Developmental asynchrony caused by steep temperature gradients does not impair pattern formation in the wasp, Pimpla turionellae L.

Abstract

Generating developmental gradients by temperature gradients established within a developing organism is an easy, non-invasive technique to study physiological interdependencies between locally separated subsystems. A linear temperature gradient of about 10° C/mm was maintained up to 5 h in either direction along the long axis of a long-germ-type hymenopteran egg, which was simultaneously filmed by the 16 mm timelapse technique. The result was a dramatic desynchronization of development, which between the egg poles could reach up to 9.3 h relative to normal development. Within the same egg, up to seven mitotic waves (i.e. eight different nuclear generations) were observed at the same time, and the subsequent cellularization process was extremely asynchronous. The "initial regions" of the mitotic waves, the "fountain flow" of the ooplasm, and the gastrulation process were shifted towards the egg pole kept at higher temperatures. Developmental processes occurring successively in normal development now took place simultaneously, with either direction of the temperature gradient. For instance, while gastrulation had started in the warm region, midblastula transition and cellularization were in progress in the middle of the egg, and intravitelline nuclear multiplication occurred at the cold pole, by rapid and still biphasic cell cycles. In some respects, development resembled that of a short-germ-type insect egg. Nevertheless, the developmental processes were resynchronized after the temperature gradient was switched off. Surprisingly, the extreme desynchronization during early development did not affect the segment pattern of the resulting embryos. The technique of inducing well-defined developmental asynchronies might be applied in Drosophila to analyse the subtle interplay between maternal and zygotic gene activities described in this species.