On the Mechanism of Heterosis. I. Metabolic Capacity of Different Races of Drosophila melanogaster for Egg Production

Abstract

This paper deals with the lifetime egg productions of one hybrid and 8 inbred races of Drosophila. Lifetime egg productions of roughly 50 females composed the data for each race. Individual females range in egg production from 0 to 3,168 eggs. The lifetime egg records of all races are similar in form. Egg laying begins on the second day, rises rapidly to a maximum between the 5th and 10th day, then declines as age advances at a slow but steady rate. Marked differences in racial egg production exist. The range is from 263 to 1,701 eggs for the inbred races; 2,034 for the Hybrid. These racial differences in egg production are associated with similar significant differences in female duration of life, days eggs were laid and male duration of life. The variances for the different racial egg yields, days eggs were laid and duration of life of the males and females are likewise characteristic. A marked correlation exists between the means and variances such that the larger the mean racial value the greater the variance. The correlation between the means and variances has the effect of setting the individuals in the high-producing group further apart, thus facilitating any selective procedure. Both racial and individual lifetime egg productions are correlated with such variables as duration of life of the female, the days the female laid, the maximum egg production in the cycle and the rate of senescence in egg production. In terms of variance controlled, the importance of these variables to lifetime egg production is in the following order-maximum egg production, days female laid, age at maximum production and length of survival of female. The rate of senescence is of minor importance. Hereditary variations are responsible for a large part of the racial differences. Something like 50 per cent. of the variances are fixed in the different racial groups as contrasted with 50 per cent. which still remains within the races. A similar pattern of correlated physiological functions for lifetime egg production is found within races. Again maximum egg production followed by days the female laid eggs are the important variables. The age at maximum and survival of the female are of minor significance. Heterotic increase in egg production, associated with hybridity, is shown to turn largely on greater maximum daily egg production. The lengths of life of the female and their rates of senescence may be comparable in hybrids or inbreds. The correlations show that by taking the production on the 5th to 10th days of imago life we gain most of the information on metabolic capacity which is pertinent to heterosis.