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 ho-
molog is a different shade of grey. Early and late recombination nodules are depicted
as dots of different size.Images on the leftare 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 reproduc-
tive 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. Theameiotic1(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et al., unpublished).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 mostam1mutants, instead of enter-
ing 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 dynam-
ics requiream1. In addition to its role in initiating meiosis,am1regulates
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et al. 1993) (Pawlowski et al., unpublished).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 unusualam1allele,am1-praI, in which pre-meiotic cells enter
meiosis but arrest during meiotic prophase.
TheArabidopsishomolog ofam1,SWITCH1(SWI1), also known asDYAD,
has been shown to regulate several meiotic prophase processes, including es-