PUPAL DIAPAUSE IN TROPICAL FLESH FLIES: ENVIRONMENTAL AND ENDOCRINE REGULATION, METABOLIC RATE AND GENETIC SELECTION

Abstract

1. Pupal diapause in flesh flies collected within 10° of the equator may be induced by low temperature but not by photoperiod. The threshold for diapause appears to be near the fringe of ecologically relevant temperatures in each geographic area. 2. Cool temperature during the photophase elicits both a higher incidence of diapause and longer diapause than cool temperature coinciding with the scotophase. 3. Diapause persists for several months at a constant cool temperature, but four days at 25° C is sufficient to break diapause in P. spilogaster. The four-day exposure can be allocated either as a single block of time or as four one-day units spread over one month, suggesting that the fly pupa has an accurate mechanism for temperature summation. 4. Water content of the larval food exerts a minor effect on pupal diapause incidence: 10% addition of water elevated the response by 10%, and 10% decrease of water reduced the response by 18%. 5. Males and females enter diapause in approximately equal proportion. 6. When non-diapausing flies were selected for propagation, the level of diapause in P. spilogaster at 15° C dropped from 80 to 10% in ten generations. 7. Metabolic rate during diapause (7.9-10.9 µl O2/g/hr) is less than 15% of the lowest rate measured in non-diapause pupae. Infradian cycles of O2 consumption similar to, but more variable than, the pattern in temperate zone flies are observed during diapause. 8. Adult development can be initiated readily with an injection of ecdysterone. A juvenile hormone analogue is ineffective by itself but greatly enhances the ecdysterone response. Development also can be triggered with topical application of several organic solvents. 9. Diapause in the tropical flies is proposed as the evolutionary basis for the long over-wintering diapause in flesh flies that have radiated into temperate zones.