RESUMEN
A reversal of sexual phenotype resulting in the appearance of neo-males and neo-females occurs in the populations of two marine subspecies of Idotea balthica. Sex inversions are rare in I. b. basteri but more frequent in I. b. tricuspidata. By crossing these two subspecies, the rate of neo-males (ti) was considerably increased, allowing us to study the determination of sex inversion. The I. b. tricuspidata used in this study came from the offspring (27 p. 100 neo-males) of one gravid albafusca female (A1) collected in La Rochelle (LR); these offspring had been studied for five generations. The I. b. basteri came from offspring of females collected in Marseille (M), Racou (eastern Pyrenees) and Tunis. The chi 2 or Mann-Whitney test was used to compare the results of the different crosses. The female Idotea balthica is heterogametic (WZ); the male is homogametic (ZZ). The loci of the genes which determine the colored phenotypes, albafusca (A) and bilineata (B), are situated on the W heterochromosome. The fundamental phenotype, uniformis (U), is the wild type, according to classical genetics. The males used were either ZZ and uniformis or WZ (neo-males) and albafusca. 1) Ten crosses: female A basteri (M) X male U tricuspidata (LR) gave a variety of offsprings (chi 2 = 56 - d.f. = 9), two having an A1-like ti and the other eight no or very few neo-males. 2) Backcrosses (from CRF2 to CRF5) between hybrid females and U La Rochelle males: in CRF2, the ti increased significantly and in CRF3, CRF4 and CRF5 it reached the ti of the A1 lineage. 3) In the female F1 X male UM backcrosses, the ti was nearly zero, thus being comparable with the basteri lineage of Marseille. 4) When female ALR X male UM of some lineages was crossed, the expressivity of the phenotype A was reduced (from A to U) only in neo-males. The penetrance of gene A did not vary significantly in seven crosses from F1 to CRF2. 5) The crossing of male male ULR with female female basteri B or U from various populations showed that only some B and U females were likely to favor sex inversion in their daughters; the resulting ti was ten times higher with A females. These results led to the following conclusions: --crosses 1, 2 and 3 showed that reinforcement of the tricuspidata genotype of La Rochelle, by means of a regular addition of mainly autosomal genes, induced an increase in the ti, whereas reinforcement of the basteri genotype induced a decrease in that rate. The results supported the hypothesis of a polyfactorial determination of sex involving autosomal, or partly sex-linked, factors acting as inhibitors of the major female gene(s); --crosses 4 showed that the reduction of gene A expressivity was always associated with the sex inversion of genetic hybrid females; that process also seemed to be controlled by some autosomal or partly sex-linked recessive genes, different from those of the preceding complex and acting as inhibitors of the major female-determining factor(s) situated on the W heterochromosome...