Chromosome axis defects induce a checkpoint-mediated delay and interchromosomal effect on crossing over during Drosophila meiosis
Meiotic recombination occurs during prophase I when homologous chromosomes are synapsed along their entire length. Synapsis is defined as the close and stable association of homologous chromosomes through a proteinaceous structure called the synaptonemal complex (SC). In most organisms, this complex is composed of two main parts: lateral elements that attach along the axis of each homologous chromosome and transverse elements that span the central region of the SC and function to tether the homologs [1,2]. At the leptotene/zygotene stages of meiotic prophase, these structural proteins begin to load onto the chromosome axes, and are completely assembled at pachytene, when homologous chromosomes are synapsed along their entire length. Recombination between the homologous chromosomes initiates with DNA double-strand breaks (DSBs) that are repaired as either crossovers or noncrossovers [3–5]. Crossovers establish chromatin linkages called chiasmata, which, along with sister chromatid cohesion, hold homologs together after recombination has been completed and chromosomes have dissociated their SC proteins. Chiasmata help orient the homologous chromosomes on the metaphase I spindle and ensure their proper segregation at anaphase I. The failure to establish a crossover/chiasma can result in the nondisjunction of homologs and lead to aneuploid gametes.