| Summary: | Sex-ratio (proportion of males) in a species is related to a not entirely explained mechanism of sex determination. In Drosophila, sex is determined by the proportion of X chromosomes vs. autosomes. A decrease in the proportion of males vs. females, known as sex-ratio, is characteristic to several Drosophila species, and is related to an inversion in the sexual chromosome the so called “sex-ratio” (SR) condition. In this occasion we study the presence of that inversion in several populations of Drosophila pseudoobscura from Mexico. With this purpose we did collections of this species on Nevado de Colima,Col., Valparaíso, Zac, Zirahuén, Mich., Tulancingo, Hgo. and Amecameca, Mex. Flies were captured in nature and carried to the laboratory were individual cultures of each female were established, when the offsprig emerged salivary glands of a single larva from each culture were extracted and stained with an aceto-orcein solution and the corresponding genotype for the third and sex chromosome determined and their relative frequencies calculated. The corresponding frequencies of the “sex-ratio” inversion in the localities analyzed were: Nevado de Colima 21.8 %, Valparaiso 18.8 %, Zirahuén 11.4 %, Tulancingo 6.8 % and Amecameca 6.8 %. Apparently an East-West cline distribution is present in these populations. Relative frequencies for inversions in the third chomosome were recorded and relationships between third and X chromosomes inversions performed. Comparisons with similar studies on this and other species are pointed out.
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