Aspetti e problemi della vita coloniale

Abstract

Mnnomorphic and polymorphic colonics arc representative of two different levels in the physiological coordination of zooids. The division of labour anion the members of a polymorphic colony may go so far as to remind of the organ integration into one organism. The aggregation of individual tnyxamochac of Dictyostolium to form the pscudoplasuiodium, which then differentiates into an organized fruiting Imdy, could exemplify the possible origin of the multicellular organism through an integration process of individual cells in the protistan colony. The life-cycle study of the colony of the ascidian Dotryllus schlossvri show that also the succeeding generations of zooids in the colony may be in a very strict coordination. The various ways of colonial development and its relations with sexual reproduction, embryogcncsis ami regeneration are reviewed and the validity of « reserve cells » Concept discussed. Some recent data cn the physiology of development and regeneration can throw some light on the problem of the regional organization in the colonies. The main factors which can put in motion the mechanisms of hud development and control its result arc considered. Examples taken from Ilydrozoa (stolon mutation in Hydra) and Aseidiacca (« situs inversus vi«ecrtim » in Bairyllus schlosseri) give the opportunity of illustrating and discussing the transmission to the buds of spontaneous or experimentally induced variations, which arc not heritable through the germ cells. The adaptive significance of colonial life is emphasized. It finds its expression in the somatic plasticity of colonics which allows a continuous repetition of developmental cycles, with external and internal conditions controlling the results. This plasticity confers to the colonics remarkable homoeostatie powers, as shown by the regulation of the number of generations and individuals in the colony of Botryllus. and the interchangeability of the zooids in the polymorphic colonies of Ilydrozoa and Rryozoa. © 1965 Taylor & Francis Group, LLC. All Rights Reserved.