The study of mammalian meiosis is complicated by the timing of meiotic events in females and by the intermingling of meiotic sub-stages with somatic cells in the gonad of both sexes. In addition, studies of mouse mutants for different meiotic regulators have revealed significant differences in the stringency of meiotic events in males versus females. This sexual dimorphism implies that the processes of recombination and homologous chromosome pairing, while being controlled by similar genetic pathways, are subject to different levels of checkpoint control in males and females. This review is focused on the emerging picture of sexual dimorphism exhibited by mammalian germ cells using evidence from the broad range of meiotic mutants now available in the mouse. Many of these mouse mutants display distinct differences in meiotic progression and/or dysfunction in males versus females, and their continued study will allow us to understand the molecular basis for the sex-specific differences observed during prophase I progression.

Improper chromosome pairing, synapsis, and segregation impair meiotic progression in the absence of the BLM helicase in mammalian cells.

Events occurring during meiotic prophase I are critical for the successful production of haploid gametes. Many prophase I events are mediated by a meiosis-specific structure called the synaptonemal complex. To date, the limited knowledge we have about the dynamics of these prophase I events in mice comes from fixed, two-dimensional preparations of meiotic cells making it impossible to study the three-dimensional (3D) arrangement of meiotic chromosomes. The current study involves the development of an imaging system to view prophase I events in live mammalian spermatocytes by generating a transgenic mouse, Sycp3-Eyfp ( 21HC ), expressing a fluorescently tagged synaptonemal complex protein, SYCP3. Using this live imaging system, the 3D structural arrangement of chromosomes in the different prophase I substages has been characterized in live spermatocytes, and aspects of the 3D architecture of spermatocytes have been observed that would not be possible with existing techniques. Additionally, chromosome movement in prophase I spermatocytes and meiotic progression from pachynema to diplonema were observed following treatment with the phosphatase inhibitor, okadaic acid (OA), which accelerates the progression of cells through late prophase I. These studies demonstrate that the Sycp3-Eyfp ( 21HC ) live imaging system is a useful tool for the study of mammalian prophase I dynamics.