The process of meiosis involves two rounds of chromosome segregation that follow a single round of chromosome duplication leading to the production of haploid gametes. Accurate segregation of chromosomes during meiosis is essential for the long-term survival

Meiosis is a central feature in the reproductive program of plants and all other sexually reproducing eukaryotes. In plants such as Arabidopsis thaliana, reproduction initiates during the development of the male (anther) and female (ovule) reproductive structures from somatic tissue. During anther development, a group of cells differentiate to form archesporial cells, which in turn give rise to the primary sporogenous cells. These then differentiate into pollen mother cells, the male meiocytes in which meiosis occurs, and primary parietal cells, which give rise to the tapetum, endothecium and middle layer of the anther (Yang et al., 1999). In A. thaliana each anther locule contains around 30 meiocytes that enter and proceed through meiosis with a high degree of synchrony to produce tetrads of haploid microspores (Armstrong et al., 2001). These then mature to form pollen grains, the male gametophytes. During female reproductive development in the ovule, an archesporial cell at the top of the ovule primordium differentiates from a single hypodermal cell. This cell then differentiates forming the megaspore mother cell in which meiosis occurs, leading to the formation of four haploid megaspores. The three megaspores closest to the micropyle of the ovule then undergo programmed cell death, whilst the remaining chalazal megaspore develops into the female gametophyte (Yang et al., 1999). The process of meiosis involves two rounds of chromosome segregation that follow a single round of chromosome duplication leading to the production of haploid gametes. Accurate segregation of chromosomes during meiosis is essential for the long-term survival of individual species, since any error may produce infertility or aneuploid offspring. During meiotic prophase I several crucial events contribute to and determine the subsequent correct partition of genetic material. One of these is the juxtaposition of homologous chromosomes during early prophase I to form bivalents. This process commences at leptotene or in some cases meiotic interphase with the alignment of homologous chromosomes, a phenomenon called pairing (review by Zickler and Kleckner, 1998). During leptotene, each chromosome develops a linear proteinaceous structure called an axial element (AE). In the following zygotene stage, the homologues closely associate via the polymerization of a central element between the two homologous AEs, which are then referred to as lateral elements. This tripartite structure forms the synaptonemal complex (SC). The polymerization of the SC, or synapsis, continues throughout zygotene until pachytene at which stage it is complete. The SC is then disassembled during diplotene (reviewed by Heyting, 1996; Roeder, 1997; Zickler and Kleckner, 1999). To ensure accurate chromosome segregation at anaphase I 3309 Development 130, 3309-3318 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00550