Abstract
In Drosophila uirilis, phototactic selections to both positive and negative directions were performed for 40 generations by using a classification maze. The figure of responses to both directional selections was asymmetrical. The realized heritabilities of these photo-positive and negative populations were calculated by the regression slopes for the first 20 generations and these values were fairly low.
Reverse and disruptive selections were initiated from generation 34 of both photopositive and photonegative populations, and mean photoscores tended slightly to the photonegative side. From the results, the polygene, manifesting negative phototaxis, seemed to be partially dominant over polygene, manifesting positive phototaxis, or the number of polygene, responsible for both phototaxis was assumed to be different.
As several morphological and physiological characters correlated with the effect of selection, body size (weight and length), fecundity (number of eggs per fly per day) and walking behavior to light of unselected, photonegative and photopositive populations were compared with one another.
Photopositive flies had smaller body and laid lower mean number of eggs per day than photonegative flies. On the other hand, photopositive flies showed fast response to right, but their sensitivity to the change of light environments was dull and their life time was presumed to be short. Photonegative flies were close to unselected flies on such morphological and physiological characters. The correlative effects of directional selections for phototactic behavior would be either linkage relationship between some polygenic systems or pleiotropic functions of some polygene.
