Meiotic segregation of compound-3 chromosomes in Drosophila

Analysis of crossos between genetically marked stocks of Drosophila melanogaster showed, that the compound-3 chromosomes C(3L)RM and C(3R)RM segregate preterentially in female meiosis, and the following two types of eggs are formed predominantly: C(3L)RM; 0 and 0; C(3R)RM. In male meiosis segregation is almost random and four types of sperm are formed: 1. C(3L)RM; C(3R)RM, 2. 0; 0, 3. C(3L)RM; 0, 4. 0; C(3R)RM. The frequencies of these sperm types vary with the genotypes tested. In the stock C(3L)RM, st; C(3R)RM, p ^p, males produce 76.8% type 1 and 2, and 23.2% type 3 and 4; males of the stock C(3L)RM, ri; C(3R)RM, sr form 63.2% type 1 and 2, and 36.8% type 3 and 4.The segregational behaviour of compound-3 chromosomes found in female meiosis is expected according to the distributive pairing hypothesis. In the male however, where there is no distributive pairing, the stock-specific segregation of compound-3 chromosomes may be due to the presence of small homologous chromosome segments near the centromere which influence chromosome distribution.     

Congression of achiasmate chromosomes to the metaphase plate in Drosophila melanogaster oocytes

Chiasmata established by recombination are normally sufficient to ensure accurate chromosome segregation during meiosis by physically interlocking homologs until anaphase I. Drosophila melanogaster female meiosis is unusual in that it is both exceptionally tolerant of nonexchange chromosomes and competent in ensuring their proper segregation. As first noted by Puro and Nokkala [Puro, J., Nokkala, S., 1977. Meiotic segregation of chromosomes in Drosophila melanogaster oocytes. A cytological approach. Chromosoma 63, 273-286], nonexchange chromosomes move precociously towards the poles following formation of a bipolar spindle. Indeed, metaphase arrest has been previously defined as the stage at which nonexchange homologs are symmetrically positioned between the main chromosome mass and the poles of the spindle. Here we use studies of both fixed images and living oocytes to show that the stage in which achiasmate chromosomes are separated from the main mass does not in fact define metaphase arrest, but rather is a component of an extended prometaphase. At the end of prometaphase, the nonexchange chromosomes retract into the main chromosome mass, which is tightly repackaged with properly co-oriented centromeres. This repackaged state is the true metaphase arrest configuration in Drosophila female meiosis.     

Heterologous interchange and nondisjunction of distributively paired chromosomes in Drosophila melanogaster mature oocytes

In the mature oocyte C(1)-4 and Y-4 interchanges do not direct disjunction of the centromeres of the involved heterologs. C(1)-4 detachments are recovered in either sex and almost every time ($\text{13}\text{14}$) with a free fourth chromosome that is the sister of the involved four. Y-4 fragments are recovered preferentially in males ($\text{28}\text{28}$) and with a free, sister four ($\text{26}\text{28}$). In the mature oocyte segregation patterns are determined and C(1)-4 and Y-4 interchange usually involve heterologs segragating to the same pole.     

Differential mechanisms governing segregation of a univalent in oocytes and spermatocytes of Drosophila melanogaster

In tricomplex heterozygotes in Drosophila melanogaster three metacentric autosomes (the TRI chromosomes) appear as a trivalent in meiosis while one autosome consisting of two homologous arms attached to the same centromere (a compound) behaves as an obligatory univalent. Cytological analysis of meiosis of tricomplex heterozygotes indicates that in oocytes the univalent compound behaves non-independently in relation to segregation of the trivalent. The compound is distributed preferentially to the same pole as one TRI chromosome. In spermatocytes the compound is distributed at random. In some oocytes the directed segregation is shown to be due to a disjunctional interaction between the compound and one partner of the trivalent at the same time as the other two chromosomes of the trivalent are separating from each other. The basic difference between the segregational mechanisms in the two sexes is discussed with a review of evidence indicating that in males segregation is determined by physical linkage that produces a stable orientation of the homologues at metaphase I. On the other hand, both genetic and cytological evidence indicate that in females a physical linkage (a chiasma) is non-essential for maintenance of co-orientation and stability after the onset of prometaphase. Genetic and cytological evidence support the hypothesis that disjunction is predetermined by non-random arrangement of the centromeric regions of chromosomes in the chromocentre — a suprachromosomal organization characteristic of maturing oocytes.by D. Schweizer     

Heterologous interchange and non-disjunction of distributively paired chromosomes in Drosophila melanogaster: immature oocytes

The relationships between interchange-mediated disjunction and segregation of distributively paired chromosomes have been analyzed. Even when an interchange generates a quasi-bivalent, one component of which is either the compound-X or the Y chromosome, the uninvolved sex chromosome disjoins from its regular disjunctive partner more often than not. Interchange between distributively paired heterologs does not remove these chromosomes from the distributive pool, a consequence of which would be regular disjunction of those elements remaining in the distributive pool.     

