Unusual Patterns: A professor studies the genomic underpinings of species evolution in fruit flies
Photomicrograph of two different salivary chromosomes showing the difference in gene order among the chromosomes. The lines between the chromosomes connect similar bands between the two gene arrangements.
The formation of new species usually occurs when two populations of the same species are separated by a physical barrier, such as an ocean or an isthmus, and prevented from interbreeding. Stephen Schaeffer, professor of biology and assistant department head for graduate education, studies genomic changes in an insect—a fruit fly (Drosophila pseudoobscura) that may lead to new species by a very different mechanism.
“We believe that some of the fruit fly populations in the southwestern United States are evolving genomic barriers to gene flow rather than geographical barriers to gene flow,” he said.
According to Schaeffer, these fruit flies have accumulated genome rearrangements that could lead to a less common form of speciation, called parapatric speciation. He is asking what genetic mechanisms are responsible for the establishment of a gene rearrangement polymorphism. These mutations arose within the flies about a million years ago by mutations known as chromosomal inversions.
“We’ve found that when you go out into nature and examine different populations of fruit flies, you find specific gene arrangements that vary among populations,” said Schaeffer. “You also find different numbers of these rearrangements; for example, in the four corners region of the western United States, the flies have only one gene rearrangement, but in Texas they have two arrangements and in California they have three.”
What is driving these differences in gene arrangements? According to Schaeffer, it may be the climate. “All of the differences we are seeing among populations are correlated with changes in local climate,” he said. “We think that because this species lives in many different environments, particular beneficial combinations of genes are being held together for the fly to live in different environments. What’s interesting is that these rearrangements actually can impede genetic exchange between individuals in the same population and may lead to the formation of new species.”
Schaeffer said that the genes being affected by reorganization are odorant-receptor genes, gustatory genes (taste-receptor genes), and genes responsible for dealing with environmental toxins.
To visualize the rearranged genes, Schaeffer uses an old microscopic technique developed in the 1930s, called polytene squashes. “Basically, you dissect the salivary glands of the fly larvae, put them on a slide, squash them to spread the large chromosomes out, and photograph them,” he said. “What you see are chromosomes with banding and puffing patterns [the gene arrangements] that differ among fly populations.”
Schaeffer uses a broad array of modern, sophisticated techniques to identify the genes targeted by selection in these rearrangements; where precisely on the chromosomes the rearrangements occur; and whether protein function or amounts alter rearrangements frequencies; among other things.
The next phase of work, Schaeffer said, is to investigate why odorant, gustatory, and environmental-toxin genes are being targeted and why the rearrangements ultimately are beneficial to the fruit flies.”
“It turns out these genes we’re seeing being selected in this fruit fly are the same ones that are enriched in other insect genomes,” said Schaeffer. “So our work may be important in terms of the evolution and diversification of insects in general.”