Some genetic aspects of meiosis in males of Drosophila melanogaster,carriers of In(1)sc 4 sc 8

In order to verify the theory of non-homologous pairing in the male meiotic metaphase of Drosophila melanogaster, particular crosses were carried out with In(1)sc ⁴ sc ⁸ males, with or without the inversion In(2)SM ₅, Cy on the second chromosome. The results seem to indicate the existence of the above mechanisms, which would appear to contribute, not infrequently, to the overall non-disjunction of the In(1)sc ⁴ sc ⁸+Cy males.The work has been financially supported by the Consiglio Nazionale delle Ricerche (CNR).     

Bivalent behavior in Drosophila melanogaster males containing the In(1)sc 4Lsc8RX chromosome

The sex chromosome bivalent was examined in Drosophila melanogaster males possessing the In(1)sc ^4Lsc^8R X chromosome. Three-dimensional reconstructions from electron micrographs of serially cut thin sections were made. A large proportion of the kinetochores of In(1)sc ^4Lsc^8R/Y bivalents did not face opposite poles during metaphase I and anaphase I. This suggests that In(1)sc ^4Lsc^8R/Y bivalents may have difficulty achieving bipolar stability. Delay in achieving bipolar stability could contribute to the nondisjunctional behavior found in In(1)sc ^4Lsc^8R/Y males.     

Meiosis in Drosophila melanogaster. V. Univalent behavior in In(1)sc4Lsc8R/BsY males

Univalent behavior during meiosis has been examined in Drosophila melanogaster males possessing the In(1)sc4Lsc8R X chromosome using light microscopy and serial section electron microscopy. Males from two stocks, displaying high (0.40) and low (0.14) frequencies of sex chromosome nondisjunction, have been investigated. The results demonstrate that (i) sex chromosomes are more intimately paired during prometaphase I in males from the low nondisjunction stock than in males from the high nondisjunction stock, and (ii) the univalents are distributed to the poles in an unbiased manner during meiosis rather than by directed segregation of both univalents to the same pole as previously determined for other In(1)sc4Lsc8R/Y males.     

Cytological studies of heterochromatin function in the Drosophila melanogaster male: autosomal meiotic pairing

In Drosophila melanogaster it is now documented that the different satellite DNA sequences make up the majority of the centromeric heterochromatin of all chromosomes. The most popular hypothesis on this class of DNA is that satellite DNA itself is important to the pairing processes of chromosomes. Evidence in support of such a hypothesis is, however, circumstantial. This hypothesis has been evaluated by direct cytological examination of the meiotic behaviour of heterochromatically and/or euchromatically rearranged autosomes in the male. It was found that neither substantial deletions nor rearrangements of the autosomal heterochromatin cause any disruption of meiotic pairing. Autosomal pairing depends on homologs retaining sufficient euchromatic homology. This is the first clear demonstration that the highly repeated satellite DNA sequences in the heterochromatin of the second, third and fourth chromosomes are not important in meiotic pairing, but rather that some euchromatic homology in the autosomes is essential to ensure a regular meiotic process. These results on the autosomes, when taken in conjunction with our previous studies on sex chromosome pairing, clearly indicate that satellite DNA is not crucial for male meiotic chromosome pairing of any member of the D. melanogaster genome.     

Non-homologue pairing and spontaneous meiotic interchanges in Drosophila melanogaster females

Spontaneous interchange between the X chromosomes and the C(2L) autosomal compound in their centromeric regions was studied in y/XY;C(2L);C(2R) and In(1)dl-49+BM1/XY;C(2L);C(2R) Drosophila melanogaster females. These females were mated with F(2L)/F(2L);C(2R) males. Interchange occurrence was recorded as the appearance of an F1 individual with a half-translocation of either X . 2L or Y . 2L type. 37 interchanges were recovered in y/XY and 67 in In(1)/XY females. The majority of the interchanges were of meiotic origin. The interchanges were mainly C(2L)-XY; the most frequent type of half-translocation was Y . 2L;dl-49+BM1. Inversion increased about 5-fold the interchange frequency. In the course of C(2L)-XY interchange, the other X chromosome and C(2R) compound regularly paired and disjoined. In y/XY females, 8 crossover half-translocations of meiotic origin were recovered. The results obtained indicate that meiotic pairing between the X's and C(2L) occurred in the females examined. According to our estimates, XY-C(2L) pairing is associated with interchange in the heterochromatic centromeric regions with a frequency of 10(-3). The recovery of crossover half-translocations supports the chromocentral model of non-homologous pairing and allows us to assume that a chromosome may simultaneously pair with a homologue and a non-homologue. The disjunction pattern of this trivalent depends on its structure in each particular case. The chromosome-segregation pattern resulting from spontaneous interchanges was similar to that resulting from radiation-induced interchanges in the immature oocytes described by Parker. This similarity suggests that non-homologue pairing occurs in the immature oocytes too. The non-homologue-pairing pattern established by the interchange test conformed well with that previously established in y/XY and In(1)XY females by the distribution test.     

