Molecular mechanisms that initiate meiosis have been studied in fungi and mammals, but little is known about the mechanisms directing the meiosis transition in other organisms. To elucidate meiosis initiation in plants, we characterized and cloned the ameiotic1 (am1) gene, which affects the transition to meiosis and progression through the early stages of meiotic prophase in maize. We demonstrate that all meiotic processes require am1, including expression of meiosis-specific genes, establishment of the meiotic chromosome structure, meiosis-specific telomere behavior, meiotic recombination, pairing, synapsis, and installation of the meiosis-specific cytoskeleton. As a result, in most am1 mutants premeiotic cells enter mitosis instead of meiosis. Unlike the genes involved in initiating meiosis in yeast and mouse, am1 also has a second downstream function, whereby it regulates the transition through a novel leptotene-zygotene checkpoint, a key step in early meiotic prophase. The am1 gene encodes a plant-specific protein with an unknown biochemical function. The AM1 protein is diffuse in the nucleus during the initiation of meiosis and then binds to chromatin in early meiotic prophase I when it regulates the leptotene-zygotene progression.chromosomes ͉ plant development ͉ genetics ͉ recombination

Recombination is a crucial component of evolution and breeding, producing new genetic combinations on which selection can act. Rates of recombination vary tremendously, not only between species but also within species and for specific chromosomal segments. In this study, by examining recombination events captured in recombinant inbred mapping populations previously created for maize, wheat, Arabidopsis, and mouse, we demonstrate that substantial variation exists for genomewide crossover rates in both outcrossed and inbred plant and animal species. We also identify quantitative trait loci (QTL) that control this variation. The method that we developed and employed here holds promise for elucidating factors that regulate meiotic recombination and for creation of hyperrecombinogenic lines, which can help overcome limited recombination that hampers breeding progress.A LTHOUGH natural selection is a powerful evolutionary process, it utilizes only the existing variation present in a population. Recombination of alleles is required to efficiently evolve new genetic varieties. Not surprisingly, theoretical predictions (Otto and Michalakis 1998) and empirical studies (Saleem et al. 2001) indicate that populations experiencing directional or strong selection pressures evolve increased recombination rates. Similarly to the natural evolutionary processes, combining many positive alleles into a single germplasm is the main objective of plant and animal breeding. The stacking of the favorable alleles is limited by the time and the number of meioses required to recombine numerous alleles from multiple parents. Consequently, a better understanding of the factors controlling recombination holds numerous implications for both academic and applied realms.To date, many of the genes involved in meiotic recombination have been identified and the mechanistic basis of recombination have begun to emerge (Krogh and Symington 2004;Cohen et al. 2006;Li and Ma 2006). However, the mechanisms that regulate recombination are poorly understood. Particularly, little is known about the control of genomewide recombination rates. Variation in recombination rates has been documented both within and between species, as well as between particular chromosomal regions (Rees 1961;Säll 1990;Beavis and Grant 1991;Tulsieram et al. 1992;Fatmi et al. 1993;Korol et al. 1994;Williams et al. 1995;Sanchez-Moran et al. 2002;Anderson et al. 2003;De Massy 2003;Myers et al. 2005; YandeauNelson et al. 2006). A minimum of one obligatory crossover per chromosome, or chromosome arm, occurs during meiosis as a requirement for proper chromosome segregation (Pardo-Manuel De Villena and Sapienza 2001). However, factors that control whether just this one or multiple crossovers occur per chromosome are poorly understood. Even though the idea that recombination frequencies can be genetically dissected, as any other quantitative trait, was first proposed long ago (Rasmusson 1927), to our knowledge, no quantitative trait loci (QTL) affecting recombination rates have been ...