Segmentation during Anterior Regeneration
In contrast to posterior regeneration, segment addition during anterior regeneration is more limited. All anteriorly regenerating species regenerate the anterior non-seg- mental regions (prostomium and peristomium). Most species also regenerate one or more segments: exactly how many varies according to each species, the amount of anterior segments amputated, and often also with the position of the cut along the main body axis (Berrill 1952). In many cases, fewer anterior segments regenerate than the number removed by amputation, generating a positional mismatch at the boundary between the regenerate and the stump and triggering a morphallactic shift of the segmental identities of the stump segments to restore the missing axial identities (see later). Development of regenerated anterior segments is either simultaneous or shows an anteroposterior gradient, suggesting the transient formation of a growth zone at the posterior margin of the blastema, where it joins the stump (Allen 1923; Berrill 1952).
Reports of gene expression patterns associated with formation of segmental tissues during anterior regeneration are much sparser than for posterior regeneration (Ozpolat and Bely 2016). Segmental tissue precursor cells share the blastema-wide expression of GMP genes and Wnt pathway genes (Bely and Wray 2001; Tadokoro et al. 2006; Nyberg et al. 2012; Ozpolat et al. 2016). Interestingly, in the clitellate Pristina leidyi, overactivation of the Wnt pathway during anterior regeneration by treatment with the GSK3beta inhibitor azakenpaullone results in formation of fewer, larger, and less developed anterior segments (Balarezo, Zattara, and Bely 2011). This result is similar to that reported during posterior regeneration in the nereid Perinereis nuntia (Niwa et al. 2013) and hints that the role of Wnt signaling in the segmentation clock might be common to most if not all annelids.
Out of the several genes putatively associated with the segmentation process in nereids, only engrailed expression has been characterized during anterior regeneration, in the clitellate Pristina leidyi, where it shows a salt-and-pepper pattern in likely ventral nerve cord cells of the blastema (Bely and Wray 2001). However, this same pattern was found for posterior regeneration, suggesting this gene might not play the same role in Pristina and nereids. Given the scarcity of molecular data of anteriorly regenerating species, it is currently not possible to adequately assess the commonalities and differences in gene expression between posterior and anterior regeneration. This knowledge gap will have to be addressed if we are to gain a better understanding of segment formation during anterior regeneration, and gain insight into wTiy anterior regeneration has showm to be much more labile than posterior regeneration throughout annelid evolutionary history (Bely 2010; Zattara and Bely 2оГб).