Fine structure of chromosome pairing in ten Ascomycetes: meiotic and premeiotic (mitotic) synaptonemal complexes

The meiotic prophase of ten Ascomycetes was studied by both light and electron microscopy. All ten species show a Neurospora type of meiosis. Two Podospora have no distinct synaptonemal complexes but synaptic structures in their nucleolus. The synaptonemal complexes of the eight other species are well differenciated and have uniform dimensions. The banded pattern of the lateral components seems to be a characteristic of Ascomycetes. The most striking observation is the specificity of this banding which can be used as a marker in interspecific crosses. Ascophanus carneus shows two kinds of synaptonemal complexes: normally between the pachytene bivalents and unusually in the ascogenous hyphae mitoses before meiosis and even before caryogamy.     

The distribution of male meiotic pairing sites on chromosome 2 of Drosophila melanogaster: meiotic pairing and segregation of 2-Y transpositions

The distribution of meiotic pairing sites on a Drosophila melanogaster autosome was studied by characterizing patterns of prophase pairing and anaphase segregation in males heterozygous for a number of 2-Y transpositions, collectively coveringall of chromosome arm 2R and one-fourth of chromosome arm 2L. It was found that all transpositions involving euchromatin from chromosome 2, even short stretches, increased the frequency of prophase I quadrivalents involving the sex and second chromosome bivalents above background levels. Quadrivalent frequencies were the same whether the males carried both elements of the transposition or just the Dp (2;Y) element along with two normal chromosome 2s, indicating that pairing is non-competitive. The frequency of quadrivalents was proportional to the size of the transposed region, suggesting that pairing sites are widely distributed on chromosome 2. Moreover, all but the smallest transpositions caused a detectable bias in the segregation ratio, in favor of alternate segregations, indicating that the prophase associations were effective in orienting centromeres to opposite poles. One transposition involving only heterochromatin of chromosome 2 had no effect on quadrivalent frequency, consistent with previous evidence that autosomal heterochromatin lacks meiotic pairing ability in males. One region at the base of chromosome arm 2L proved to be especially effective in stimulating quadrivalent formation and anaphase segregation, indicating the presence of a strong pairing site in this region. It is concluded that autosomal pairing in D. melanogaster males is based on general homology, despite the lack of homologous recombination.by A.C. Spradling     

Specific recognition and differential affinity of meiotic X-Y pairing sites in Lucilia cuprina males (Diptera: Calliphoridae)

Meiotic pairing of X and Y chromosomes in male Lucilia cuprina was studied by cytological observation of normal, rearranged and deficient sex chromosome karyotypes in spermatogenesis. Two X-Y pairing regions located distally in each arm of the X and Y chromosomes were defined. Contrasting with findings in Drosophila melanogaster, these pairing regions show specific recognition of their partners. By studying rearranged sex chromosomes short arm pairing was localised to their distal ends, closely associated with secondary constrictions containing nucleolar organisers in both sex chromosomes. Short arm pairing is very tight and not greatly disrupted by chromosome rearrangement, deficiency for the Y chromosome long arm or the presence of supernumerary X chromosomes. The pairing region of the long arms could not be precisely localised but probably also occurs at their distal ends. Pairing between the long arm sites is much weaker and is easily disrupted by chromosome rearrangement, failing completely in flies deficient for the Y chromosome short arm. No cytologically visible pairing was seen between X chromosomes and the remainder of the Y. In males with an extra X chromosome, the ends of both X chromosomes pair to form multivalents with normal and rearranged Y chromosomes provided the Y short arm is present, otherwise an independent X chromosome bivalent is formed. The mechanism of pairing in male Lucilia sex chromosomes thus seems to depend on specific loci of distinctive structure within the X and Y heterochromatin. Comparison of cytological and genetic data shows that increasing cytological pairing failure is matched by higher genetic X-Y nondisjunction but that the former occurs at much higher levels. In some karyotypes cytologically observed X-Y pairing failure is not matched by high frequencies of nondisjunction presumably because weak pairing associations are disrupted during slide preparation.     

