On the hormonal control of posterior regeneration in the annelid Platynereis dumerilii

Developmental stage dependent effects of posterior and germline regeneration on sexual maturation in Platynereis dumerilii

Regeneration, regrowing lost and injured body parts, is an ability that generally declines with age or developmental transitions (i.e. metamorphosis, sexual maturation). Regeneration is also an energetically costly process, and trade-offs occur between regeneration and other costly processes such as growth, or sexual reproduction. Here we investigate the interplay of regeneration, reproduction, and developmental stage in the segmented worm Platynereis dumerilii. P. dumerilii can regenerate its whole posterior body axis, along with its reproductive cells, thereby having to carry out the two costly processes (somatic and germ cell regeneration) after injury. We specifically examine how developmental stage affects the success of germ cell regeneration and sexual maturation in developmentally young versus developmentally old organisms. We hypothesized that developmentally younger individuals (i.e. with gametes in early mitotic stages) will have higher regeneration success than the individuals at developmentally older stages (i.e. with gametes undergoing meiosis and maturation). Surprisingly, older amputated worms grew faster and matured earlier than younger amputees. To analyze germ cell regeneration during and after posterior regeneration, we used Hybridization Chain Reaction for the germline marker vasa. We found that regenerated worms start repopulating new segments with germ cell clusters as early as 14 days post amputation. In addition, vasa expression is observed in a wide region of newly-regenerated segments, which appears different from expression patterns during normal growth or regeneration in worms before gonial cluster expansion.

Annelids as models of germ cell and gonad regeneration

Germ cells (reproductive cells and their progenitors) give rise to the next generation in sexually reproducing organisms. The loss or removal of germ cells often leads to sterility in established research organisms such as the fruit fly, nematodes, frog, and mouse. The failure to regenerate germ cells in these organisms reinforced the dogma of germline-soma barrier in which germ cells are set-aside during embryogenesis and cannot be replaced by somatic cells. However, in stark contrast, many animals including segmented worms (annelids), hydrozoans, planaria, sea stars, sea urchins, and tunicates can regenerate germ cells. Here I review germ cell and gonad regeneration in annelids, a rich history of research that dates back to the early 20th century in this highly regenerative group. Examples include annelids from across the annelid phylogeny, across developmental stages, and reproductive strategies. Adult annelids regenerate germ cells as a part of regeneration, grafting, and asexual reproduction. Annelids can also recover germ cells after ablation of germ cell progenitors in the embryos. I present a framework to investigate cellular sources of germ cell regeneration in annelids, and discuss the literature that supports different possibilities within this framework, where germ-soma separation may or may not be preserved. With contemporary genetic-lineage tracing and bioinformatics tools, and several genetically enabled annelid models, we are at the brink of answering the big questions that puzzled many for over more than a century.

Nerves and availability of mesodermal cells are essential for the function of the segment addition zone (SAZ) during segment regeneration in polychaete annelids

Most of annelids grow all over their asexual life through the continuous addition of segments from a special zone called "segment addition zone" (SAZ) adjacent to the posterior extremity called pygidium. Amputation of posterior segments leads to regeneration (posterior regeneration-PR) of the pygidium and a new SAZ, as well as new segments issued from this new SAZ. Amputation of anterior segments leads some species to regeneration (anterior regeneration-AR) of the prostomium and a SAZ which produces new segments postero-anteriorly as during PR. During the 1960s and 1970s decades, experimental methods on different species (Syllidae, Nereidae, Aricidae) showed that the function of SAZ depends on the presence and number of mesodermal regeneration cells. Selective destruction of mesodermal regeneration cells in AR had no effect on the regeneration of the prostomium, but as for PR, it inhibited segment regeneration. Thus, worms deprived of mesodermal regeneration cells are always able to regenerate the pygidium or the prostomium, but they are unable to regenerate segments, a result which indicates that the SAZ functions only if these regeneration cells are present during PR or AR. Additionally, during AR, nerve fibres regenerate from the cut nerve cord toward the newformed brain, a situation which deprives the SAZ of local regenerating nerve fibres and their secreted growth factors. In contrast, during PR, nerve fibres regenerate both during the entire regeneration phase and then in normal growth. This review summarizes the experimental evidence for mesoderm cell involvement in segment regeneration, and the differential impact of the digestive tube and the regenerated nerve cord during PR vs AR.

The cost and payout of age on germline regeneration and sexual maturation in Platynereis dumerilii

Regeneration, regrowing lost and injured body parts, is an ability that generally declines with age or developmental transitions (i.e. metamorphosis, sexual maturation) in many organisms. Regeneration is also energetically a costly process, and trade-offs occur between regeneration and other costly processes such as somatic growth, or sexual reproduction. Here we investigate the interplay of regeneration, reproduction, and age in the segmented worm Platynereis dumerilii. P. dumerilii can regenerate its whole posterior body axis, along with its reproductive cells, thereby having to carry out the two costly processes (somatic and germ cell regeneration) after injury. We specifically examine how age affects the success of germ cell regeneration and sexual maturation in developmentally young versus old organisms. We hypothesized that developmentally younger individuals (i.e. lower investment state, with gametes in early mitotic stages) will have higher regeneration success and reach sexual maturation faster than the individuals at developmentally older stages (i.e. higher investment state, with gametes in the process of maturation). Surprisingly, older amputated worms grew faster and matured earlier than younger amputees, even though they had to regenerate more segments and recuperate the more costly germ cells which were already starting to undergo gametogenesis. To analyze germ cell regeneration across stages, we used Hybridization Chain Reaction for the germline marker vasa. We found that regenerated worms start repopulating new segments with germ cell clusters as early as 14 days post amputation. In addition, vasa expression is observed in a wide region of newly-regenerated segments, which appears different from expression patterns during normal growth or regeneration in worms before gonial cluster expansion. Future studies will focus on determining the exact sources of gonial clusters in regeneration.