SEX REVERSAL IN APLOCHEILUS LATIPES AND A NEW EXPLANATION OF SEX DIFFERENTIATIO

Abstract

In Aplocheilus latipes where the male is digametic XY two allelomor- phic genes concerning the body color, orange-red (R) and white (r) are located in the X and Y chromosomes (AIDA 1921). Heterozygous orange-red males XRYr in crosses with white females XrXr produce orange-red females XRXr and white males XrYr in criss-cross manner, though sometimes a few exceptional orange-red males may be found. Some of these abnormal males produce in crosses with normal females offspring with an abnormal sex ratio, the females always predominating in number and the males which are all abnormal like their male parent being very few or not present at all. By carrying on the breeding of these males gen- eration after generation I found that the proportion of males gradually in- creases (AIDA 1930), and I tried in the former report to explain these abnormal males as the product of non-disjunction in sex chromosomes having XXY constitution. WINGE (1930) found in Lebistes reticulatus sex-reversed males of female genotype XX which in crosses with normal females produced female off- spring only. On account of the similarity of breeding results between his observation on sex reversal and ours on abnormal males he suggested that our male fish are also produced by sex reversal, but not by non-disjunction. To determine which one of these alternative conceptions is more valid further research has been continued. A new fact irreconcilable with the supposition of non-disjunction was found, and at present I am inclined to believe that our males are, as WINGE has assumed, sex-reversed males of the female genotype XX. Moreover I was able to produce at the same time females which are unquestionably due to sex reversal, viz. females of male genotype XY. The possibility of sex alteration in so well differentiated a gonochorist as Aplocheilus and the breeding results induced me to make a new hypo- thesis on sex differentiation in our fish.