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Study of fruit fly chromosomes improves understanding of evolution and fertility

August 10, 2012
Arizona State University
New research explores the evolution of sperm structure and function, through an analysis of Drosophila genes and gene products. The research has important implications for the study of human infertility as well.


These as-yet-unresolved issues fascinate Timothy Karr, a developmental geneticist and evolutionary biologist at Arizona State University's Biodesign Institute. To probe them, he uses a common fruit fly, Drosophila melanogaster -- an organism that has provided science with an enormous treasure-trove of genetic information.

"My research focuses on the evolution of sex and in gamete function," Karr says. "I focus primarily on the sperm side of the sexual equation. I'm interested in how they originated and how they are maintained in populations."

Karr's current study, in collaboration with researchers at the University of Chicago, recently appeared in the journal BMC Biology. The study reexamines an earlier paper that analyzed the sex chromosomes of fruit flies during spermatogenesis -- the process that produces mature sperm from germ cells.

While the previous paper, by Lyudmila M Mikhaylova and Dmitry I Nurminsky, argued against the silencing of sex-linked genes on the X chromosome in Drosophila during meiosis -- a process referred to as Meiotic Sex Chromosome Inactivation (MSCI) -- the reanalysis presented by Karr suggests MSCI is indeed occurring.

The work sheds new light on the evolution of sperm structure and function, through an analysis of Drosophila genes and gene products. As Karr explains, the research has important implications for humans as well: "The more direct, biomedical aspect is that when we learn about the function of a gene that encodes a protein in Drosophila sperm, we can immediately see if there's a relationship between these genes and their functions and known problems with fertility in humans."

Super Fly

Perhaps no other model organism has yielded more insights into human genetics than the tiny fruit fly Drosophila melanogaster. In 1906, Thomas Hunt Morgan of Columbia University began work on D. melanogaster, (one of over 1500 species contained in the Drosophila genus) capitalizing on the species' ease of breeding, rapid generation time and ability to readily produce genetic mutants for study. Morgan's efforts with Drosophila led to the identification of chromosomes as the vector of inheritance for genes, and earned him the 1933 Nobel Prize in Medicine.

Drosophila are yellow-brown in color, have reddish eyes and transverse black rings across their abdomen. Females are about 2.5 millimeters long, while males are slightly smaller and may be easily identified by their darker color.



Chromosomes: genetic storehouses




"There are certain aspects to the composition of these sex chromosomes that have intrigued evolutionary biologists for a long time," Karr notes. One such issue involves an apparent reduction in the number or the level of expression of sex-linked genes on the X chromosome during spermatogenesis. It is believed that this reduction or silencing of genes on the X chromosome may have profound implications for the evolution of sex chromosomes.


The relocation of male-biased genes to the autosomes may be due to a selective advantage favoring genes that move off the X chromosome and therefore avoid X-inactivation during meiosis. Such theories remain controversial however, as statistical analyses are used to evaluate gene frequencies and expression levels, making the proper categorization of genes particularly challenging. "The data we create and generate to support our ideas and hypotheses are messy, there's noise in them," Karr says. "Such noise is inherent in the history of evolution."


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Journal Reference:

  1. Maria D Vibranovski, Yong E Zhang, Claus Kemkemer, Hedibert F Lopes, Timothy L Karr, Manyuan Long. Re-analysis of the larval testis data on meiotic sex chromosome inactivation revealed evidence for tissue-specific gene expression related to the drosophila X chromosome. BMC Biology, 2012; 10 (1): 49 DOI: 10.1186/1741-7007-10-49

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Arizona State University. "Study of fruit fly chromosomes improves understanding of evolution and fertility." ScienceDaily. ScienceDaily, 10 August 2012. <>.
Arizona State University. (2012, August 10). Study of fruit fly chromosomes improves understanding of evolution and fertility. ScienceDaily. Retrieved August 31, 2015 from
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