The effect of 5-azacytidine (5-azaCR) on head regeneration and budding in hydra are reported. Hydra attenuata were exposed to various doses of 5-azaCR for 48 h and then decapitated and cultured. Head regeneration and bud formation were observed for 12 days after decapitation. Untreated control hydra regenerated heads within 7 to 8 days of decapitation with a budding index of 0·2. Buds invariably arose in the normal budding zone (below the gastric region).

In the group treated with 0·8mM-5-azaCR, 9 days after decapitation head regeneration was seen in only 13% of animals, and an average of two buds per hydra were formed, most of which were in the vicinity of the distal end. Induction of budding was also seen in the animals that regenerated heads.

In animals exposed to 1 mM-5-azaCR three main types of responses were observed 9 days after decapitation. (1) 44% of the animals regenerated normal heads; about half of them developed at least one bud and these buds originated in the budding zone. (2) 17·5% of the animals developed abnormal, long hypostome-like structures with single or bifurcated tentacles at theirtips. There were at least two buds per animal and they were invariably at abnormal sites. (3) 32% of the animals failed to regenerate heads, although they developed two buds. 87% ofthese buds originated in abnormal sites of the body column and a large number (72%) did not detach even by the 12th day after decapitation.

Both 5 and 10 mM of 5-azaCR were toxic to the animals; the survivors formed large globeshaped heads. Bud induction was seen in 60% and 28% of animals in the 5 and 10 mM groups, respectively.

These observations demonstrate that 5-azaCR induces bud formation in hydra at doses that inhibit head regeneration. This bud induction might be due to a specific expression of gene products responsible for bud formation.

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