Standing on the shoulders of giants

Abstract

Few scientists have had a greater impact on our understanding of biological processes than Barbara McClintock. Her discovery of transposable elements and her unorthodox ideas about genome organization and function have provided the intellectual foundation for the current genomics revolution.Using genomics technology, Jeff Bennetzen (Purdue University, West Lafayette, USA) has begun to develop a comprehensive view of the prevalance and distribution of transposable elements in maize and the distantly related grasses, rice and sorghum. In 1996, Bennetzen's lab made the surprising discovery that most of the huge stretches of intergenic DNA in maize are comprised exclusively of one class of element, retrotransposons that can apparently attain phenomenal copy numbers by inserting harmlessly into each other. The ability to date retrotransposon insertion times through the divergence of the long terminal repeat (LTR) sequences of the elements led to another surprising finding, that most retrotransposition has occurred within the past five million years.The idea that the transposable elements in the maize genome have undergone recent and massive amplification is also consistent with the findings of Qiang Zhang (University of Georgia, Athens, USA) who reported on the Heartbreaker (Hbr) family of miniature inverted repeat elements (MITEs). The finding that the 3000 to 4000 Hbr elements are both highly conserved (<90%) and polymorphic in maize suggests that they have recently been amplified. Zhang also reported that over 70% of the examined Hbr insertion sites were in single or low copy sequences (regions thought to contain the genes), despite 80–90% of the maize genome being repetitive. Taken together, the data on the recent amplification of retrotransposons and MITEs indicate that the maize genome has a very dynamic recent history, and this may have contributed to its remarkable success as an easily adaptable crop plant.This meeting described the use of genes isolated from Antirrhinum to yeast to explore maize biology. As the era of functional genomics and interspecific investigations advances, exciting new challenges and opportunities await this generation of maize geneticists.