The genetics of Artemia salina. V. Crossing over between the X and Y chromosomes.

N wild populations of brine shrimps, the eye color is black. A preliminary I study of a spontaneous recessive mutation, w, which governs white eyes, indicated that the white locus is partially sex-linked; that is, it lies on the homologous segments of the X and Y chromosomes (BOWEN 1963). The female was found to be the heterogametic sex. Females yere designated XY, males XX. In matings of +/w females to w/w males, crossirig over between the white locus and the sex locus could be detected. The frequency of recombination was low (1/1625 or O.OGO/,) in the earlier study in which females derived from a mutant laboratory stock were outcrossed to males from two wild populations. The purpose of the present investigation was to mate white-eyed shrimps to wild-type shrimps from eight bisexual populations to determine if the female is heterogametic in all eight populations and to determine whether the frequency of crossing over is the same on the different genetic backgrounds. One European population appears to be reproductively isolated from the mutant stocks (see Experiment 6). However, when the seven American races are mated to the mutant stocks, fertile hybrids are obtained which can be testcrossed to mutant shrimps (Experiments 1, 4, and 5 ) . It will be shown that the female is heterogametic in the seven American populations and that there is a surprisingly great amount of variation in frequency of recombination between the white locus and the sex locus (values of 0.03% to 20%). Experiments 1,2, and 3 are designed to demonstrate that a characteristic frequency of crossing over can be transmitted from mother to daughter and that neither the X chromosome nor the autosomes affect the frequency. The sex chromosomes of Artemia: There are two possible modes of sex determination in Artemia. (1) Sex may be determined by one true sex locus; either by dominance of the gene for femaleness to the allele for maleness at this locus or by a balance between a gene for maleness at the sex locus on the X with genes for femaleness on the autosomes. (2) Sex may be determined by one or more loci on the differential segment of the X and/or the Y chromosome. The apparent “sex locus” would be the junction of the differential segment (carrying the true sex loci) with the homologous segment of the sex chromosome (Figure 1 ) . Both concepts of a sex locus would be in accord with the data obtained from genetic experiments (BOWEN 1963). However, the second model now appears more probable