The species Drosphilia melanogaster has been critical to the development of genetics for generations. Their simple life cycle and easily identifiable characteristics have made the species one of the most essential model organisms in the field. One of the species defining traits includes the ability with which one can easily identify the sex of these flies by simple physical features. This has allowed them to be important to the study of dosage compensation and sex determination. However, a study conducted at the Janelia Farm Research Campus of the Howard Hughes Medical Institute may forever change the views of sex determination in the cells in Drosphilia.
Until recently, it was believed that all of the cells in the fruit fly express the genes that determine maleness or femaleness but that only some regions of the body exhibit external characteristics while other regions seem physically identical regardless of sex. This study suggests that not all the cells in a fruit fly's body are the same but are mosaic, with different cells expressing the doublesex gene in different patterns while other cells not expressing it at all. Doublesex (dsx) is a critical gene for sex determination along with other genes such as fru. By using molecular biology, the scientists observed dsx expression in both larval and adult flies in order to study the location and conditions required for gene activation. The researchers began by inserting a GAL4 gene into a dsx locus through homologous recombination. They then partnered this sequence with a reporter gene, allowing them to track the cells expressing dsx. Their findings were quite significant.
Overall, this study resulted in several important findings. In addition to shedding light on how dsx regulates sexual development by controlling transcription factors and cell-cell signaling molecules in specific cells and tissues, the study illustrated that dsx acts on different aspects of sex at different times, even within one cell lineage. Also, up to this point, sex differentiation was considered to be an adult characteristic in flies but the results demonstrate that dsx is even expressed in the late third instar larval fat bodies. However the most significant finding was that dsx was not expressed in all cells. Looking at the evolutionary level, the results suggest that dsx and fru arose through modifications of the genes’ cis-regulatory regions. This implies that the pattern of sex determination present in current Drosphilia resulted from evolutionary selection for individuals who did not express sex determining genes throughout the body. Such expression of dsx may perhaps even hinder an individual’s overall fitness.
As the article in the Science Daily explains, the consequences of this find are huge on both a molecular and evolutionary level. If the cells in Drosphilia are mosaic and do not all undergo sex determination, then some of the XX and XY cells in one sense are not sex specific. What does this mean for other species? Many other species control sex determination through genes analogous to doublesex. How would such findings in C.elegans, zebafish or mice affect our understanding of sex differences? To then apply such considerations to higher species such as humans or other larger mammals may entirely change the way we consider sex differences. Only time and further research will determine what the implications of this finding will mean for our understanding of sex differences.
Link to article: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000365
Citations:
Public Library of Science. "Male or Female? In Flies, Some Cells Can't Tell." ScienceDaily 5 May 2010. 30 May 2010
Robinett CC, Vaughan AG, Knapp J-M, Baker BS. Sex and the Single Cell. II. There Is a Time and Place for Sex. PLoS Biology, 2010; 8 (5): e1000365 DOI: 10.1371/journal.pbio.1000365
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