1(2)41Aa,a heterochromatic gene of Drosophila melanogaster,is required for mitotic and meiotic chromosome condensation

Genetic and cytological approaches have yielded significant insight into the mapping and organization of genes located in the heterochromatin of Drosophila melanogaster. To date, only a few of these genes have been molecularly characterized in detail, and their function unveiled. As a further step towards the identification of heterochromatic gene functions, we have carried out a cytological analysis of mitotic and meiotic cell divisions in mutants carrying different allelic combinations of 1(2)41Aa, a gene located in the proximal heterochromatin of chromosome 2. Our results showed that larval brains of 1(2)41Aa mutants display a high frequency of cells with irregularly condensed chromosomes. In addition, defective chromosome condensation was detected in male meiosis, consequently affecting chromosome segregation and giving rise to irregular spermatids. Taken together, these findings indicate that 1(2)41Aa is a novel cell cycle gene required for proper chromosome condensation in both somatic and germ line cells.     

Meiosis VIII: Segregation matrix methods applied to meiotic drive in Drosophila melanogaster males with an SC 4-SC 8 inverted X

Two alternative matrix solutions of drive in Drosophila males with sc ⁴-sc ⁸ as studied by Peacock (1965) poses questions concerning meiosis in such flies. The two solutions are based primarily on the observations that sc ⁴-sc ⁸ males with low levels of X, Y non-disjunction produce twice as many female-as male offspring; that 50% of the sperm in this Drosophila line may be dysfunctional; that at all levels of non-disjunction studied, the difference between the % of female and male offspring produced by normal disjunction is about 33; and finally that, among non-disjunction types, nullo sperm werer covered more frequently than those with X and Y.The first matrix solution suggests a new model involving production of one infertile- and one fertile daughter cell at meiosis 1 in 1/3 of the cases and at meiosis 2 in the other 2/3. The second solution is already well known and would require the production of one fertile and one infertile secondary cyte at meiosis 1, in all cases. The unique feature of the first solution is that all four anaphase 2 poles (or genes) are present in primary spermatocytes as primordia. Two of these would predetermine fertility and two, infertility; and by assuming a random 2-by-2 assortment of these four poles at M1, together with physical binding of one of them with an X-or a Y chromatid, all known sc ⁴-sc ⁸ data as listed above, can be explained. The second model is necessarily more cumbersome, as it is difficult to account for a 2/1, female/male, ratio being produced at one cell division (meiosis 1).Discussed are possible applications of the segregation matrix method as a tool for determining degrees of freedom (complexity) in any biological system; implication of the models for segregation distorter (SD), for heterozygous translocations analyzed in Drosophila by Glass (1935) and by Zimmering (1955), and in other known or suspected cases of drive.     

End-to-end chromosome attachments in mitotic interphase and their possible significance to meiotic chromosome pairing

Cytological studies on telophase and early prophase in roottip cells of several plant species (Allium cepa, 2n=16; four Crepis species, including Crepis capillaris, 2n=6; Callitriche hermaphroditica, 2n=6; Nigella arvensis, 2n=12; Secale cereale, 2n=14) revealed that chromosome ends are attached two by two forming chains of chromosomes (interphase associations). In these chains homologous chromosomes are presumably located adjacent to each other. In Crepis capillaris it was observed that the two nucleolar chromosomes form a separate ring one end attached to the ring of the four remaining chromosomes and the other end attached to the nucleolus. It is proposed that these end-to-end attachments have significance for chromosome pairing in meiosis. The adjacent location of homologous chromosomes in the interphase associations would facilitate rapid and regular synapsis.     

Ecdysterone responsive functions in the mutantl(1)su(f) ts67g ofDrosophila melanogaster

Summary Transferring the temperature sensitive mutantl(1)su(f) ^ts67g from 25° C to 30° C before or early in the third larval instar blocks the increase in the ecdysterone titer that normally occurs at the end of the larval period. Feeding exogenous ecdysterone to these hormone-deficient larvae results in the formation of pseudopupae. The mutant was used to study ecdysterone-inducible functions in late larval salivary glands by preparing three animal samples with different hormone titers: the titer was low in one sample because of an earlier temperature shift, high in a second sample because the larvae were subsequently transferred to ecdysterone-supplemented food, and also high in a third sample that was kept at 25°C, providing a control for normal development. The effect of the different hormone conditions was studied by³⁵S-methionine labeling of the salivary gland proteins during the larval to prepupal transition and the prepupal period. The results indicate that synthesis of several of the proteins normally appearing during the transition and prepupal period is induced by exogenous ecdysterone.     

