The International Journal of Developmental Biology

Int. J. Dev. Biol. 46: 1023 - 1033 (2002)

Vol 46, Issue 8

Autoaggressive, multi-headed and other mutant phenotypes in Hydractinia echinata (Cnidaria: Hydrozoa)

Published: 1 December 2002

Werner A Müller

Institute of Zoology, University of Heidelberg, Germany.


In an inbreeding program conducted with the colonial hydroid Hydractinia echinata, each F1 mating produced up to 50% F2 offspring displaying an aberrant, clone-constant phenotype, hence referred to as mutant strain. In autoaggressive strains, in one or several areas of the colony autoreactive stolons direct their aggressive devices (stolon tips filled with cytotoxic stinging cells), normally used to kill allogeneic competitors for living space, towards neighboring stolons or polyps (hydranths) of their own colony. In these areas tumor-like masses of self-aggressive stolons were formed, in severe cases causing the death of the colony. Based on previous genetic studies, the interpretation proposed here attributes autoaggressive behavior to a mosaic-type alternative expression of arl (allorecognition) alleles in heterozygous individuals. Developmental mutant strains termed He-mh form supernumerary heads during regeneration and normal development as well. Common to all He-mh phenotypes isthe production of additional headsalong the bodycolumn of fully-grown polyps. The heads give rise to complete hydranths connected by a tube that derives from the gastric region of the original polyp and eventually transforms into a stolon. In bastol strains, polyps convert the basal region of their body column into a periderm-covered stolon from which the residual apical hydranth detaches. Colonies expressing both the He-mh and the bastol (bst) phenotype frequently lose detaching multi-headed hydranths and the colony disintegrates. The large number of mutant F2 offspring reveals high genetic variability in Hydractinia.

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