Fig. 17.7. Effect of X/A ratio on sex; different X/A ratios were obtained experimentally by varying the number of X chromosomes (2X, 3X, 4X) and the sets of autosomes (2A, 3A, 4A); at ratios too low (< 0.3) or too high (> 1.5) the dosage compensation is unable to prevent death; the threshold range (over which intersex develops) is slightly different in two species i.e. at X/A ratio 0.67, *Drosophila* is intersex, while *C. elegans* is male (modified from Hodgkin, 1990).

*Coenorhabditis elegans* is a nematode, in which following two types of individuals are found : (i) XX self fertilizing hermaphrodites and (ii) XO males (Fig. 17.6; compare with

*Drosophila*)

*.* The hermaphrodites are believed to be female in its soma (somatic tissues) and mixed (male and female) in its germ line. When the effect of X/A ratio was examined in this nematode, a pattern similar to

*Drosophila* with minor variations was observed. In this nematode an X/A ratio less than 0.67 (0.50 in

*Drosophila*)leads to male sex and a ratio greater than 0.75 leads to a hermaphrodite (this ratio is 1.0 for female in

*Drosophila*)

*.* Acomparison of these ratios in the nematode and fruitfly (Fig. 17.7) suggests that a threshold is in operation in each case, which is an excellent example of threshold response in developmental biology. One may, of course, like to find out the manner by which the embryo is able to compute this ratio accurately and reliably leading to sex differentiation (see later).

Fig. 17.7. Effect of X/A ratio on sex; different X/A ratios were obtained experimentally by varying the number of X chromosomes (2X, 3X, 4X) and the sets of autosomes (2A, 3A, 4A); at ratios too low (< 0.3) or too high (> 1.5) the dosage compensation is unable to prevent death; the threshold range (over which intersex develops) is slightly different in two species i.e. at X/A ratio 0.67, *Drosophila* is intersex, while *C. elegans* is male (modified from Hodgkin, 1990).