The role of the chromocenter in nonrandom meiotic segregation of nonhomologous chromosomes in Drosophila melanogaster females

The evidence supporting universal significance of physical links between pericentromeric regions of homologous chromosomes for their bipolar orientation during the first meiotic division is discussed. The pericentromeric chiasmata between homologs or (in the absence of the latter) chromocentric links between nonhomologs, which are preserved until prometaphase, compensate for the disturbed binding between homologous pericentromeric regions in both structural or locus mutants. When the links between nonhomologs are involved, interchromosomal effects on chromosome disjunction and nonhomologous pairing were revealed by the genetic methods. An explanation suggested for genetic events observed during Drosophila meiosis conforms with the original, cytogenetically proved model of the orderly two-ring chromocenter formation and reorganization.     

Linkage and segregation distortion in Drosophila melanogaster

Segregation distortion is caused by a group of genetic elements in and near the centric heterochromatin of chromosome 2 of Drosophila melanogaster. These elements promote their preferential recovery in heterozygous males by rendering sperm bearing the homologous chromosome dysfunctional. Previous work has shown that numerous Y-autosome translocations are associated with the suppression of the segregation distorter phenotype. The present study examined the effects of translocations between the major autosomes upon the expression of segregation distortion. Autosomal translocations involving either the segregation distorter chromosome or its sensitive homologue had no significant effect upon the expression of segregation distortion. These results argue that linkage arrangement per se may not have a major effect on segregation distortion. The suppression of SD by specific Y-autosomal translocations may be due to the disruption of elements on the Y chromosome that are important for the expression of SD.     

GFP-tagged balancer chromosomes for Drosophila melanogaster

We constructed green fluorescent protein (GFP)-expressing balancer chromosomes for each of the three major chromosomes of Drosophila melanogaster. Expression of GFP in these chromosomes is driven indirectly by a Kruppel (Kr) promoter, via the yeast GAL4-UAS regulatory system. GFP fluorescence can be seen in embryos as early as the germ band extension stage, and can also be seen in larvae, pupae, and adults. We show the patterns of GFP expression of these balancers and demonstrate the use of the balancers to identify homozygous progeny.     

Elementary structures in polytene chromosomes of Drosophila melanogaster

Whole-mounted polytene chromosomes were isolated from nuclei by microdissection in 60% acetic acid and analyzed by electron microscopy. Elementary chromosome fibers in the interchromomeric regions and individual chromomeres can be distinguished in polytene chromosomes at low levels of polyteny (2⁶–2⁷ chromatids). Elementary fibers in the interbands are oriented parallel to the axis of the polytene chromosome. Their number roughly corresponds to the expected level of polyteny. These fibers have an irregular beaded structure, 100–300 Å in diameter, and there is no apparent lateral association between them in the interchromomeric regions. Most bands, in contrast, form continuous structures crossing the entire width of the chromosome. Polytene chromosomes isolated in 2% or 10% acetic acid can be reversibly dispersed in a solution for chromatin spreading. The spread chromosomes consist of long uniform deoxyribonucleoprotein (DNP) fibers with a nucleosome structure. This supports the notion that continuous DNA molecules extend through the entire length of a polytene chromosome and that the nucleosome structure exists both in bands and interbands. Analysis of the band shape and of the fibrillar pattern in the interbands emphasizes that the polytene chromosome assumes a ribbonlike structure from which the more complex three-dimensional structure of the polytene chromosome at higher levels of polyteny develops.     

No multistrandedness in mitotic chromosomes of Drosophila melanogaster

Feulgen cytophotometric measurements of neuroblasts in the first and third instar larvae of Drosophila melanogaster reveal the same DNA content for metaphases with chromosomes of different size. The total absorbance of all measured metaphases gives the four-fold value of that of the spermatids. Accordingly there seem to be no reasons to retain the assumption of a multistranded structure for the large chromosomes of metaphases in the third instar larvae.     

