Pattern regulation during the development of the ventral abdomen in the flesh fly Sarcophaga agryostoma

The adult abdominal epidermis of the flesh fly, Sarcophaga agryostoma, arises from small nests of diploid histoblasts which grow out and replace the polytene larval epidermal cells during metamorphosis. Extirpation experiments are used to investigate the roles of interactions with neighbouring nests or larval epidermis in determining the fates of the ventral histoblast nests. The extirpation of both left and right ventral histoblast nests deletes the sternite in the corresponding adult segment, but when only one nest is extirpated the hemisternite on that side of the adult is regenerated by the remaining nest. Regenerated hemisternites are smaller and bear fewer bristles than normal hemisternites. Extirpation of the larval epidermis between the two ventral nests leads to the production of small duplicate hemisternites, which resemble the regenerated hemisternites produced by single ventral nests. These midventral extirpations were also found to delay ventral nest fusion. The results suggest that each ventral nest has an autonomous capacity to form both hemisternites, but that interactions between the nests after fusion normally limit their development and prevent pattern duplication. The possible nature of such interactions is discussed and related to models of pattern regulation and growth control in imaginal disc development.