A Highlights from MBoC Selection: Aurora B–mediated localized delays in nuclear envelope formation facilitate inclusion of late-segregating chromosome fragments

To determine how chromosome segregation is coordinated with nuclear envelope formation (NEF), we examined the dynamics of NEF in the presence of lagging acentric chromosomes in Drosophila neuroblasts. Acentric chromosomes often exhibit delayed but ultimately successful segregation and incorporation into daughter nuclei. However, it is unknown whether these late-segregating acentric fragments influence NEF to ensure their inclusion in daughter nuclei. Through live analysis, we show that acentric chromosomes induce highly localized delays in the reassembly of the nuclear envelope. These delays result in a gap in the nuclear envelope that facilitates the inclusion of lagging acentrics into telophase daughter nuclei. Localized delays of nuclear envelope reassembly require Aurora B kinase activity. In cells with reduced Aurora B activity, there is a decrease in the frequency of local nuclear envelope reassembly delays, resulting in an increase in the frequency of acentric-bearing, lamin-coated micronuclei. These studies reveal a novel role of Aurora B in maintaining genomic integrity by promoting the formation of a passageway in the nuclear envelope through which late-segregating acentric chromosomes enter the telophase daughter nucleus.     

Information for targeting to the chloroplastic inner envelope membrane is contained in the mature region of the maize Bt1-encoded protein.

Based on the protein sequence deduced from a cDNA clone, it has been proposed that the maize bt1 locus encodes an amyloplast membrane metabolite translocator protein (Sullivan, T. D., Strelow, L. I., Illingworth, C. A., Phillips, R. L., and Nelson, O. E., Jr. (1991) Plant Cell 3, 1337-1348). The present work provides further evidence for this hypothesis by showing that the gene product of Bt1 could be imported into chloroplasts in vitro and processed to lower molecular weight mature proteins. More importantly, the imported mature proteins were localized to the inner envelope membrane, where metabolite translocators are located in plastids. In addition, the location of information for targeting to the inner membrane was investigated by constructing and analyzing the import of chimeric precursor proteins. A chimeric protein with the transit peptide of the precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase fused to the mature region of the Bt1-encoded protein was targeted to the inner envelope membrane of chloroplasts. Moreover, a chimeric protein with the transit peptide of the Bt1-encoded protein fused to the mature protein of the light-harvesting chlorophyll a/b binding protein was targeted to the thylakoid. These results indicate that the transit peptide of the Bt1-encoded protein functions primarily as a stromal targeting sequence. The information for targeting to the chloroplastic inner envelope membrane is contained in the mature region of the protein.     

Acute and chronic effects of an aromatase inhibitor on territorial aggression in breeding and nonbreeding male song sparrows

Many studies have demonstrated that male aggression is regulated by testosterone. The conversion of testosterone to estradiol by brain aromatase is also known to regulate male aggression in the breeding season. Male song sparrows (Melospiza melodia morphna) are territorial not only in the breeding season, but also in the nonbreeding season, when plasma testosterone and estradiol levels are basal. Castration has no effect on nonbreeding aggression. In contrast, chronic (10 day) aromatase inhibitor (fadrozole) treatment decreases nonbreeding aggression, indicating a role for estrogens. Here, we show that acute (1 day) fadrozole treatment decreases nonbreeding territoriality, suggesting relatively rapid estrogen effects. In spring, fadrozole decreases brain aromatase activity, but acute and chronic fadrozole treatments do not significantly decrease aggression, although trends for some behaviors approach significance. In gonadally intact birds, fadrozole may be less effective at reducing aggression in the spring. This might occur because fadrozole causes a large increase in plasma testosterone in intact breeding males. Alternatively, estradiol may be more important for territoriality in winter than spring. We hypothesize that sex steroids regulate male aggression in spring and winter, but the endocrine mechanisms vary seasonally.     

Antennular sensilla of the brine shrimp, Artemia salina.

1. Scanning electron microscopy was used to characterize the external morphology of setae found on the antennules of adults and nauplii of the brine shrimp, Artemia salina (L.). The permeability of the antennular setae was studied by means of Slifer's crystal violet method. 2. Each antennule of an adult brine shrimp possessed a terminal cluster of sensory setae. Within a cluster there were two morphologically distinct kinds of sensilla, here designated type 1 and type 2. Three type 1 sensilla were observed on every antennule examined. The number of type 2 sensilla per antennule was usually four or five. 3. Type 1 sensilla of adults were 43 to 80 micrometer long and simple in external morphology. They were widest at the base, decreased in diameter gradually, and terminated as a finely tapered tip. No pores were resolved by scanning electron microscopy. 4. Type 2 sensilla of adults were shorter (shaft length, 12 to 23 micrometer) and displayed a single pore at the tip (average pore diameter, 0.4 micrometer). In thin section they were seen to possess a distinctive articular specialization of the cuticle at the base of the seta. 5. Dye penetration experiments indicated that type 2 sensilla were permeable to aqueous crystal violet, whereas type 1 sensilla were not. 6. The antennular setae of nauplii resembled type 1 sensilla in general shape, in being impermeable to crystal violet, and in lacking a terminal pore and basal articular specialization. Moreover, a total of three setae was normally present on each naupliar antennule, and the same number of type 1 sensilla was found on each adult antennule examined. If the three naupliar setae represent a developmental stage in the formation of three adult sensilla, available observations suggest that the larval setae are developmentally related to type 1, rather than to type 2 adult sensilla.