TRANSFER OF A PARASITIC SEX FACTOR TO THE NUCLEAR GENOME OF THE HOST - A HYPOTHESIS ON THE EVOLUTION OF SEX-DETERMINING MECHANISMS IN THE TERRESTRIAL ISOPOD ARMADILLIDIUM-VULGARE LATR

In A. vulgare sex is usually determined either by a cytoplasmic femini zing factor (F symbiotic bacteria) or by another feminizing factor (f) which behaves like a mobile element of DNA and which seems to corresp ond to a fragment of bacterial DNA. By inhibiting the expression of ma le genes carried by the Z heterochromosome, these feminizing factors i nduce differentiation of neo-females [ZZ(+F) or ZZ(+f)]. Such a mechan ism leads to the production of progenies whose sex ratio is highly fem ale biased. In some populations in which F and/or f factors are presen t, genetic females (WZ) have disappeared and all individuals (males an d females) are genetic males. However in other populations, cohabitati on of ZZ(+f) neo-females and females in all points similar to genetic females is observed. Such a situation may be unstable and is not likel y to be explainable only by migrations of individuals from distinct po pulations. Owing to certain types of crosses, in particular those whic h involve an artificial neo-male (= female reversed into a functional male by an implant of androgenic gland) we show here that the f factor can be transmitted as a Mendelian gene. In these progenies Z(f)Z fema les may appear: like WZ females, they breed broods whose sex ratio is unbiased. The hypothesis that the ''F bacteria A. vulgare'' symbiosis may have led, after a complex co-evolutive process (F bacteria --> f m obile element --> insertion of f on Z heterochromosome), to the creati on (from a male genotype) of a female genotype, is put forward. The co nsequences of such a phenomenon on the composition and the evolution o f A. vulgare populations are examined.