Biologists have thought that new genes appear when evolution copies existing genes and then adapts the copies to new tasks. However, a recent study shows that new genes often form from scratch. Analysis of genes from mice, humans and fish shows that new genes are shorter than old ones and simpler in structure. These and other differences between young and old genes indicate that completely new genes can also form from previously unread regions of the genome. Moreover, the new genes often use existing regulatory elements from other genes before they create their own.
The study found many exceptions to the previous belief. The researchers analysed over 20,000 mouse genes and traced their origins. According to their findings, genes that appeared later in evolution are often shorter than those that have been in existence longer. Moreover, younger genes have fewer exons and fewer protein domains. A new gene needs time to acquire additional exons and introns. So genes become longer with time and consist of numerous exons and introns. Analyses of human, zebrafish and stickleback genes confirm the correlations discovered in the mouse.
The researchers also studied another way in which new genes can arise from existing genes: through a change in the reading frame. The genetic reading frame is a triplet of nucleotides. Each of these triplets corresponds, by the genetic code, to an amino acid. If this reading frame is shifted, subsequent triplets correspond to completely different amino acids. About 60% of modern genes originate from genes of early unicellular ancestors. Many new genes appeared during the advent of fundamental evolutionary innovations, such as the transition from unicellular to multicellular organisms and the emergence of vertebrates. However, the researchers only found a few locations on chromosomes in which newly formed genes accumulate. Instead, new genes are relatively evenly distributed across the entire genome. One of the few exceptions is a cluster of genes on chromosome 14 which control the activity of neurons, among other things.
New genes thus frequently arise from scratch in the course of evolution, forming in the gene-free sections of the genome. Yet this often requires only minimal changes. “For example, genes need elements known as promoters which control their activity. It appears that new genes can appropriate promoters belonging to other genes and use them for their own purposes,” explained researcher Diethard Tautz.