July 21, 1999 Nature has required species to adapt themselves to their environment through changes in their genetic makeup. These changes, known as mutations, can come about by various means, from a base substitution in a DNA molecule to major alterations in one or more chromosomes. Most often, these mutations are prejudicial and are eliminated by natural selection, but they can be favourable increasing the carrier's capacity for survival and reproduction and, in time, lead to new species.
One of the organisms which scientists have studied in great depth with regard to these changes is the Drosophila fly. This organism has giant sized chromosomes in the salivary glands of its larvae which are relatively easy to view with a microscope. This characteristic make it an excellent source for studying genetic changes that take place in the course of evolution. One of the most typical genetic changes in the Drosophila type species are the chromosomal inversions. In these, a segment of the DNA molecule, which corresponds to a chromosome, adopts an inverted orientation with regard to the original order. Despite many years of study, the molecular mechanisms that generate these inversions have remained unknown.
Researchers from the Department of Genetics and Microbiology at the Universitat Autònoma de Barcelona (UAB), in collaboration with a researcher from the University of Chicago, have demonstrated that the origin of natural chromosomal inversions present in the Drosophila are found in small independent DNA sequences. These sequences are known as transposons, or transposable elements, that are able to reproduce themselves and insert themselves freely in different points of the chromosomes. Transposons were discovered some fifty years ago but the implications of these with regard to cellular operations have only recently begun to be uncovered.
The research team is made up of Mario Cáceres, José María Ranz, Antonio Barbadilla and Manyuan Long under the direction of Alfredo Ruiz. They have been studying the origin of one of the chromosomal inversions in the species Drosophila buzzatii. By using molecular markers they have been able to isolate and sequence the extremes of the chromosome 2 inversion which is found together with the normal organisation in all populations of the species. The comparison of chromosome 2 with, and without the inversion, has allowed them to demonstrate that the inversion is generated by the insertion of two homologous DNA fragments in different locations of the chromosome and the subsequent recombination between them.
The researchers have identified the DNA fragments responsible for generating the inversion, as two copies of the new transposon which has been given the name Galileo. In this way scientists have demonstrated that transposons, traditionally considered mere parasites, are implicated in the appearance of favourable mutations in natural populations of the fly Drosophila, i.e. they have a positive role in evolution. The fact that transposons are found in the entire species, including the human species, and that there are many chromosomal changes that have taken place in the course of evolution of all living organisms, suggest that transposable elements could also be responsible of chromosomic changes associated with the evolution of species.
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