Genetic assimilation of part of a mutant phenotype

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Abstract
SUMMARY: When the mutant veinlet was removed from a Drosophila melanogaster line selected for shortened vein length it was found that the selected polygenic background produced vein gaps in the absence of the major mutant. This example of genetic assimilation was unusual, however, in that the phenotype of the assimilated line was not exactly the same as the phenotype of the original selection line. It differed in two respects: only one of the longitudinal veins was affected in the assimilation line whereas all veins are shortened in the selected ve line, and vein gaps were sub-terminal in the assimilated line whereas they are terminal in veinlet. Modifiers of L 4 vein length were found to be located on both chromosomes II and III, though the chromosome II factor had a larger effect and was required for gap formation. The chromosome II gene was mapped to the same position as a similar L 4 vein length polygene (PL(2)L4a) found in another veinlet selection line and reported elsewhere. The pattern of expression of the ve mutant and the assimilated line genotype were compared using profiles of vein frequency at given points. The vein pattern profiles are discussed in relation to an hypothesis to explain the observed selection response and the effectiveness which these polygene combinations showed in modifying the much more complex veinlet phenotype.