High-resolution QTL Mapping Reveals Sign Epistasis Controlling Ovariole Number between two Drosophila Species

¹ Princeton University
² Johns Hopkins University

^* To whom correspondence should be addressed. E-mail: vorgogoz{at}princeton.edu.

Submitted on November 30, 2005
Revised on January 30, 2006
Accepted on 11 February 2006

	Abstract

Identifying the genes underlying genetically complex traits is of fundamental importance for medicine, agriculture and evolutionary biology. However, the level of resolution offered by traditional quantitative trait locus (QTL) mapping is usually coarse. We analyze here a trait closely related to fitness, ovariole number. Our initial interspecific mapping between D. sechellia (8 ovarioles/ovary) and D. simulans (15 ovarioles/ovary) identified a major QTL on chromosome three and a minor QTL on chromosome two. To refine the position of the major QTL, we selected 1038 additional recombinants in the region of interest using flanking morphological markers (selective phenotyping). This effort generated approximately one recombination event per gene and increased the mapping resolution by approximately seven times. Our study thus shows that employing visible markers to select for recombinants can efficiently increase the resolution of QTL mapping. We resolved the major QTL into two epistatic QTLs, QTL3a and QTL3b. QTL3a shows sign epistasis: it has opposite effects in two different genetic backgrounds, the presence versus the absence of the QTL3b D. sechellia allele. This property of QTL3a allows us to reconstruct the probable order of fixation of the QTL alleles during evolution

Key Words: D. sechellia, QTL, selective genotyping, selective phenotyping, sign epistasis

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