Gene conversion: a possible mechanism for eliminating selfish DNA.

Abstract

A strong case has recently been made for the existence of selfish DNA in higher organisms (Doolittle and Sapienza, 1980; Orgel and Crick, 1980). Given that genetic elements exist which have the means of reproducing themselves by transposition to new locations, they would be expected to gradually accumulate in the genome, especially in diploid organisms. The essence of the argument is that each insertion of a nonfunctional DNA sequence, which only very slightly increases the size of the genome, would have a neglible phenotypic effect on the individual and therefore would not be selected against. For the amount of DNA to increase, all that is required is that the probability of a DNA sequence being added to the genome is greater than the probability of it being eliminated by natural selection or by random deletion. There will also be positive selection for genetic elements which increase by mutation their efficiency of transposition. Not only will genomes increase in size, but also most of the DNA will become “junk,” with no function for the organism. This conclusion may help solve the C-value paradox, namely, that higher organisms appear to have much more DNA than is required for the synthesis and control of gene products, and that there is enormous variation of DNA content between different taxonomic groups or even between quite closely related species of higher organisms.