Asexual Reproduction and Segmental Regeneration, but not Morphallaxis, are Inhibited by Boric Acid in Lumbriculus variegatus (Annelida: Clitellata: Lumbriculidae)

Abstract

Body fragmentation, in some animal groups, is a mechanism for survival and asexual reproduction. Lumbriculus variegatus (Mu¨ller, 1774), an aquatic oligochaete worm, is capable of regenerating into complete individuals from small body fragments following injury and reproduces primarily by asexual reproduction. Few studies have determined the cellular mechanisms that underlie fragmentation, either regenerative or asexual. We utilized boric acid treatment, which blocks regeneration of segments in amputated fragments and blocks architomic fission during asexual reproduction, to investigate mechanistic relationships and differences between these two modes of development. Neural morphallaxis, involving changes in sensory fields and giant fiber conduction, was detected in amputated fragments in the absence of segmental regeneration. Furthermore, neural morphallactic changes occurred as a result of developmental mechanisms of asexual reproduction, even when architomy was prevented. These results show that fragmentation in L. variegatus, during injury or asexual reproduction, employs developmental and morphallactic processes that can be mechanistically dissociated by boric acid exposure. In regeneration following injury, compensatory morphallaxis occurred in response to fragmentation. In contrast, anticipatory morphallaxis was induced in preparation for fragmentation during asexual reproduction. Thus, various forms of regeneration in this lumbriculid worm can be activated independently and in different developmental contexts.

Effects of nerve injury and segmental regeneration on the cellular correlates of neural morphallaxis

“…Worms were then returned to normal culture at 16°C. Ninety percent of worms exposed to this temperature shift undergo asexual reproduction within 3 weeks (Martinez et al, 2006). In experiments investigating the ability of amputation to suppress asexual reproduction, worms of 150 segments in length were cut into two body pieces, at segment 100, 2 days after temperature shift.…”

“…Morphallaxis, the reorganization of original (intact) structures without the recruitment of cell proliferation (Morgan, ’01; Gilbert, 2006; Sanchez-Alvarado and Tsonis, 2006), remodels existing neural networks. Neural morphallaxis involves the transformation of the adult nervous system as regenerating fragments acquire new anterior–posterior neurobehavioral identities (Drewes and Fourtner, ’90; Lesiuk and Drewes, 2001a; Martinez et al, 2005, 2006). This rare form of neural plasticity is highly adaptive, because segmental regeneration in Lumbriculus is asymmetric; that is, body fragments stereotypically regenerate only eight new head segments on anterior ends and tails of variable length on posterior ends (Drewes and Fourtner, ’90; Lesiuk and Drewes, 2001a; Martinez et al, 2005).…”

“…Neural morphallaxis is also activated earlier to body fragmentation by architomic fission during asexual reproduction (Martinez et al, 2005, 2006). Although the use of the terms epimorphosis and morphallaxis has recently come into question (Agata et al, 2007), in lumbriculid worms these terms appropriately define distinctive regenerative processes that underlie the replacement of lost structures and the remodeling of others.…”

“…The replacement of lost segments in Lumbriculus occurs as a process of epimorphosis, which involves the formation of a wound blastema that further differentiates into new head and tail body parts (Morgullis, ’07; Berrill, ’52; Drewes and Fourtner, ’91). As amputated fragments always regenerate a head of 7–8 segments in length, the original fragment segments undergo a morphallactic regeneration, which involves anatomical, physiological, and behavioral reorganization of those segments (Drewes and Fourtner, ’91; Martinez et al, 2005, 2006). Specifically, worm fragments demonstrate changes in nervous system structure; in rapid-escape reflexes; and in regeneration-specific protein expression (Drewes and Fourtner, ’91; Martinez et al, 2005, 2006).…”

“…We have utilized changes in rapid escape reflex behavior (Drewes and Fourtner, ’91; Martinez et al, 2006) and a molecular marker of morphallaxis (Martinez et al, 2005) to examine the contributions of nerve injury and segmental regeneration in the remodeling of the original body fragment. A neural glycoepitope, which is labeled by a monoclonal antibody, Lan 3-2 (Martinez et al, 2005), allowed the monitoring of several proteins whose expression is enriched during morphallaxis.…”