Sex inversion in domesticated strains of the swordtail,Xiphophorus helleriHeckel (Pisces, Osteichthyes)

In Xiphophorus helleri several alleged instances of sex inversion in females were described. Later researchers, doubting such findings, regarded the species as having separate sexes, of males, strong and weak, and females, strong and weak, in relation to the differing ratios of male and female sex determiners, and they considered the alleged sex inverted specimens to be cases of arrhenoidy of old females or weak males, in which the male secondary sex characters appear late so that in the male immature stage they could be mistaken for young females. The present investigation confirms these last two observations but it also shows that exceptional individuals are firstly in a female phase and produce broods, and secondly they enter a functional male phase. Virgin females mated with these hermaphrodites in male phase produce broods with a low percentage of males, in accordance with the hypothesis on the polyfactorial sex determination in Xiphophorus helleri. From our experiments it is suggested that in Xiphophorus helleri there is great variability in the sexual genotypes and phenotypes: some populations are strictly gonochoric; others have a high percentage of protogynic hermaphrodites; again others exceptionally may show sex inverted specimens.