The effect of theDrosophilaX chromosome on the number of male germ cells

Abstract

Summary Fertile Drosophila subobscura males having the X chromosomal gene arrangement A2+3+5+7 produce predominantly female offspring. These males are called “sex ratio” males. Irrespective of the unisexual offspring, the total number of offspring of a sex ratio male can be as high as the bisexual offspring of control males. Thus the absolute number of female offspring is higher in A2+3+5+7 males than in control males. One reason for this high number of female offspring could be increased sperm production by sex ratio males. In the present study we analyzed spermatogenesis of sex ratio and control males. Sperm differentiation in Drosophila occurs in so-called cysts. In D. subobscura they contain, after spermatogonial divisions and meiosis, 128 synchronously developing spermatids each. We found that the testes of sex ratio males contained double the amount of cysts as control males. However, this is not the explanation for the great amount of female offspring of those males. In the final stage of spermatid maturation, in the coiling stage, the number of cysts was the same in sex ratio males as in control males. Because in sex ratio males only the X sperm are released into the seminal vesicle, these males transfer only half the number of functional sperm. The mechanism which is in charge of the high female offspring of sex ratio males could be a higher frequency of copulation and/or a better utilization of sperm by the female, or something else. The same high cyst number as in fertile A2+3+5+7 males was also found in sterile males with the same A chromosomal gene arrangement but different autosomal gene arrangements as fertile A2+3+5+7 males. This result indicates that the total number of cysts is controlled only by the A chromosome, independent of male fertility. In sterile A2+3+5+7 males sperm differentiation stopped during individualization. No sperm was found to enter the seminal vesicle.