X-ray quality and the induction of meiotic recombination in Drosophila melanogaster males.

This paper reports a set of experiments designed to determine whether the radiation-quality effect, reported for the induction of somatic recombination, could be also demonstrated for meiotic recombination. Males heterozygous for markers on the 2nd chromosome were given an exposure of 600 R with either 55 kV or 100 kV X-irradiation. Treated spermatocytes were sampled by irradiating young pupae and then taking the first day sperm from the emergent adult males. When 2-h old pupae were irradiated there were significantly fewer centric and more non-centric recombinants, than when 4--6 h old pupae were irradiated. No radiation-quality effect was found.     

The conserved kinase NHK-1 is essential for mitotic progression and unifying acentrosomal meiotic spindles in Drosophila melanogaster

Conventional centrosomes are absent from the spindle in female meiosis in many species, but it is not clear how multiple chromosomes form one shared bipolar spindle without centrosomes. We identified a female sterile mutant in which each bivalent chromosome often forms a separate bipolar metaphase I spindle. Unlike wild type, prophase I chromosomes fail to form a single compact structure within the oocyte nucleus, although the integrity of metaphase I chromosomes appears to be normal. Molecular analysis indicates that the mutant is defective in the conserved kinase nucleosomal histone kinase-1 (NHK-1). Isolation of further alleles and RNA interference in S2 cells demonstrated that NHK-1 is also required for mitotic progression. NHK-1 itself is phosphorylated in mitosis and female meiosis, suggesting that this kinase is part of the regulatory system coordinating progression of mitosis and meiosis.     

The effects of recombination-defective meiotic mutants in Drosophila melanogaster on gonial recombination in males.

Recombination-defective female meiotic mutants representing 7 loci in Drosophila melanogaster have been examined for effects on gonial recombination in males. These loci were chosen for study because they represent a broad range of the known types of defects in processes necessary for meiotic recombination and somatic chromosome stability. Alleles at 6 of the loci studied did not increase the frequency of gonial recombination in males, whereas a mutant at one locus was associated with an increase (about 10-fold) in gonial recombination. These results suggest that the defects in chromosomal metabolism caused by these recombination, and in some cases repair, defective mutants are distinct from those of the male-recombination promoting elements (Mr) recently isolated from many natural populations. Analysis of the spontaneous events detected in this study showed that a third to a half of the events detected are actually of mutational rather than recombinational origin.     

Effect of the l(1)su(f)ts67g mutation ofDrosophila melanogaster on glue protein synthesis

Summary The l(1)su(f)^ts67g mutation has been shown to suppress the developmentally regulated expression of glue protein genes at 30°C. Transferring mutant larvae to the restrictive temperature before the end of the second larval instar results in the absence or extreme reduction of glue protein synthesis while general protein synthesis is unaffected. At the same time, the three glue protein correlated chromosomal regions 3C, 25B, and 68C continue to show prominent puffs. The results suggest that the mutation may be affecting the processing or translatability of specific mRNAs rather than the translational machinery itself.     

The influence of heterochromatin, inversion-heterozygosity and somatic pairing on x-ray induced mitotic recombination in Drosophila melanogaster

Summary Mitotic recombination has been induced with X-rays in Drosophila melanogaster larvae and assayed later as twin mosaic spots in the adult eyes. When the X-chromosomes are marked with zeste and white and the third chromosomes with roughoid and sepia, the frequency of twin spots was about 20 times higher for the X-chromosome than for the third chromosome. The greater amount of heterochromatin in the X-chromosome was considered responsible for the difference.Experiments with different inversion heterozygotes support this interpretation. Euchromatic inversions of different lengths have, when heterozygous, little or no influence on the twin spot frequency. The shorter the heterochromatic segment between the kinetochore and the proxomal break point of the inversion the stronger is the reduction of the twin spot frequency.The heterozygotes for the long sc ⁸ and sc ^S1 inversions gave exceptionally low twin spot frequencies. It seems possible that potential twin spot daughter cells die after recombination because of genetic imbalance and/or lack of proper cell separation resulting from the persistence of the dikinetic chromosome elements.To test whether inaccurate somatic pairing in inversion heterozygotes could help explain the low twin spot frequencies in those of sc ⁸ and sc ^S1, neuroblast chromosomes were investigated. They show that chromosomal arrangement during metaphase is determined exclusively by the location of the kinetochore, which always points, irrespective of earlier somatic pairing, toward the center of the metaphase plate. It is possible that there is a lack of proper chromosome alignment at the X-ray sensitive stage for mitotic recombination.