Chromosome Dynamics in Meiosis

Meiosis encompasses a large number of dynamic processes. Some of them are biochemical, such as formation and repair of meiotic double-strand breaks, while others are physical in nature, such as homologous chromosome segregation in anaphase I. Plants have been used as model species in meiosis studies for over 80 years. However, the past decade brought a dramatic improvement in the understanding of meiosis in plants at the mechanistic level, thanks to the adoption of genetic and molecular biology techniques in chromosome research and new microscopy methods. 1 Overview of Meiosis Meiosis consists of two consecutive nuclear divisions (Fig. 1), a reductional division (meiosis I) and an equational division (meiosis II), without an intervening S phase between them. While meiosis II is essentially similar to a mitotic division, meiosis I is a specialized division, whose aim is to reduce the number of chromosomes in the nucleus and allow exchange of genetic material between maternal and paternal chromosomes. Based largely on chromosome dynamics, meiosis I is subdivided into four stages, prophase I, metaphase I, anaphase I, and telophase I. Meiotic prophase I, the most eventful of these stages, is further subdivided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis (Fig. 2). During leptotene, which follows the pre-meiotic S phase, decondensed chromatin becomes organized into chromosomes by the assemblage of a proteinaceous core. Meiotic recombination is initiated at this step by formation of double-strand breaks (DSBs) in chromosomal DNA (Pawlowski et al. 2004; Zickler and Kleckner 1999). In zygotene, homologous chromosomes pair. Pairing is followed by synapsis, when the central element of the synaptonemal complex (SC) is installed between the paired homologs and stabilizes pairing interactions (Page and Hawley 2004). By pachytene, SC formation is complete and meiotic recombination between homologs is resolved. In diplotene, the SC disassembles and chiasmata are visible. Chiasmata are the sites of crossovers (COs) and are responsible for holding homologous chromosomes together until their segregation in anaphase. Finally, in diakinesis, the chromosomes undergo the final stage of condensation. 104 A. Ronceret et al. Fig. 1 A general overview of meiosis. Only one pair of chromosomes is shown and each homolog is a different shade of grey. Early and late recombination nodules are depicted as dots of different size. See Fig. 2 for a detailed list of prophase I substages and events Recombination, pairing and synapsis are three main processes of meiotic prophase I. Recombination encompasses formation and repair of meiotic DSBs, including reciprocal chromosome arm exchanges. Pairing includes interactions between chromosomes, which involve homology recognition and lead to juxtaposition of homologs. Pairing is followed by synapsis, which is defined as the process of installation of the central element (CE) of SC, which binds the paired chromosomes along their entire length. These three processes are formally distinct but genetic and molecular analyses are now drawing a more complex scenario where these processes are intimately interconnected and show a great deal of coordination (Pawlowski and Cande 2005). 2 Initiation of Meiosis in Plants The switch from the mitotic to the meiotic cell cycle in plants is preceded by a developmental pathway that assigns germ cell identity. First, archesporial Chromosome Dynamics in Meiosis 105 Fig. 2 Pre-meiotic interphase and meiotic prophase I in plants: substages, main events and key genes that regulate them. Only one pair of chromosomes is shown and each homolog is a different shade of grey. Early and late recombination nodules are depicted as dots of different size. Images on the left are chromosomes of maize male meiocytes stained with DAPI. The images are flat projections of three-dimensional image stacks collected with three-dimensional deconvolution microscopy cells are differentiated from hypodermal cells in male and female reproductive organs. Then, the archesporial cells develop into sporocytes, which show features distinct from other cells and are destined to undergo meiosis (Ma 2005). In contrast to the developmental events preceding the switch to meiosis, the initiation of meiosis in plants is less understood. The ameiotic1 (am1) gene has been identified as a master controller of the switch from mitotic to meiotic cell cycle in maize (Golubovskaya et al. 1997, 1993) (Pawlowski et al., unpublished). Pre-meiotic cells in most am1 mutants, instead of entering meiosis, undergo mitotic divisions. In severe cases, the progression of the cell cycle is arrested during interphase. All known specific aspects of meiosis, such as establishment of meiosis-specific chromatin structure, chromosome pairing, synapsis, recombination, and meiosis-specific chromosome dynamics require am1. In addition to its role in initiating meiosis, am1 regulates the progression through the early stages of meiotic prophase (Golubovskaya et al. 1993) (Pawlowski et al., unpublished). This conclusion is based on the analysis of an unusual am1 allele, am1-praI, in which pre-meiotic cells enter meiosis but arrest during meiotic prophase. The Arabidopsis homolog of am1, SWITCH1 (SWI1), also known as DYAD, has been shown to regulate several meiotic prophase processes, including es106 A. Ronceret et al. tablishment of meiotic chromosome structure, recombination and synapsis (Agashe et al. 2002; Mercier et al. 2001, 2003). However, the phenotypes of swi1 mutants are less obvious than those of the maize am1 mutants. Male meiocytes in swi1 mutants either undergo a normal meiosis or show meiotic sister-chromatid cohesion (SCC) defects. Female swi1meiocytes undergo an equational division. However, this division is an abnormal meiosis rather than mitosis, since the meiosis-specific cohesin REC8 is loaded onto chromosomes, and DMC1, a gene encoding a meiosis-specific recombinase, is expressed. None of the swi1mutations result in meiocytes undergoing a normal mitotic division, as is the case in am1mutants. This suggests that initiation at the mechanistic level differs between maize and Arabidopsis. Homologs of the AM1/SWI1/DYAD proteins are confined to plants. The molecular functions of AM1 and SWI1 are not known and the pathway downstream from these proteins that results in the transition from mitotic to meiotic cell cycle is poorly understood. The decision to enter meiosis is probably made before or at the beginning of the pre-meiotic S phase, although the evidence is mostly indirect. This timing is suggested by observations that female meiocytes in several am1 mutants arrest at interphase and that the Arabidopsis SWI1 shows expression exclusively during pre-meiotic G1 and S. Overall, the mechanisms of meiosis initiation in plants is likely to corroborate conclusions from yeast and mammals indicating that the signaling cascade leading to meiosis initiation shows great diversity among species while the timing of meiosis initiation is a universal feature shared by all eukaryotes (Pawlowski et al. 2007). 3 Regulation of Meiosis Progression Several candidates for meiotic cell cycle regulators have been identified in Arabidopsis based on their functions and/or similarity to known cyclins. These proteins, CDC45, SOLO DANCERS (SDS), and TARDY ANSYNCHRONOUS MEIOSIS (TAM), are proposed to act at different times in meiosis. CDC45 functions during pre-meiotic S-phase (Stevens et al. 2004). SDS regulates chromosome pairing and synapsis in prophase I (Azumi et al. 2002), although the molecular mechanism of its function is not known. TAM has been proposed to regulate progression of both, meiotic prophase I and meiosis II, and its absence also leads to meiotic nuclear division becoming asynchronous with cytokinesis (Wang et al. 2004). Chromosome Dynamics in Meiosis 107 4 Meiotic Chromosome Structure