Asymmetric cell divisions in the early embryo of the leech Helobdella robusta

The small glossiphoniid leech Helobdella robusta is among the best-studied representatives of the super-phylum Lophotrochozoa in terms of early development. The Helobdella embryo undergoes a modified version of spiral cleavage, characterized by stereotyped cell lineages comprising multiple examples of equal and unequal divisions, many of which are well-conserved with respect to those of other clitellate annelids, such as the oligochaete Tubifex. Here, we review the early development of Helobdella, focusing on the variety of unequal cell divisions. We then summarize an experimental analysis of the mechanisms underlying the unequal first cleavage in Helobdella, concluding that the unequal first cleavages in Helobdella and Tubifex proceed by different mechanisms. This result demonstrates the evolvability of the basic cell biological mechanisms underlying well-conserved developmental processes. Finally, we propose a model in which the unequal second cleavage in Helobdella may be regulated by the polarized distribution of PAR protein homologs, convergent with the unequal first cleavage of the nematode Caenorhabditis elegans (super-phylum Ecdysozoa).