Abstract
In addition to five pairs of euchromatic chromosomes common to all individuals, three different morphological types of heterochromatic chromosomes were found in Hydrotaea meridionalis: Two acrocentric chromosomes of different length and a smaller submetacentric chromosome that occur in different combinations. The most common type, the larger of the acrocentric chromosomes, occurred as a pair in 149 out of 197 larvae. Mother-offspring combinations revealed that the larvae progeny of several mothers carried morphologically only the common karyotype. Despite existing morphological variation in karyotypes the same karyotypes were found in male and female adult flies. This observation is interpreted by an autosomal sex-determining mechanism. As there was also found a sex-specific karyotype in males, a simple hypothesis to explain the observed distribution of karyotypes among sexes is suggested: one male locus located on one of the autosomes with a dominant heterogametic sex determination mechanism. Only in homozygous recessives for that locus is sex determined by the independent XX/XY sex-determining system.
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