Evolutionary Aspects of Annelid Regeneration

Abstract

Annelids, like many other invertebrate animals, replace lost body parts in a process called regeneration. However, the ability to regenerate lost segments is present in some groups and not others; for example, leeches do not regenerate lost segments. Anterior and posterior regeneration involves the formation of a bud containing stem cells that differentiate into the new head or tail segments. Annelid regeneration also involves remodelling of surviving body fragments. The ability of annelids to regenerate tail segments appears to be nearly universal among species capable of regeneration. The ability to regenerate head segments, although common, is not universal and can depend on the number of segments lost. The absence and presence of regeneration across annelid groups, including closely related species, suggests that regeneration ability may be an ancient trait that has been lost in some species during annelid evolution. Key Concepts Annelids vary in their capability for regenerating body segments, including among closely related species. The ability of annelids to regenerate posterior segments appears to be nearly universal. The ability of annelids to regenerate anterior segments, although common, is not universal and is often limited depending on the number of segments lost. Annelid regeneration may involve both epimorphic and morphallactic mechanisms. Multiple losses and gains of regeneration ability have likely occurred during annelid evolution. Why regenerative ability among annelids varies extensively remains unclear. With development of new techniques for genetic analysis and microscopy, annelids are becoming important model systems for the study of regeneration.

Annotation of nerve cord transcriptome in earthworm Eisenia fetida

“…Recently, Nino et al, 2016 demonstrated that during the anterior regeneration in the brain amputated earthworm Eudrilus eugeniae the nerve cord takes control of the neurological activities and regulates the segmental nerves to assist in worm's survival [7] . The process of regeneration in annelids is a combinatorial outcome of epimorphosis and morphallaxis and the formation of blastema during epimorphosis is supported by the neoblast stem cells residing particularly in the epidermal cell layers [103] , [104] , [105] . Especially the post amputation and wound healing process enables the synthesis and activation of several growth factors and signaling molecules, which assist in signal transduction by triggering the stem cell proliferation to facilitate the process of tissue regeneration and organogenesis [106] .…”

Large scale changes in the transcriptome of Eisenia fetida during regeneration

“…In annelids, work on marine naidine annelids has yielded that the position and developmental stage of amputations made, can ‘naturally’ rescue regenerative capacities[ 34 ]. The ability to regenerate anterior segments has been lost at least 12 times while the posterior regenerative ability was lost at least 4 times during the evolution of annelids [ 53 ]. The genome of E .…”

A Comprehensive Ultrastructure and Transcriptome Analyses of Early Regeneration in Ophryotrocha Xiamen Sp. Nov. (Annelida: Dorvilleidae)

“…In the early stages of regeneration, in ammation and apoptosis factors launch the downstream process of development [41,42]. All of them were reported to play important roles in early embryonic development which contained several genes playing integral roles in re-speci cation of regenerated structures in several annelid species [14]. Due to the scarcity of annelid genomic data used for annotation, a large set of potentially novel DEGs in O.…”

“…In recent decades, dozens of studies have proven that almost each species of annelids owns segmental regeneration ability which made it to be an excellent group to study the regeneration mechanisms [12,13]. The ability to regenerate segments in annelids after amputation varies from one species to another; however, anterior regeneration is less common than posterior regeneration which has been documented in many annelid groups [12,14]. In Ophryotrocha, no evidence has been shown for the ability of anterior regeneration, although species such as Ophryotrocha puerilis and O. notoglandulata have been shown to regenerate posteriorly [15].…”