Karyotype evolution - Insights from a D. melanogaster strain with unusual sex chromosome karyotypes
Authors: Duojia Li 1; Dhyey Gandhi 1,2,3; Yukiko Yamashita 1,2,4
Affiliations: 1) Whitehead Institute for Biomedical Research; 2) Department of Biology, Massachusetts Institute of Technology; 3) Department of Chemistry, Massachusetts Institute of Technology; 4) Howard Hughes Medical Institute, Massachusetts Institute of Technology
Keywords: c. chromosome structural variation; h. selection
Drosophila melanogaster utilizes the X/Y sex determination system where the X to autosome ratio determines sex. Their sex chromosomes, X and Y, each harbors a ribosomal DNA (rDNA) locus consisting of hundreds of tandemly repeated rRNA genes whose sufficient copy number and transcription is essential for cellular functions. Females carrying Y chromosomes are rare occurrences which usually results from low rates of nondisjunction events. Here we discovered a strain of D. melanogaster from geographically isolated Seychelles archipelago whose X chromosomes barely contains rDNA, necessitating all females to carry at least one Y chromosomes. We found that this strain has two kinds of Y chromosomes, an intact one (Y) and a truncated one (YS) that lacks the majority of the long arm, the latter of which cannot support male fertility. Investigating why such a non-functional YS chromosome may be maintained in this strain, we found that Y chromosome makes female sterile when present in multiple copies. Therefore, YS serves as the essential source of rDNA for female. Our work on the Seychelles strain may hint at how karyotype may evolve, eventually leading to the diversification of species.