No abstract

The recombination nodule is a transient structure present during pachytene in intimate association with the synaptonemal complex. The total numbers of these structures per nucleus and their locations along the bivalents correspond to the total numbers and distributions of genetic exchanges. It is suggested that this structure may be involved in the recombination process. The mechanism of recombination in eukaryotes has been the subject of considerable speculation by both geneticists (in terms of crossing-over) and cytologists (in terms of chiasma formation) for over 70 years. Classical investigations of the mechanism of recombination have been hampered by the fact that only the end products of recombination can be studied (crossover chromosomes in the progeny or chiasma configurations at diplotene-diakinesis). The development of the technique of electron microscopy has permitted the fruitful observation of the fine structure of meiotic stages before chiasmata become visible, in particular of pachytene, the stage during which crossing-over is generally thought to occur (1). The most striking observation derived from electron microscopic studies of pachytene is that of the presence of the synaptonemal complex (SC), a structure which lies between the synapsed homologues (2).In most organisms, the presence of the SC appears to be necessary, but not sufficient, to insure normal meiotic recombination (3-5). Moreover, observations of structural differences of the SC at pachytene between relatively long portions of the bivalent chromosomes have been correlated with the presence or absence of recombination in such portions (6-9). For example, the SC of the Drosophila melanogaster X chromosome shows longitudinal differentiation in that the distal portion associated with the euchromatic segment is morphologically different from the proximal portion associated with the heterochromatic segment. These differences correlate with the presence and absence, respectively, of meiotic crossing-over in those segments. However, although the SC of the euchromatic portion of the bivalent is quite long [up to 20 Im long (9) by 0.1 ,um wide], there are only an average of 1.32 exchanges per meiosis (66 total map units for the X/50 map units per exchange). Thus, whereas such longitudinal differentiations may determine (or reflect) restrictions affecting the portion of a bivalent in which recombination may occur, additional structures, structural modifications, and/or processes would seem to be necessary both to determine the precise site of recombination and to effect the recombinational event itself.This report describes a structure, the recombination nodule, which is present at pachytene in Drosophila melanogaster females in intimate association with the SC. The agreement of the total numbers of nodules per nucleus and their locations along the bivalents with the total numbers and distributions of genetic exchanges suggests that this structure may, in some manner, be involved in the recombinational process. MATERIALS AND METHODSD...

Polo is a conserved kinase that coordinates many events of mitosis and meiosis, but how it is regulated remains unclear. Drosophila females having only one wild-type allele of the polo kinase gene and the dominant Scant mutation produce embryos in which one of the centrosomes detaches from the nuclear envelope in late prophase. We show that Scant creates a hyperactive form of Greatwall (Gwl) with altered specificity in vitro, another protein kinase recently implicated in mitotic entry in Drosophila and Xenopus. Excess Gwl activity in embryos causes developmental failure that can be rescued by increasing maternal Polo dosage, indicating that coordination between the two mitotic kinases is crucial for mitotic progression. Revertant alleles of Scant that restore fertility to polo–Scant heterozygous females are recessive alleles or deficiencies of gwl; they show chromatin condensation defects and anaphase bridges in larval neuroblasts. One recessive mutant allele specifically disrupts a Gwl isoform strongly expressed during vitellogenesis. Females hemizygous for this allele are sterile, and their oocytes fail to arrest in metaphase I of meiosis; both homologues and sister chromatids separate on elongated meiotic spindles with little or no segregation. This allelic series of gwl mutants highlights the multiple roles of Gwl in both mitotic and meiotic progression. Our results indicate that Gwl activity antagonizes Polo and thus identify an important regulatory interaction of the cell cycle.

SummaryThe nature of the relationship between the two types 05 meiotic recombination outcomes, exchange (crossing-over) and simple gene conversion, has been debated for years. Z here propose that these two types of events are not necessarily causally related and hypothesize that the primary role of events detected as simple gene conversion is in the recognition of homology during synapsis.

Protein phosphatase 2A (PP2A) plays a major role in dephosphorylating the targets of the major mitotic kinase Cdk1 at mitotic exit, yet how it is regulated in mitotic progression is poorly understood. Here we show that mutations in either the catalytic or regulatory twins/B55 subunit of PP2A act as enhancers of gwlScant, a gain-of-function allele of the Greatwall kinase gene that leads to embryonic lethality in Drosophila when the maternal dosage of the mitotic kinase Polo is reduced. We also show that heterozygous mutant endos alleles suppress heterozygous gwlScant; many more embryos survive. Furthermore, heterozygous PP2A mutations make females heterozygous for the strong mutation polo11 partially sterile, even in the absence of gwlScant. Heterozygosity for an endos mutation suppresses this PP2A/polo11 sterility. Homozygous mutation or knockdown of endos leads to phenotypes suggestive of defects in maintaining the mitotic state. In accord with the genetic interactions shown by the gwlScant dominant mutant, the mitotic defects of Endos knockdown in cultured cells can be suppressed by knockdown of either the catalytic or the Twins/B55 regulatory subunits of PP2A but not by the other three regulatory B subunits of Drosophila PP2A. Greatwall phosphorylates Endos at a single site, Ser68, and this is essential for Endos function. Together these interactions suggest that Greatwall and Endos act to promote the inactivation of PP2A-Twins/B55 in Drosophila. We discuss the involvement of Polo kinase in such a regulatory loop.