Sexual reproduction predominates among eukaryotic organisms on our planet. While debate continues over why this should be so, burgeoning genomic and functional information now allows us to begin to think reasonably about some of the events that may have occurred to make sex possible in the first place.

A central event in sexual reproduction is the reduction in chromosome number that occurs at the meiosis I division. Most eukaryotes rely on crossing over between homologs, and the resulting chiasmata, to direct meiosis I chromosome segregation, yet make very few crossovers per chromosome pair. This indicates that meiotic recombination must be tightly regulated to ensure that each chromosome pair enjoys the crossover necessary to ensure correct segregation. Here, we investigate control of meiotic crossing over in Caenorhabditis elegans, which averages only one crossover per chromosome pair per meiosis, by constructing genetic maps of end-to-end fusions of whole chromosomes. Fusion of chromosomes removes the requirement for a crossover in each component chromosome segment and thereby reveals a propensity to restrict the number of crossovers such that pairs of fusion chromosomes composed of two or even three whole chromosomes enjoy but a single crossover in the majority of meioses. This regulation can operate over physical distances encompassing half the genome. The meiotic behavior of heterozygous fusion chromosomes further suggests that continuous meiotic chromosome axes, or structures that depend on properly assembled axes, may be important for crossover regulation.

Sexual reproduction requires the production of haploid gametes (sperm and egg) with only one copy of each chromosome; fertilization then restores the diploid chromosome content in the next generation. This reduction in genetic content is accomplished during a specialized cell division called meiosis, in which two rounds of chromosome segregation follow a single round of DNA replication. In preparation for the first meiotic division, homologous chromosomes pair and synapse, creating a context that promotes formation of crossover recombination events. These crossovers, in conjunction with sister chromatid cohesion, serve to connect the two homologs and facilitate their segregation to opposite poles during the first meiotic division. During the second meiotic division, which is similar to mitosis, sister chromatids separate; the resultant products are haploid cells that become gametes. In C. elegans (and most other eukaryotes) homologous pairing and recombination are required for proper chromosome inheritance during meiosis; accordingly, the events of meiosis are tightly coordinated to ensure the proper execution of these events. In this chapter, we review the seminal events of meiosis: pairing of homologous chromosomes; the changes in chromosome structure that chromosomes undergo during meiosis; the events of meiotic recombination; the differentiation of homologous chromosome pairs into structures optimized for proper chromosome segregation at Meiosis I; and the ultimate segregation of chromosomes during the meiotic divisions. We also review the regulatory processes that ensure the coordinated execution of these meiotic events during prophase I.

Most sexually reproducing organisms depend on the regulated formation of crossovers, and the consequent chiasmata, to accomplish successful segregation of homologous chromosomes at the meiosis I division. A robust, chromosome-wide crossover control system limits chromosome pairs to one crossover in most meioses in the nematode Caenorhabditis elegans ; this system has been proposed to rely on structural integrity of meiotic chromosome axes. Here, we test this hypothesis using a mutant, him-3(me80), that assembles reduced levels of meiosis-specific axis component HIM-3 along cohesin-containing chromosome axes. Whereas pairing, synapsis, and crossing over are eliminated when HIM-3 is absent, the him-3(me80) mutant supports assembly of synaptonemal complex protein SYP-1 along some paired chromosomes, resulting in partial competence for chiasma formation. We present both genetic and cytological evidence indicating that the him-3(me80) mutation leads to an increased incidence of meiotic products with two crossovers. These results indicate that limiting the amount of a major axis component results in a reduced capacity to communicate the presence of a (nascent) crossover and/or to discourage others in response.

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