In meiosis I, the centromeres of sister chromatids are pulled, together, toward the same pole of the spindle. In mitotis, the opposite is true—sister centromeres are pulled apart. This study explores the manner in which mitotic outer kinetochores are shed to allow assembly of meiotic kinetochore structures.

Background In zebrafish and many other organisms, specification of primordial germ cells (PGCs) requires the transmission of maternally‐derived germ plasm. Zebrafish germ plasm ribonucleoparticles (RNPs) aggregate along the cleavage furrows during the first several cell cycles, segregate asymmetrically during the cleavage stages, and undergo cytoplasmic dispersal in the late blastula. Results For all tested germ plasm RNAs [carbonic anhydrase 15b (ca15b), deleted in azoospermia‐like (dazl), dead end (dnd), nanos 3 (nos3), regulator of G‐protein signaling14a (rgs14a), and vasa/DEAD box polypeptide 4 (vasa/ddx4)], RNPs are homotypic (containing a single RNA type), with RNPs packing tightly yet remaining distinct within germ plasm aggregates. Homotypic clustering of RNAs within RNPs is observed before aggregation in the cortex and is maintained through germ plasm recruitment, asymmetric segregation and RNP dispersal. We also identify a step of germ plasm fragmentation during the cleavage stages that precedes RNP dispersal. Conclusions Our findings suggest that germ plasm aggregates act as subcellular compartments that temporarily collect and carry single RNA‐type RNPs from fertilization until their cytoplasmic dispersal in PGCs at the end of the blastula period, and describe a previously unknown fragmentation step that allows for an increase in the pool of germ plasm‐carrying cells, presumably PGCs. Developmental Dynamics 248:306–318, 2019. © 2019 Wiley Periodicals, Inc.

The differentiation of primordial germ cells (PGCs) occurs during early embryonic development and is critical for the survival and fitness of sexually reproducing species. Here, we review the two main mechanisms of PGC specification, induction, and preformation, in the context of four model vertebrate species: mouse, axolotl, Xenopus frogs, and zebrafish. We additionally discuss some notable molecular characteristics shared across PGC specification pathways, including the shared expression of products from three conserved germline gene families, DAZ (Deleted in Azoospermia) genes, nanos-related genes, and DEAD-box RNA helicases. Then, we summarize the current state of knowledge of the distribution of germ cell determination systems across kingdom Animalia, with particular attention to vertebrate species, but include several categories of invertebrates – ranging from the “proto-vertebrate” cephalochordates to arthropods, cnidarians, and ctenophores. We also briefly highlight ongoing investigations and potential lines of inquiry that aim to understand the evolutionary relationships between these modes of specification.

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