Selfish DNA

Selfish DNA
Earlier in this section we discussed that in higher organisms there is considerable proportion of DNA represented by, repetitive sequences. Although it has been speculated that these repetitive sequences might play a regulatory role, their definite function is largely unknown. On the other hand, there may be some repetitive sequences which contain definite genetic information and the most important example of such genes is ribosomal genes which are present in thousands within the same cell for the synthesis of large number of ribosomes needed for protein synthesis. Nevertheless, ribosomal genes make such a small proportion of repetitive sequences, that they alone cannot explain the occurrence of these sequences. In 1980s, it was suggested that these repetitive sequences may not confer any definite advantage to the organisms which contain them. Therefore, sometimes it is said that repetitive sequences represent the DNA which is a repository from which new functional sequences can be obtained. There is, however, no evidence to prove or disprove this hypothesis and it may not be necessary to argue that these DNA sequences, if not of definite advantage to organism, should have been eliminated through natural selection. Actually, the cells themselves provide an environment and the repetitive sequences may be subjected to selection pressure within the cell. It can, therefore, be argued that these sequences may be maintained through non-phenotypic selection, and by a mechanism directed for the self preservation of these sequences. It is in this context that the term selfish DNA, was used for this repetitive DNA.

One of the example of selfish genes is found in transposable genetic elements in prokaryotes. These transposable genetic elements are not indispensable and can be transposed from one place to the other or from one organism to the other. It is difficult to find reason for selection of these sequences represented by transposable genetic elements on the basis of phenotypic fitness. On the other hand, it has been argued that the transposable genetic elements are maintained due to their property of transposability which permits them to escape the pressure of selection exercised on the organism. Such transposable elements are also found in case of eukaryotes, for example Tyl element in yeast and the copia like and inverted repetitive elements of Drosophila (for details consult Plasmids, IS Elements, Transposons and Retroelements)

In Lethality and Interaction of Genes, we described selfish genes like SD in Drosophila, 't' in mouse, and M in flour beetle. The term selfish genes were used there for those genes, which are successful in propagating themselves, while being detrimental to the organisms that carry them. Therefore, a distinction needs to be made between these selfish genes described in Lethality and Interaction of Genes and selfish DNA described in this section; here it refers to repeated sequences with unknown function.