‘Dark flies’ and experimental evolution

By Elof Carlson

In 1907 a graduate student at Columbia University, Fernandus Payne, did a project supervised by his mentor, T.H. Morgan. He spent two years growing fruit flies in the dark. That’s 69 generations of fruit flies (or about 1,500 years if it were done on humans). Payne tested samples every 10 generations and found there was no change in eye color, a robust red, and there was no change in the flies’  attraction to light. They moved toward light.

In 1954 at Kyoto University, Syuti Mori placed some fruit flies in darkened containers and they have been bred and raised in the dark ever since. That’s about 1,500 generations (in humans it would be about 40,000 years in the dark).   

Mori wondered what changes would take place in the dark that would differ from the original control flies from which they were separated. He and his colleagues found that there were changes. The flies developed larger bristles (which can detect contact with objects and sense what they are) and they developed a greater sensitivity to hormones that are released as sex attractants.

Mori is now retired, but his colleagues continue to follow the new generations raised in the dark. They found 84 differences in their genes and they have already detected those affecting the bristles and those affecting sex hormone production and detection. Each gene difference is being isolated and its function is being worked out. They hope eventually to identify those genes that are random events that have no role in the adaptation to living in the dark and those that do have a role to play in living in the dark. They also hope, when the project is completed, to copy the appropriate mutations and insert them into control flies not raised in the dark, to see if these altered flies are as efficient as the 1,500th generation flies living in the dark.

This would be a nice contribution to the analysis of an evolutionary process because it would show the molecular basis for the differences between the two adaptive strains (one by selection and the other by genetic engineering) and how they differ from flies not grown in the dark.

Long-term experiments are relatively rare in science, especially those that are continued after the retirement or death of the original investigator. Both Payne’s experiment, more than a century ago, and Mori’s, which is ongoing, show how science is limited by what it knows and by what tools are available to advance our understanding.

In 1907 Morgan and his students had not yet worked out X-linked inheritance, mapping genes or determined mutation frequency. That genes were composed of DNA was not demonstrated until 1944. That DNA provided a mechanism for how mutations arise was not worked out until the late 1950s. Working out complete genomes of multicelled organisms did not occur until the 1990s. Inserting genes to specific places in the chromosomes was not possible until this decade. The experiments that can be done today were impossible even to imagine 100 years ago.

Elof Axel Carlson is a distinguished teaching professor emeritus in the Department of Biochemistry and Cell Biology at Stony Brook University.