Observations on the discovery of a dorso-ventral axis in Hydra
Authors: Baird Ronald V. , Burnett Allison L. 
Published: April 2021

The developmental relationship between bud morphogenesis and the parent animal was analysed by following bud development subsequent to each of five experimental treatments of the parent animals.

(1) Excision of the region of the parent animal distal to the budding region demonstrates that: (a) the tentacle pattern and body form of all buds produced are normal; (b) the number of initiated buds is not altered; (c) the number of detached buds is reduced. (2) Excision of the region of the parent animal proximal to the budding region demonstrated that (a) the tentacle pattern and body form of all buds produced are normal; (b) the number of initiated buds is not altered; (c) the number of detached buds is not altered. (3) Isolation of the budding region demonstrated that: (a) the tentacle pattern and body form of all buds produced are normal; (b) the number of initiated buds is not altered; (c) the number of detached buds is not altered. (4) Polarity reversal of the budding region demonstrated that: (a) disto-proximal polarity reversal results in a dorso-ventral reversal of the bud tentacle pattern; (b) polarity reversal of the budding region may persist. (5) The development of isolated buds demonstrated that: (a) the tentacle pattern and body form of all buds are normal; (b) the polar phenomena of all buds are normal; (c) the maturation process is extended but complete.

On the basis of the results of these experiments, the conclusion is drawn that, once bud initiation has occurred, bud morphogenesis proceeds independently of parental regulation. It is proposed that this occurs because the factors regulating body form, the tentacle pattern, and the two axes of the bud are produced by the developing bud.

The relationship of bud morphogenesis to maintenance of the adult form was analysed in two groups of experiments. Both groups of experiments were designed to determine if the dorso-ventral axis of the bud persisted after bud maturation.

(1) The effect of the dorso-ventral axis of polarity at the level of the growth region: (a) regenerated tentacles on mature animals form a pattern identical with the bud tentacle pattern; (b) the dorso-ventral axis of the regenerated tentacle pattern coincides with the dorso-ventral axis of the bud tentacle pattern; (c) chronological ageing has no effect upon the pattern of regenerated tentacles or its polar axis. (2) The effect of the dorso-ventral axis of polarity at the level of the budding region: (a) the first bud formed by recently detached animals appears on the dorsal side of the animals; (b) consecutive buds tend to form opposite the preceding bud rather than alternating between the dorsal and ventral sides of the parent animal; (c) following a non-budding period, the incidence of dorsal buds, as budding resumes, increases irrespective of the position of the last bud formed prior to the non-budding phase; (d) the position of consecutive buds is not altered by excision of the regions of the parent animal proximal or distal to the budding region.

On the basis of the results of these experiments, the conclusion is drawn that the dorso-ventral axis of the bud persists at the level of the growth and budding region after bud maturation.

The hypothesis is proposed that the growth regulatory effect of the dorso-ventral axis of polarity is demonstrated by the formation of a bilaterally symmetrical, dorso-ventrally polarized bud tentacle pattern, by the orientation of the regenerated tentacle pattern and by the determination of the site of the first bud produced following maturation or starvation of adult animals.

A possible mechanism of the action of the dorso-ventral axis of polarity in bud initiation, development and maturation is discussed in terms of a model of bud development.