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Fig. 9. Scheme showing the sequential steps of centromere and telomere redistribution that occur during transit from vegetative nuclear architecture into meiotic prophase. It is based on observations made in this and earlier work (see text). In the premeiotic nucleus, telomeres (red) form several perinuclear clusters and all centromeres (blue) locate tightly near the spindle pole body (SPB). Induction of meiosis leads to the dissolution of the centromere cluster and perinuclear telomere clusters. The dispersion of centromeres and telomeres differ relative to their dependence on premeiotic S phase. Telomeres cluster transiently near the SPB (bouquet stage) before scattering again over the periphery of the nucleus. The steps where centromere and telomere behavior are affected by the loss of Clb5 and Set1 are indicated by blue and red T-bars, respectively. The absence of Clb5 affects only the movements of telomeres with a severe impairment of vegetative telomere cluster dissolution, a delayed occurrence of meiotic telomere clustering and a persistence of meiotic telomere clustering. The lack of Set1, on the other hand, induces defects in centromere and telomere dispersion from their respective vegetative clusters and blocks meiotic telomere cluster formation. Accordingly, the thin T-bars represent a leaky effect which can be bypassed by some cells to reach the next stage of nuclear topology, whereas the bold T-bars represent a block that induces a cell to remain with a topological pattern of the previous stage, while prophase I may still be progressing. Note that the relative duration of the transitions is not drawn to scale.
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. Frequencies of nuclei displaying a single centromere FISH signal (% cen cluster), of nuclei with two to eight perinuclear vegetative telomere FISH signals (% veg tel cluster) or with a single telomere signal (% bouquet) in wild-type and clb5
