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Etudes du developpement de la drosophile: variations on a theme Grigliatti, Thomas A.

Abstract

As a holometabolous organism which has been highly exploited genetically, the fruitfly, Drosophila, is an excellent model for the study of development in higher organisms. Conditional mutations that are sensitive to temperature differences were used to investigate problems of gene action in different tissues, regulation of specific tissue determination and differentiation, and the genetic regulation of development and functional integration of the nervous system. The first problem, a classical question in developmental biology, attempts to determine whether control of the activity of a structural gene is directly imparted by the tissue in which the gene product is active or whether, in fact, a freely flowing evocator of gene action exists. A temperature-sensitive ras allele has an effective lethal phase at 29°C about 12 hours after pupation and a temperature-sensitive period (TSP) for lethality beginning midway through the third larval instar and ending around pupation. The mutation also alters the quantity of pteridines present in the eyes, testes and malpighian tubules at 29°C. The TSP for pigment production in the malpighian tubules occurs in the egg and first instar larvae, and in eyes and testes after pupation. The demonstrated autonomy of the mutant in the eyes implies the tissue-specific functioning of the gene. It is suggested that the different TSPs for a single mutation indicate tissue-specific activation of a gene at different times during development, although the possibility of activation of preformed polypeptides has not been eliminated. There exists in Drosophila a class of mutations called "homeotic" which cause changes in determination of the imaginal discs. The second problem investigated concerns the possibility of isolating a temperature-sensive homeotic mutant for the purpose of studying genes which regulate specific pathways of differentiation. The homeotic mutant, ss[sup a40a], was found to have a temperature-sensitive transformation of the arista segment of the antennal complex to a tarsus of the leg. In a selected stock, penetrance was complete so that at 29°C, normal aristae were produced, whereas at 17°C, complete tarsi developed in all flies. "Shift" studies revealed a temperature-sensitive period in the third larval instar. The temperature-initiated action of ss[sup a40a] does not appear to act on a ts receptor site within the disc cells. In combination with another homeotic mutant, Antenna-pedia, the entire antennal complex is transformed to a complete leg at 22°C. Mutants affecting the nervous system were sought for the purpose of investigating the genetic regulation of the development and function of the nervous system. A temperature-sensitive mutation, para[sup ts] of Drosophila melanogaster causes an immediate but reversible paralysis only of adults when shifted from 22°C to 29°C. The mutation is a sex-linked recessive mapping 2.8 units to the left of f. Wild-type flies observed for two hour periods exhibited normal mobility at all temperatures between 22° and 35°C. From 22° to 25°C, para[sup ts] flies were wild-type in walking, climbing and flying ability. At one degree intervals above 25°C, para[sup ts] flies became increasingly debilitated and at 29°C, complete paralysis occurred. After prolonged maintenance at 29°C, recovery of some activity could occur at that temperature. Extensive studies of behavior of mosaics at 29°C revealed a requirement of the + allele in the head for mobility and a thoracic component for proper leg movement. Normal electroretino-grams were obtained at both 22° and 30°C. The results suggest a temperature-sensitive defect in the central nervous system.

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