Fruit fly DNA detail as never before
Image: Drosophila melanogaster chromosomes; DNA (grey) and centromeres (green) [Ankita Chavan (Mellone lab)]
US-based researchers have combined cutting-edge sequencing technology with molecular and high-resolution microscopy methods to discover the sequences of all centromeres in the fruit fly Drosophila melanogaster.
Centromeres are essential chromosomal regions that mediate kinetochore assembly and spindle attachments during cell division.
Yet, despite being visible under the microscope for over a century, little is known about their organisation at the DNA level because of the difficulty of accessing highly repetitive DNA with traditional sequencing technologies.
To get around this problem, Professor Amanda Larracuente from the University of Rochester and colleagues, sequenced longer sections of DNA and purified the segments of the centromere that stick to a centromere-specific histone protein.
They also imaged chromatin fibres with an inverted Deltavision RT restoration imaging system equipped with a Cool Snap HQ2 camera and 100×/1.40 NA oil immersion lens.
In this way, the researchers generated a complete and intact picture of the fly's centromeres.
Crucially, they discovered that buried within a sea of highly repetitive sequences were 'islands' of more complex DNA sequences that might hold the key to how the centromeres function to segregate chromosomes faithfully.
The researchers also found that centromeres contain a surprisingly high number of transposable elements; sequences that jump around and selfishly proliferate throughout the genomes.
"What is exciting is that the centromere islands are rich in a type of transposable element called retroelements, which we usually consider to be genome parasites," says Larracuente.
Their findings suggest that these selfish DNA elements may have a role in centromere function across a wide range of species, as retroelements have been found to be associated with centromeres in fungi, plants, mammals, and now fruit flies.
"With the centromere sequences in hand, we are poised to leverage the powerful fruit fly genetic toolkit to understand the role these sequences play in centromere function and evolution," highlights Larracuente's colleague, Professor Barbara Mellone, from the University of Connecticut.
Research is published in PLOS Biology.