Female Germline Cysts in Clitellate Annelids: Organization, Functioning, and Diversity.

Germ cell precursors developing into functional gametes tend to form syncytial groups of cells known as cysts, clusters, or nests. Such cysts develop in the early stages of gametogenesis when gonial cells divide without full cytokineses, and cells (cystocytes) stay interconnected by relatively large cytoplasmic channels termed cytoplasmic (intercellular) bridges or ring canals. Such germline cysts occur during oogenesis in clitellate annelids (Clitellata). A distinctive feature of female cysts in clitellates is the presence of the cytophore, a shared anuclear cytoplasmic mass that occupies the cyst center. Each clustering cystocyte has one ring canal connecting it to the cytophore. This general pattern of cyst architecture is conserved in this group; however, the number of interconnected cells (from eight to 2500) and the shape and dimensions of the cytophore (reticular, ball-like, or tree-like) vary between taxa. This plasticity in cyst organization is usually conserved at the family/subfamily level and can, therefore, be a useful morphological/histological feature for phylogenetical considerations.This chapter presents the general aspects of cysts' organization and function. Moreover, we present several variants of cyst organization found in the main groups of Clitellata, such as microdriles, leech-like taxa, and megadriles. We also note the recent progress in cyst analyses and the resulting perspectives on cyst evolution in this group.