The developmental theory of ageing predicts a positive correlation between developmental time and adult longevity. Experiments that vary larval density and food level have been carried out to test this prediction. The results show differences in viability, developmental time, starvation resistance and adult longevity. It is concluded that pre-adult developmental time is not a causal factor for the determination of adult longevity in Drosophila melanogaster. The observed variation in adult longevity is discussed in relation to viability selection and changed adult physiology.

A wild-type strain of Drosophila melanogaster was successfully selected for both fast and slow larval development. The realized heritabilities (h ) ranged from 0.20 to 0.30 for the fast lines and 0.35 to 0.60 for the slow lines. The selection applied is relevant in relation to the evolution of aging. The longevity of adults, either virgin or mated, was not affected by selection for developmental time, indicating that developmental time is not a causal determinant of life span, thus confirming the results of the studies on environmental effects on aging (Zwaan et al. 1991, 1992). However, adult body weights were higher in the slow developmental lines and lower in the fast lines, relative to the control flies. Furthermore, slow females showed relatively high early fecundity and low late fecundity, as compared with control and fast females. Mated longevities and total lifetime progeny productions were not statistically different. Previous results obtained by other authors from selection experiments on age at reproduction either supported the mutation accumulation or the negative pleiotropy theory of aging (Luckinbill et al. 1984; Rose 1984b). The impact of the reported results on the interpretation of these studies is discussed, and it is noted that direct selection on adult longevity is needed to settle this issue.

Flies from a wild type strain of Drosophila melanogaster, previously kept at 25°C, were reared at either 20, 25 or 29°C. As expected, developmental time and adult body size decreased with increasing temperature. Adult longevity of flies reared at 25°C was slightly greater than that of flies raised at 20 or 29°C when measured at all three temperatures. This may reflect the laboratory history of the strain. On the whole, it appeared that longevity was independent of adult body size. These results support our previous conclusion (Zwaan et a!., 1991) that developmental time and body size are not causally related to longevity in 'environmental' studies. It is stressed, that genetic analysis is needed to investigate the reputed correlation between development and ageing.

SummaryFungi normally do not senesce, but in some species mitochondrial plasmids are known to occur that induce senescence. In this paper models for the dynamics of a senescence plasmid in a fungal population are developed and analysed. In the first model it is assumed that total fungal biomass density is constant, while in the second model the resource dynamics and its effect on fungal growth is modelled explicitly. An additional death rate describes the effect of the plasmid on the senescent subpopulation. Plasmids can be transferred to non-senescent fungus. Criteria for the coexistence of the non-senescent and senescent fungal strains are derived, all of which have a clear biological interpretation. It is shown that coexistence is not possible in the first model, but is possible in the second model for a large range of parameter values. We show that the interplay between resource dynamics, fungal growth and plasmid transmission is crucial for coexistence. We develop a biological interpretation of how these mechanisms have to interact to promote coexistence. A numerical study of the second model further clarifies the relations between the numerical value of several parameters and coexistence of non-senescent and senescent fungal strains.