Shrinking Genomes

Scientists generally believe that insertions of retroelements, or"jumping genes," once established in a population, are irreversible and are maintained throughout evolution. This unidirectional theory of retroelement evolution, which calls for ever-expanding genome size, is challenged by work that appears in the September issue of Genome Research.

Deletion of Genome elements from Rhesus to Humans
Researchers performed a whole-genome comparison of the human, chimpanzee, and Rhesus monkey sequences, and they identified 37 instances where a retroelement was present in Rhesus (a more primitive primate species) but absent in either humans or chimpanzees. This indicated that these retroelements had been deleted during the evolution of the more recent primate species.

Mediation by short identical sequences
Intriguingly, the scientists further demonstrated that these deletions were mediated by short identical sequences that flank the retroelements. They extended the study to random, non-retroelement sequences and showed that deletions caused by short identical DNAsequences were a widespread genomic phenomenon. In fact, thousands of insertion-deletion sequence differences between the human and chimpanzee genomes were likely mediated by short identical sequences.

The work strongly suggests an important role for short, non-adjacent, identical segments of DNA in genomic deletions and it lends insight into deletion mechanisms that help to counterbalance genome expansion in primates.

Conclusion: Genes don't get bigger with more complex species, they get smaller.