SEX-RATIOS IN GAMBUSIA HOLBROOKI. 2OI 



as a result of all these crosses that the dorsal fin-spot of Type B 

 was carried through the male parent only. This fact was 

 demonstrated very beautifully by Schmidt's experiments, and 

 caused him to conclude that the genetic constitution of this fish 

 is of the XX, XY type, and that the Y-chromosome carries the 

 factor for fin-spot. Since this paper was written, Winge has 

 (220, 22b) further demonstrated both cytologically and geneti- 

 cally the XX, XY constitution of the Pceciliids, and has greatly 

 extended Schmidt's genetical studies. Aida ('21) has shown by 

 his breeding experiments, also, that another Pceciliidj Aplocheilus, 

 also has an XX, XY genetic constitution. 



If, indeed, the genetic constitution of the Pceciliidse is of this 

 XX, XY type, then it follows as a corollary that in the male, 

 which is the heterozygous sex, approximately equal numbers of 

 male- and of female-determining gametes are produced. As- 

 suming that no differential chance-of-fertilization exists we 

 would infer that in the young there would be nearly equal num- 

 bers of males and females. This is exactly what we do find 

 under controlled conditions. If then in the adult there is a pro- 

 nounced excess of females it would appear that this excess must 

 be the end-result of a differential death-rate of the sexes. 



A Differential Death-Rate in Gambusia. The males are much 

 less resistant to harmful environmental factors than the females, 

 and hence have a lower survival value. This is shown by the 

 results of several lines of experiment and observation. 



In those cases where quantitative studies have been made with 

 analyzed factors, the males do not survive as well as the females. 

 Thus in the writer's experiments, it was found that high tem- 

 peratures, high H-ion concentration, , oxygen-deficiency, and 

 concentrations of KCN kill the males much more readily than 

 the females. For example, in a collection of 283 young Gam- 

 busia which was killed in hot water, practically all the fish that 

 died first were males. Bellamy (in lit.) states that in his ex- 

 periments with other Pceciliids with high temperatures, oxygen 

 deficiency KCN, etc., he obtained results that are " in complete 

 agreement " with the writer's contention that " males are more 

 susceptible to ' difficult ' conditions than the females." 

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