A Germline Clone Screen for Meiotic Mutants in Drosophila melanogaster

Using a FLP/FRT-based method to create germline clones, we screened Drosophila chromosome arms 2L and 3R for new female meiotic mutants. The screen was designed to recover mutants with severe effects on meiotic exchange and/or segregation. This screen yielded 11 new mutants, including six alleles of previously known meiotic genes (c(2)M and ald/mps1). The remaining five mutants appear to define at least four new genes whose ablation results in severe meiotic defects. Three of the novel meiotic mutants were identified at the molecular level. Two of these, mcm5A7 and tremF9, define roles in meiotic recombination, while a third, conaA12, is important for synaptonemal complex assembly. Surprisingly, five of the nine mutants for which the lesion has been identified at the molecular level are not the result of mutations characteristic of EMS mutagenesis, but rather due to the insertion of the transposable element Doc. This study demonstrates the utility of germline clone-based screens for the discovery of strong meiotic mutants, including mutations in essential genes, and the use of molecular genetic techniques to map the loci.     

Non-Mendelian segregation of sex chromosomes in heterospecific Drosophila males

Interspecific hybrids and backcrossed organisms generally suffer from reduced viability and/or fertility. To identify and genetically map these defects, we introgressed regions of the Drosophila sechellia genome into the D. simulans genome. A female-biased sex ratio was observed in 24 of the 221 recombinant inbred lines, and subsequent tests attributed the skew to failure of Y-bearing sperm to fertilize the eggs. Apparently these introgressed lines fail to suppress a normally silent meiotic drive system. Using molecular markers we mapped two regions of the Drosophila genome that appear to exhibit differences between D. simulans and D. sechellia in their regulation of sex chromosome segregation distortion. The data indicate that the sex ratio phenotype results from an epistatic interaction between at least two factors. We discuss whether this observation is relevant to the meiotic drive theory of hybrid male sterility.     

The segregation distortion (SD) phenomenon in wild populations of Drosophila melanogaster: interaction between chromosomes 3 and SD chromosomes 2

The Segregation Distortion (SD) phenomenon is a typical case of non-Mendelian segregation in Drosophila melanogaster, due to the dysfunction of sperm bearing a non-SD homologous chromosome. In nature, several factors involved in the expression of the SD phenomenon have been described; among these, a genetic modifier carried by chromosome 3, which enhances the distortion effect of the SD chromosomes. The analysis of natural Sardinian populations, carried out in order to evaluate the presence of chromosome 3 bearing these enhancer factors, has enabled us to ascertain that (a) also in these populations chromosomes 3 with enhancer factors are present, although with frequencies lower than those previously reported in other publications; (b) among these enhancer chromosomes 3, some increase the k of certain chromosomes 2 from values of chromosomes considered non-distorting (k0.66) to values typical of SD chromosomes. The data obtained also allow us to put forward some considerations regarding the dynamics of the SD phenomenon in Sardinian populations, where the frequency of SD chromosomes is fairly elevated.     

GFP-tagged balancer chromosomes for Drosophila melanogaster.

We constructed green fluorescent protein (GFP)-expressing balancer chromosomes for each of the three major chromosomes of Drosophila melanogaster. Expression of GFP in these chromosomes is driven indirectly by a Kruppel (Kr) promoter, via the yeast GAL4-UAS regulatory system. GFP fluorescence can be seen in embryos as early as the germ band extension stage, and can also be seen in larvae, pupae, and adults. We show the patterns of GFP expression of these balancers and demonstrate the use of the balancers to identify homozygous progeny.     

Meiotic drive of t-haplotypes: segregation of chromosomes in mice with partial trisomy

The properties of the t haplotypes, specific mutant states of the proximal region of chromosome 17 in the house mouse keep renewing interest. One such property is increased transmission of the t haplotype from heterozygous t/+ males to their offspring. By means of reciprocal translocation T (16; 17)43H, we have constructed males with tertiary trisomy 17 (+T43/++/RB7+) carrying Robertsonian translocation Rb(16.17)7Bnr. The offspring of these males was viable when sperm of +T43/++ and Rb7+ was used. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that in the case when the t haplotype is on the normal acrocentric (male male ++T43/+t12+/Rb7++), its presence in the gamete +t12+/++T43 does not produce meiotic drive. However, when t6 is on Rb7, meiotic drive was equal to 80%. It is concluded that the presence of a normal homolog and a t-bearing chromosome in sperm does not result in meiotic drive. Possible mechanisms of meiotic drive of the t haplotypes are discussed.