Photoperiodic induction of pupal diapause in the flesh-fly sarcophaga argyrostoma

Sar'cophaga argyrostoma has a temperature modified photoperiodic response curve typical of a long day species. In the present study both the intrauterine embryos and larvae were exposed to the same photoperiod and temperature, resulting in an increase in the incidence of pupal diapause and the value of the critical daylength when compared to curves in which only the larval stage had experienced inductive photoperiods. S. argyrostoma is thought to possess a temperature compensated photoperiodic 'counter' which adds up successive short days (or long nights), the final response being a consequence of the number of diapause inductive cycles that the insect experiences during the restricted photoperiodically sensitive period (Saunders, 1971). Gibbs's (1975) model for the photoperiodic counter was tested by altering the length of the sensitive period using methods other than temperature. Manipulations such as prevention of larviposition or starvation of early larval instars tended to increase the length of the larval period and I. resulted in an increase in the incidence of diapause. On the otherhand, crowding larvae during the feeding phase or the premature extraction of larvae from the food decreased the length of the larval period and resulted in a decrease in diapause incidence. Unexpectedly, an increase in the post feeding phase, caused by crowding larvae during this stage, was associated with a decrease in diapause incidence. Photoperiodic sensitivity.was shown to decline from a maximum during the late embryonic stage through to the mature feeding larvae and ceased shortly after the onset of the wandering stage. These results are discussed in relation to Gibbs's model, and possible refinements to the model are suggested. 3. Two artificially selected strains were produced from a stock c-ulture of S. argyrostoma by breeding from the first and last individuals to form puparia; these strains were named 'fast' and 'slow' respectively. The behaviour of the strains and hybrids produced by interstrain crosses were investigated under a range of photoperiods and temperatures, the results suggesting that both fast and slow strains were highly polygenic. The strains were found to differ only in the duration of the post feeding stage; selection had not affected the duration of any other fraction of the life cycle, although the critical photoperiod of the fast strain appeared to have shortened by 1 hour compared to that of stock and slow. In conditions of low temperature and short daylengths puparium Formation in the 'fast' strain was highly synchronised whilst that of the 'slow' strain was characterised by a long negative skew. Since 'fast' and 'slow' produced individuals with widely differing larval periods, they provided a useful tool to investigate the summation of photoperiodic cycles in S. argyrostoma, and data from the behaviour of the strains in diapause-inductive cycles was consistant with the 'refined' version of Gibbs's model. 4. Low temperature is not a prerequisite for diapause termination in S. argyrostoma; spontaneous termination of diapause can occur at constant temperatures of 20 ° C or 25 0 C. In this study, it was demonstrated that diapause termination itself is temperature independnt. The rate of diapause termination was also similar in both sexes, and was independnt of both the larval (diapause inductive) photoperiod and the degree of diapause in the group from which the diapause pupae were drawn. Results from preliminary experiments suggested that male and female flies possess different thresholds for diapause induction. 5. Pupal diapause in S. argyrostoma is a result of the inactivation of the brain-prothoracic gland system, ultimately resulting in the absence of the peak of ecdysone usually associated with pharate adult development. The levels of ecdysteroids in the haemolymph of diapause destined and non-diapause destined larvae and prepupae were measured, using a radloimmunoassay technique. The results failed to support the hypothesis that the temporary cessation of endocrine activity was a result of a negative feedback system involving ecdysteroids and the PTTH neurosecretory cells. TABLE OF CONTENT Page GENERAL INTRODUCTION: THE DIAPAUSE STATE DEFINITION AND CLASSIFICATION 1 STAGE SPECIFICITY 4 PHYSIOLOGICAL ADAPTATIONS 7 COLD HARDINESS 10 ECOLOGICAL ADAPTIONS 14 PHOTOPERIODIC REGULATION 17 FACTORS AFFECTING THE PHOTOPERIODIC RESPONSE Geographical Location 19 Diet 22 C. Temperature 24 TIME MEASUREMENT IN INSECT PHOTOPERIODISM 27 PHOTDPERIODIC SENSITIVE PERIOD 37 ENVIRONMENTAL FACTORS THAT AFFECT DIAPAUSE DEVELOPMENT 41 ENDOCRINOLOGY OF DIAPAUSE 44 PHOTOPERIODIC SUMMATION 48 INTERACTION OF THE PHOTOPERIODIC COUNTER AND ENVIRONMENTAL VARIABLES 53 THE AIMS OF THE STUDY 56 GENERAL MATERIALS AND METHODS 58 CRITERIA FOR THE DETERMINATION OF NON DIAPAUSE AND DIAFAUSE DEVELOPMENT 61 AUTOGENY IN S. argyrostoma 62 LIFE CYCLE OF S. argyrostoma at 25° C 64-65 Page CHAPTER I PHOTOPERIODIC INDUCTION OF PUPAL DIAPAUSE IN S. argyrostoma INTRODUCTION 68 MATERIALS AND METHODS 70 RESULTS: THE PHOTOPERIODIC RESPONSE CURVE 74 EARLY EMBRYONIC SENSITIVITY TO PHOTOPERIOD 731 MATERNAL INFLUENCE ON THE INCIDENCE OF DIAPAUSE AMONG THE 1st GENERATION PROGENY OF ADULTS WITH A HISTORY OF DIAPAUSE 93 DISCUSSION: GENERAL FEATURES OF THE PHOTOFERIODIC RESPONSE 95 DELAY OF PUPARIATION IN DIAPAUSE COMMITTED LARVAE 100 CHAPTER II THE PHOTOFERIODIC COUNTER INTRODUCTION 104 MATERIALS AND METHODS 112 RESULTS: MANIPULATION OF THE LENGTH OF THE LARVAL FEEDING STAGE 121 MANIPULATION OF THE LENGTH OF THE POST-FEEDING LARVAL STAGE 161 DISCUSSION 188 C H A P T E R III SELECTION FOR EARLY AND LATE PUPARIATION IN S. argyrostoma INTRODUCTION 203 MATERIALS AND METHODS 206 Page RESULTS: TIMING OF PUPARIUM FORMATION IN THE SELECTED STRAINS 208 PHOTOPERIODIC RESPONSE OF THE SELECTED STRAINS 234 DISCUSSION 249