Abstract
We have sampled wild chromosomes from two natural populations of Drosophila melanogaster and obtained flies fully homozygous for the second chromosome, the third chromosome, or both, as well as flies heterozygous for one or both wild chromosomes and balancer chromosomes. Rate of embryogenesis (egg laying to larval hatching) and rate of development from egg to adult are measured, by classifying the individuals into fast, intermediate, and slow developmental classes. The experiments indicate that variation for rate of embryogenesis and for rate of egg-to-adult development is plentiful in the natural populations. Various hypotheses are enunciated to account for the small range of phenotypic variation observed in wild-type individuals with respect to the two parameters (embryogenesis and egg-to-adult development) and for the difficulty in changing the mean rates by artificial selection. Appropriate experiments may decide among the hypotheses, helping us to understand the genetic control of rate of ontogenesis, which is an important fitness component.
Highlights
Rate of ontogenesis, i.e., the time elapsed from the egg to the adult stage, is an important fitness component: the earlier an individual reaches reproductive age, the greater its fitness (Cole, 1954, Lewontin, 1965; Parsons, 1983)
Most artificial selection experiments attempting to modify the mean rate of development in Drosophila are usually unsuccessful for a number of generations, if the selection is sustained for many generations selection gains are usually obtained (Sang & Clayton, 1957; Marien, 1958; Clarke et al, 1961;Prout, 1962; Dawson, 1966; Tigerstadt, 1969; Cavener, 1983)
In our laboratories we have been unsuccessful in selection for faster and slower rates of embryogenesis - time from egg laying to larval hatching - in Drosophila melanogaster and D. simulans (Marinkovi6 & Ayala, in press)
Summary
I.e., the time elapsed from the egg to the adult stage, is an important fitness component: the earlier an individual reaches reproductive age, the greater its fitness (Cole, 1954, Lewontin, 1965; Parsons, 1983). The interaction between the fitness gained by faster development and" the physiological constraints may be the mason for the apparent strong homeostatic control of the rate of development. Most artificial selection experiments attempting to modify the mean rate of development in Drosophila are usually unsuccessful for a number of generations, if the selection is sustained for many generations selection gains are usually obtained (Sang & Clayton, 1957; Marien, 1958; Clarke et al, 1961;Prout, 1962; Dawson, 1966; Tigerstadt, 1969; Cavener, 1983). In our laboratories we have been unsuccessful in selection for faster and slower rates of embryogenesis - time from egg laying to larval hatching - in Drosophila melanogaster and D. simulans (Marinkovi6 & Ayala, in press). The present experiments explicitly address the question whether natural populations of D. melanogaster store genetic variation for rate of embryogenesis and for rate of development from egg to adult. The opportunity for the expression of the genetic variation is enhanced by obtaining flies that are fully homozygous for the second and the third chromosomes (which together amount for 80 percent of the genome of D. melanogaster), so that recessive genes will be expressed
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