We address the relative roles of astral and central spindle microtubules (MTs) in cytokinesis of Drosophila melanogaster primary spermatocytes. Time-lapse imaging studies reveal that the central spindle is comprised of two MT populations, “interior” central spindle MTs found within the spindle envelope and “peripheral” astral MTs that probe the cytoplasm and initiate cleavage furrows where they contact the cortex and form overlapping bundles. The MT-associated protein Orbit/Mast/CLASP concentrates on interior rather than peripheral central spindle MTs. Interior MTs are preferentially affected in hypomorphic orbit mutants, and consequently the interior central spindle fails to form or is unstable. In contrast, peripheral MTs still probe the cortex and form regions of overlap that recruit the Pav-KLP motor and Aurora B kinase. orbit mutants have disorganized or incomplete anillin and actin rings, and although cleavage furrows initiate, they ultimately regress. Our work identifies a new function for Orbit/Mast/CLASP and identifies a novel MT population involved in cleavage furrow initiation.

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Magnetic superlattice peaks are observed in single-crystal neutron-diffraction measurements on orthorhombic La 1.88 Sr 0.12 CuO 4 at reciprocal points of ͑1/2Ϯ⑀,1/2,0͒ and ͑1/2,1/2Ϯ⑀,0͒ in the tetragonal notation where ⑀ϭ0.126Ϯ0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below ϳ45 K corresponding to the existence of magnetic order. The remarkable softening of longitudinal sound waves along ͓110͔ is observed in the same crystal. The features observed in the neutron diffraction, NMR, and ultrasonic measurements suggest that the dynamical incommensurate spin correlation is pinned by a lattice instability toward the low-temperature tetragonal phase. ͓S0163-1829͑98͒50706-0͔ RAPID COMMUNICATIONS R323057 T. SUZUKI et al.

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Cluster-spin glass and ferroelectric relaxors have been observed in defectcontaining ferromagnetic systems and ferroelectric systems, respectively. However, it is unclear whether or not an analogous glass state exists in the physically parallel ferroelastic (or martensitic) systems. In the 1990s, theoretical studies suggested that premartensitic tweed could be viewed as a strain glass. However, there has been no experimental verification of this hypothesis. In this paper, we provide an experimental test of this hypothesis by measuring the possible glass signatures in two well-known premartensitic tweed systems prior to their martensitic transformation: one Ni 63 Al 37 and the other Ti 50 Ni 47 Fe 3 martensitic alloy. Our experiments show that no glass signatures exist for the premartensitic tweed in both systems. There is no mechanical susceptibility/modulus anomaly in the tweed temperature regime, suggesting no glass transition exists. The tweed remains ergodic, inconsistent with a frozen glass. These two critical experiments show that premartensitic tweed is not a frozen glass state. We demonstrate that strain glass exists in ferroelastic/martensitic systems but only in defect-containing ferroelastic/martensitic systems with defect concentration exceeding a critical value. This strain glass is a mechanical analogue of cluster-spin glass or ferroelectric relaxors, and possesses all the features of a glass. We further show that the tweed is equivalent to an 'unfrozen state' of a strain glass. Finally, we demonstrate that the microscopic origin of the strain glass can be easily understood in analogy with the behavior of a 'defect-containing domino array'.