LAUSR

Meiosis is a specialized cell-division process in germ cells that ensures proper segregation of a single haploid genome into each gamete from a diploid precursor cell. This process ensures that the embryo receives the correct number of chromosomes upon fertilization. To accomplish this, meiotic germ cells duplicate their genome one time followed by two rounds of cell division (meiosis I and II) to produce haploid gametes. Therefore, there are major modifications that occur compared to mitotic cell divisions to ensure that the two successive meiotic cell divisions faithfully segregate one copy of every chromosome into each gamete [1]. In contrast to mitosis, after DNA replication, the meiotic chromosomes (each consisting of the duplicated sister chromatids) find their counterparts (homologs) and pair. Upon the first meiotic division, these paired homologous chromosomes separate from each other so that each daughter cell ends up with one set of chromosomes. To accomplish this reductional division, homologous chromosomes must find each other and form attachments—or pair, synapse, and form crossovers. Finally, at the second meiotic division, the sister chromatids segregate, resulting in the final outcome of four haploid cells. How homologous chromosomes find each other and pair during meiosis I is somewhat of a mystery but seems to involve the telomeres. In many organisms, including vertebrate animals, the telomeres attach to the nuclear envelope at the beginning of meiosis. This attachment is mediated through binding of telomere-associated proteins to a protein complex that spans the nuclear envelope and interacts with the cytoskeleton [2]. The telomeres then move and cluster to one side of the cell forming a structure called the “bouquet” (Fig 1). The movement of the chromosomes during bouquet formation is thought to facilitate pairing of homologous chromosomes. Once homologous chromosomes find each other, they make stable connections, which ensures alignment on the metaphase plate and separation of the two homologs to opposite daughter cells. These stable connections are provided by a proteinaceous structure that forms between homologous meiotic chromosomes, called the synaptonemal complex, and through direct chromosome interactions made through meiotic crossovers [3]. The mechanisms underlying meiotic chromosome segregation have been elucidated through studies on several model organisms, including yeast, fruit flies, nematodes, and mice. The zebrafish has more recently become an additional favorite model system as an intermediate between invertebrates and mammals. Zebrafish research has contributed extensively to many aspects of germ cell biology, including how primordial germ cells are guided to the developing gonad as well as the role of germ cell-specific small RNAs (pi-RNAs) in the germ line, to name a few [4–7]. The zebrafish has many attributes as a model organism, including excellent genetic and genomic tools. In zebrafish, as is true in most animals, gametes in both sexes are continuously replenished throughout the life of the animal. Therefore, the processes of germline stem cell regulation, progenitor cell-divisions, meiosis, and gametogenesis can all