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First published online May 13, 2004
Journal of Experimental Biology 207, 2133-2145 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01002

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Arrested development in Xenopus laevis tadpoles: how size constrains metamorphosis

Irena Rot-Nikcevic^* and Richard J. Wassersug

Department of Anatomy and Neurobiology, Dalhousie University, 5850 College Street, Halifax, Nova Scotia, B3H 1X5, Canada

^* Author for correspondence (e-mail: nikcevic{at}dal.ca)

Accepted 29 March 2004

Xenopus laevis tadpoles that arrest development and remain as larvae for several years sometimes occur spontaneously in laboratory populations. These tadpoles cease development at an early hindlimb stage, but continue to grow and develop into grossly deformed giants. Giant tadpoles lack thyroid glands, and differ in morphology and behaviour from normal larvae. They are negatively buoyant, typically with small and partially solidified lungs, and have greatly enlarged fat bodies. Giant tadpoles have mature gonads with eggs and sperm, whereas normal tadpoles of the same stage have undifferentiated gonads. Larval reproduction has never been reported in anurans, but gonadal development decoupled from metamorphosis brings these giants the closest of any anurans to being truly neotenic. We discuss behavioural and morphological factors that may hinder both reproduction in giant Xenopus larvae and the evolution of neoteny in anurans in general. Experimental treatment with exogenous thyroid hormone induces some, but not complete, metamorphic changes in these giants. The limbs and head progress through metamorphosis; however, all tadpoles die at the stage when the tail would normally be resorbed. The disproportionate growth of tissues and organs in giant tadpoles may preclude complete metamorphosis, even under exogenous thyroid hormone induction.

Key words: Xenopus laevis, giantism, thyroid gland, growth, development, neoteny

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