Dicentric chromosome formation and spermatogenesis.

A. Mechanism to generate a dicentric Y chromosome. FLP catalyzes recombination between inverted FRTs on sister chromatids of a Y chromosome marked with Bar ^Stone (B^S) and yellow⁺ (y⁺) to produce a dicentric chromosome marked with y⁺ and an acentric chromosome carrying both copies of B^S. During mitosis, breakage of the dicentric at a non-central site produces a short centric fragment Y lacking B^S and FRTs and a long centric fragment Y lacking B^S but carrying inverted FRTs. The acentric chromosome is not expected to segregate reliably. B. Overview of early spermatogenesis in the Drosophila melanogaster testis. Germline stem cells (GSC) at the apical tip divide asymmetrically to produce another stem cell and a primary spermatogonial cell, which becomes surrounded by two somatic cyst cells which do not divide further. A spermatogonial cell normally undergoes four rounds of mitosis followed by the two meiotic divisions to produce a cyst of 64 haploid spermatids. After meiosis the spermatids differentiate and elongate, followed by individualization and release of mature sperm into the seminal vesicle (not shown).