Introgressing marked chromosome segments

   Initially, a large number of lines were established.  Each line had a unique portion of one chromosome marked with a transposable marker.  For each line, individuals of the two species were crossed to yield hybrids.  The hybrids were then repeatedly backcrossed to D. simulans, each generation choosing offspring individuals that displayed the marker.  This procedure ensured that the individuals backcrossed to D. simulans in the next generation carried a portion of the D. mauritiana chromosome surrounding the marker.  Because of repeated recombination, however, the length of that segment surrounding the marker became shorter and shorter, until after 15 generations of backcrossing, the average length of the marked segment was approximately 4.7 centiMorgans.
 
 
 
 
 
 
 
 
 
 

    The locations of the introgressed segments are shown on the following map (one segment was introgressed per line).  Note that the introgressed segments are well-dispersed over the entire genome.:

    (Figures from J. R. True, B. S. Weir and C. C. Laurie. 1996.  A genome-wide survey of hybrid incompatibility
     factors by the introgression of marked segments of Drosophila mauritiana chromosomes into Drosophila
     simulans.  Genetics 142: 819-837.)

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