Mean Total Fecundity Research Articles

Influence of preimaginal environment on fecundity and ageing in Drosophila melanogaster hybrids—II. Preimaginal temperature

Reciprocal hybrids of two highly inbred strains of Drosophila melanogaster, Gabarros 4 and Abeele, were cultured, at a preimaginal population density of 30 eggs per standard vial at the temperatures of 16, 19, 22, 25, 28 and 31°C. Afterwards for every preimaginal temperature considered and for the whole life, the daily egg-production of females of known size and duration of development was counted, at 25°C. Life-span was also recorded. The mean total fecundity does not vary in function of the preimaginal temperature, exception made for the extreme 31°C. The mean daily egg-production, the size and the duration of development decrease with the increase in preimaginal temperature. Life-span at 25°C is prolonged when that temperature is decreased; life-span is thus positively correlated with size and duration of development. In a previous experiment, the life-span at 25°C of hybrids cultured at 25°C and at various preimaginal population densities was positively correlated with duration of development and negatively with size. The regressions longevity-duration of development observed in both experiments are of an identical sign; they differ however significantly. It is therefore concluded that longevity is not determined solely by preimaginal duration of development and that more of one developmental factor must be involved. The next paper in the series will compare and discuss the results of both experiments.

Influence of preimaginal environment on fecundity and ageing in Drosophila melanogaster hybrids I. Preimaginal population density

Reciprocal hybrids of two highly inbred lines of Drosophila melanogaster, Gabarros 4 and Abeele, were cultured at 25°C at preimaginal densities of 3, 7, 15, 30, 60, 120, 240 and 480 eggs per standard culture. As previously shown by different authors, the size of the emerging adults diminishes with increasing density, while the duration of development, as measured by the eclosion time of the adults, is greater. From each series of experiments some female flies were tested for daily egg-production throughout their life. With increasing density the mean total fecundity remains constant up to density 240 (80 times the lowest density used); at density 480 it decreases significantly. With increasing density, however, the mean daily egg-production decreases steadily, the number of days of egg-laying increases, while the maximum number of eggs laid in a 24-hr period decreases sharply. When pre-imaginal density increases, the longevity of the emerging imagos, isolated by pairs, increases as well. This increase in imaginal lifespan is not a simple prolongation of the egg-laying period; indeed at high density the length of the final period without egg-production is larger by a factor of six or seven than at low density. With increasing density, the imaginal age at which the maximal egg-production occurs is considerably delayed. The data are discussed in relation to genetical studies where fecundity is measured as an important character related to fitness, to genetical and physiological studies on ageing, and, more generally, in relation to theories of ageing; it is suggested that the data support the delayed maturity hypothesis of Medvedev.

Fecundity, Reproductive Rates, and Innate Capacity for Increase of Three Parasites of Therioaphis Maculata (Buckton)

Three species of hymenopterous parasites of the spotted alfalfa aphid were reared over a range of constant temperatures. At each thermal level, life—table data were obtained for each species. These data were used in computing certain statistics concerning reproduction and rates of potential population increase which are valuable in assaying the effectiveness of each species as an aphid parasite. The temperatures studied ranged from 10 to 35°C, and the relative humidity was held as constant as possible between 40 and 60%. Artificial illumination and photoperiods were identical in all studies. The studies showed that the braconid, Trioxys utilis Muesebeck, generally has the highest mean total fecundity of the three species, although the aphelinid, Aphelinus semiflavus Howard, produced more eggs in the temperature range of 18 to 22°C. The gross reproductive rate and net reproductive rate of T. utilis were also higher than the other parasites throughout most of the temperature range studied, although again A. semiflavus showed higher reproductive rates between 24 and 27°C. The innate capacity for increase, rm, of T. utilis was decidedly the highest of the three parasites at all temperatures. The braconid, Praon palitans Muesebeck, was inferior tot he other two parasites in nearly all phases of these life—table statistics. Its inclination to enter a facultative hibernal diapause at relatively mild temperatures, plus its intolerance of higher temperatures, which the other two species endured, limited its thermal range of effectiveness. Statistics such as total fecundity, gross reproduction, rate, net reproduction rate, and innate capacity for increase are discussed relative to their value in indicating the potential effectiveness parasites can be expected to possess in the field.