the finches. The birds exploited the new resources on the islands, and groups of birds gradually specialized and diverged into new species. By the time Charles Darwin stepped onto the islands in , there were many different finch species to be found on the various islands of the archipelago, feeding on seeds, fruits, insects, pollen, or even blood.

The diversity of living creatures was a source of wonder for humans long before scientists sought to understand its origins. The extraordinary insight handed down to us by Darwin, inspired in part by his encounter with the Galapagos finches, provided a broad explanation for the existence of organisms with a vast array of appearances and characteristics.


It also gave rise to many questions about the mechanisms underlying evolution. Answers to those questions have started to appear, first through the study of genomes and nucleic acid metabolism in the last half of the twentieth century, and more recently through an emerging field nicknamed evo-devo—a blend of evolutionary and developmental biology. In its modern synthesis, the theory of evolution has two main elements: mutations in a population generate genetic diversity; natural selection then acts on this diversity to favor individuals with more useful genomic tools, and to disfavor others.

Advantageous mutations in singlecelled organisms or in the germ line of multicellular organisms can be inherited, and they are more likely to be inherited that is, are passed on to greater numbers of offspring if they confer an advantage. It is a straightforward scheme. But many have wondered whether it is enough to explain, say, the many different beak shapes in the Galapagos finches, or the diversity of size and shape among mammals. Until recent decades, there were several widely held assumptions about the evolutionary process: that many mutations and new genes would be needed to bring about a new physical structure; that more-complex organisms would have larger genomes, and that very different species would have few genes in common.

All of these assumptions were wrong. Modern genomics has revealed that the human genome contains fewer genes than expected—not many more than the fruit fly genome, and fewer than some amphibian genomes. The genomes of every mammal, from mouse to human, are surprisingly similar in the number, types, and chromosomal arrangement of genes. Meanwhile, evo-devo is telling us how complex and very different creatures can evolve within these genomic realities. The kinds of mutant organisms shown in the figure above of the fruit fly were studied by the English biologist William Bateson in the late nineteenth century.

Bateson used his observations to challenge the Darwinian notion that evolutionary change would have to be gradual. Subtle changes in regulatory patterns during development, reflecting just one or a few mutations, can result in startling physical changes and fuel surprisingly rapid evolution. What it does, is creating monstrosities The Galapagos finches provide a wonderful example of the link between evolution and development.

There are at least 14 some specialists list 15 species of Galapagos finches, distinguished in large measure by their beak structure. The ground finches, for example, have broad, heavy beaks adapted to crushing large, hard seeds. The cactus finches have longer, slender beaks ideal for probing cactus fruits and flowers.

Clifford Tabin and colleagues carefully surveyed a set of genes expressed during avian craniofacial development. In each organism, these genes are part of the much larger regulatory cascade that ultimately creates the correct structures in the correct locations in each organism. They seemed to fit into the Gouldian mode of evolution punctuated equilibrium because a small mutation in a hox gene could have profound effects on an organism.

Theoretical limitations of molecular phylogenetics and the evolution of relaxins. Research in the six years since Schwabe wrote this has only born out his statement. Changes to homeotic genes cause monstrosities two heads, a leg where an eye should be, etc. And the mutations do not add any information, they just cause existing information to be mis-directed to produce a fruit-fly leg on the fruit-fly head instead of on the correct body segment, for example.

However, commonality of such features is to be expected with their origin from the same supremely intelligent Creator.