Speculations on the loss of regeneration derived from developmental modifications during land adaptation in some evolutionary lineages of animals

Abstract

Regeneration varies largely among metazoans. Aside molecular processes, this epiphenomenon depends on the biological complexity and evolutive history of each species during the adaptation to their specific environment. While most species adapted to marine or freshwater conditions can extensively regenerate, those adapted to terrestrial conditions and parasitism lost the ability to regenerate. They are mainly represented from ascelmintes evolving eutely and numerous arthropods and amniotes. High regeneration can only occur in water-adapted species and requires high tissue hydration, indirect development through metamorphosis and often also presence of asexual propagation. Metamorphosis allows the anatomical-physiological transformation of a larva in an adult through an initial destructive phase followed by a constructive (regenerative) phase. Invertebrates and vertebrates that possess genomes including metamorphic genes can re-utilize in part or largely similar genes for the regeneration of lost organs. I submit that during land adaptation in both invertebrates and vertebrates the initial larval stages and metamorphosis were lost or altered as some key genes, including those for telomerases, could no longer be expressed in the dry environment. Consequently, also the initial regenerative ability was lost while other epiphenomena were gained, including complex immunity and behaviour but also an evident process of ageing.