Abstract
A complete profile of the 20-hydroxyecdysone (20-HE) titer, development and endocrine events from 1st instar to pupation of the larvae of non-diapause-destined (NDD) and diapause-destined (DD) tasar silkworm, Antheraea mylitta Drury (Lepidoptera: Saturniidae) was studied. Diapause is induced by short days of 11 hr photophase coupled with = 12h light, >= 26°C) and one generation is completed by August. The growth rate during the course of development of larval instars decreases and instar durations are inversely related to the body weight at the time of initiation of a larval instar. A growth compensation mechanism operates during the development of the larval instars. The growth rate was higher in early instars (1st to 4th) in both generations. The DD larvae complete the final instar in 16 days followed by a spinning stage of 13 days. The NDD larvae complete the final larval instar in 9 days followed by spinning stage of 6 days and spend 14 days in the pupal stage. The signal to release the prothoracicotropic hormone (PTTH) is related to critical body weight of larvae. From 1st to 4th instar, pre-ecdysial peaks of 20-HE were recorded in both NDD and DD generations. The programme for undergoing diapause was initiated during 3rd instar and induced by a sudden decrease in the level of 20-HE in the DD generation. Two peaks of 20-HE are required for the larval-pupal transformation, first at the wandering stage and the second at cuticle formation.
Highlights
Diapause is an arrest in development accompanied by a major shutdown of metabolic activities, which is genetically programmed and occurs at a specific stage in each species (Denlinger et al, 2004)
It is presumed that the external stimuli are stored in the brain-retro-cerebral complex during larval-pupal development and diapause behaviour expressed during the pupal stage is due to the cessation of prothoracicotropic hormone (PTTH) release (Bowen et al, 1985), which stops the activation and release of ecdysone from prothoracic gland into the haemolymph that induces moulting
In the bivoltine ecorace of tasar silkworm A. mylitta, the fresh larval body weight gain was higher in NDD than DD individuals
Summary
Diapause is an arrest in development accompanied by a major shutdown of metabolic activities, which is genetically programmed and occurs at a specific stage in each species (Denlinger et al, 2004). It is presumed that the external stimuli are stored in the brain-retro-cerebral complex during larval-pupal development and diapause behaviour expressed during the pupal stage is due to the cessation of prothoracicotropic hormone (PTTH) release (Bowen et al, 1985), which stops the activation and release of ecdysone from prothoracic gland into the haemolymph that induces moulting. This is confirmed by the ecdysone titer assays of several insect species viz., Galleria melonella (Cymborowski et al, 1991); Manduca sexta (Bowen et al, 1985); Sarcophaga argyrostoma (Richards et al, 1987) and Bombyx mori (Sakurai et al, 1998). There are differences in titer patterns in diapause-bound and nondiapause-bound larvae as well as diapausing and nondiapausing pupae of Heliothis virescens (Loeb, 1982)
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