Sequential deposition of yolk components during oogenesis in an insect, Aedes aegypti (Diptera: Culicidae)

Vitellogenesis in Aedes aegypti of uniform body size was followed at 27 degrees C in narrow time intervals throughout their first reproductive cycle by measuring the length, diameter, and volume of follicles and oocytes, the latter as an expression of the yolk mass (vitellus). Independent of all experimental conditions, a two-step process of elongation was recognized for both follicle length and yolk length, so that growth curves were consistently composed of two linear regressions with different slopes against time. Follicle lengths started to increase immediately after the blood meal, while oocytes took up to 6 h to show a measurable increase in yolk length. The first linear phase continued until 30 h, when yolk length reached 268+/-22 micro m. At this point, a transition occurred where the linearity shifted sharply for the next 6 h to 2-4-times higher slopes for both regressions. This second growth phase represented a 40% elongation of oocytes and follicles. Then, both curves leveled off at their final size, characteristic of mature ovaries: 462+/-10 micro m for oocytes, 489+/-11 micro m for follicles. These values remained constant until oviposition. The first linear growth phase was associated with an equicaloric and synchronous protein and lipid incorporation into the oocytes; levels of these substances reached their maximum by the end of this first phase and remained constant until oviposition. The second linear growth phase was characterized by rapid glycogen incorporation into oocytes from 20 to 100% of the maximum. Subsequently, the surface pattern of the exochorion became visible, marking the end of yolk incorporation. Since eggs are always laid on moist substrates, within 2-3 h of oviposition they double in volume and fresh weight, driven by more than tripling of their water content. When blood-fed females were exposed to five different temperatures between 17 and 37 degrees C, the distinction between the two linear growth phases persisted, but the slopes of the respective regressions, and therefore their durations, were affected. Eggs still matured at 37 degrees C but never hatched and at 12 degrees C only 18% hatched, whereas at all the intermittent temperatures hatching was 80-90%. Oogenesis appears to be limited to the range between 12 and about 32 degrees C. The effects of age, maternal body size and the source of the blood on vitellogenesis were also examined. These parameters affected the onset and/or extent of oogenesis in various ways.