Meiotic drive for B-chromosomes in the primary oocytes of Myrmekotettix maculatus (Orthoptera: Acrididae)

Using a modified technique which allowed observation of chromosome orientation in the primary oocyte of grasshoppers at the onset of anaphase, it has been possible to establish that the B-chromosome is distributed preferentially on the egg side of the metaphase plate rather than the polar body side. The frequency of this preferential orientation matches very closely the level of preferential transmission determined from breeding experiments using individuals from the same population. The spindle is asymmetrical in the primary oocyte of this species, and a possible explanation of the meiotic drive is proposed as a result of the conical shape of nucleoplasm surrounding this spindle. The autosomal chiasma frequency of these females is generally lower than comparable males and is increased by the presence of B chromosomes; but the chiasma frequencies of the sexes respond differently to the addition of 1 and 2 B-chromosomes.     

Integration of a B chromosome into the A genome of a wasp,revisited

A previous study showed that in the haplodiploid solitary wasp Trypoxylon albitarse, most individuals carry one B chromosome per haploid genome, the same dosage as the standard (A) chromosomes, indicating a possible regularization of B-chromosome meiotic behaviour and its integration into the A genome. In a new sampling, we have analysed 15 populations (including 9 out of the 10 previously analysed) to test the evolution of this integration process. The new results provide a direct report of the invasion process in the Porto Firme population, where B frequency has dramatically increased in only four generations. In the populations from the Viçosa region, however, B frequency has remained stable, although the principal B type, the metacentric one, has increased in frequency at the expense of the acrocentric one in several populations. The implications of these new results on the hypothesis of the integration of these B chromosomes, as regular members of the A genome, are discussed.     

Presence of a centromeric filament during meiosis.

Spermatocytes at meiotic metaphase I and anaphase I have a characteristic centromeric filament in a variety of vertebrate organisms. This centromeric filament was first demonstrated on mouse spermatocytes and its presence is now extended to spermatocytes from the human, rat, golden hamster, bull, and chicken. The visualization of this filament was possible through the use of a novel silver-staining technique, which allows a high contrast between the filament and the centromeric chromatin. In the species cited, the centromeric filament shares an intense staining, a short (0.2-0.6 micron) length, a curved and branched shape, and location inside the centromeric chromatin of seemingly every homologue of the complement. The similarity of staining reactivity and the observation of transitional structures during first meiotic prophase strongly suggest that the centromeric filament is a remnant of a lateral element of the synaptonemal complex, which stays specifically at both centromeric regions of each bivalent. This filament is not found at the second meiotic division or at the centromeres of mitotic chromosomes. It is assumed that this centromeric filament joins the two sister chromatids of each homologue at the centromere and thus ensures the proper coorientation of sister kinetochores at metaphase I. Further testable assumptions on the functions of this filament are presented.     

Achiasmate Segregation of a B Chromosome from the X Chromosome in Two Species of Psyllids (Psylloidea, Homoptera)

The segregation of a B chromosome from the X chromosome was studied in male meiosis in two psyllid species, Rhinocola aceris (L.) and Psylla foersteri (Flor.) (Psylloidea, Homoptera). The frequency of segregation was determined from cells at metaphase II. In R. aceris, the B chromosome was mitotically stable and segregated quite regularly from the X chromosome in four geographically distant populations, while it showed less regular, but preferential segregation in one population. This was attributed to the presence of B chromosome variants that differ in their ability to interact with the X chromosome in segregation. In P. foersteri, the B chromosome was mitotically unstable and segregated preferentially from the X chromosome in spermatocyte cysts, which displayed one B chromosome in every cell. Behaviour of the B chromosome and X chromosome univalents during meiotic prophase and at metaphase I in R. aceris, and during anaphase I in P. foersteri suggested that the regular segregation resulted from the incorporation of B chromosomes in achiasmate segregation mechanisms with the X chromosome in the place occupied by the Y chromosome in species with XY system. The regular segregation of a B chromosome from the X chromosome may obscure the distinction of a B chromosome and an achiasmate Y chromosome in some cases. This revised version was published online in August 2006 with corrections to the Cover Date.     

Arrangement of centromeres in mouse cells

Applying a staining procedure which reveals constitutive heterochromatin to cytological preparations of the mouse (Mus musculus), one detects heterochromatin pieces at the centromeric areas of all chromosomes except the Y. The Y chromosome is somewhat heteropyenotic in general but possesses no intensely stained centromeric heterochromatin. The arrangement of the centromeric heterochromatin in interphase cells is apparently specific for a given cell type. In meiotic prophase, centromeric heterochromatin may form clusters among bivalents. From the location of the centromeric heterochromatin of the X chromosome in the sex bivalent, it is concluded that the association between the X and Y (common end) in meiosis is limited to the distal portions of the sex elements.     

Cytomolecular analysis of the male sex chromosome of Tropidacris cristata grandis (Thunberg, 1824) (Orthoptera: Romaleidae)

Many species of grasshopper have an XX/XO sex chromosome system, including Tropidacris cristata grandis (23, XX/XO). The X chromosome behaves differently from the autosomes, but little is known about its origin and molecular composition. To better understand the genomic composition and evolutionary processes involved in the origin of the sex chromosomes, we undertook an analysis of its meiotic behavior, heterochromatin distribution and microdissection in T. c. grandis. Analysis of meiotic cells revealed a difference in the behavior of the X chromosome compared to the autosomes, with different patterns of condensation and cellular arrangement. Heterochromatic terminal blocks were predominant. The chromosome painting revealed a bright block in the centromeric/pericentromeric region of the X chromosome and slight markings in the other regions. In the autosomes, the X chromosome probe hybridized in the centromeric/pericentromeric region, and hybridization signals on terminal regions corresponding to the heterochromatic regions were also observed. The results showed that the X chromosome contains a significant amount of repetitive DNA. Based on the hybridization pattern, it is possible that the autosomes and sex chromosomes of T. c. grandis have a similar composition of repetitive DNAs, which could mean that the X chromosome has an autosomal origin.     

Sex chromosomes and speciation in Drosophila

Two empirical rules suggest that sex chromosomes play a special role in speciation. The first is Haldane's rule - the preferential sterility and inviability of species hybrids of the heterogametic (XY) sex. The second is the disproportionately large effect of the X chromosome in genetic analyses of hybrid sterility. Whereas the causes of Haldane's rule are well established, the causes of the 'large X-effect' have remained controversial. New genetic analyses in Drosophila confirm that the X is a hotspot for hybrid male sterility factors, providing a proximate explanation for the large X-effect. Several other new findings -- on faster X evolution, X chromosome meiotic drive and the regulation of the X chromosome in the male-germline -- provide plausible evolutionary explanations for the large X-effect.     

Centromeric dots in crane-fly spermatocytes: meiotic maturation and malorientation

During the first meiotic division in crane-fly spermatocytes, the two homologs of a metaphase bivalent each bear two sister kinetochores oriented toward the same pole. We have previously reported treatments that increase the percentage of metaphase bivalents in which one or both homologs have bipolar malorientations: kinetochore microtubules] extending from a homolog toward both poles. The maloriented homologs lag at anaphase. Treatments that induce this behavior include: (a) recoverey from exposure to low temperatures or Colcemid or Nocodazole concentrations that prevent spindle formation but allow nuclear membrane breakdown, and (b) exposure to 6° C, a temperature that permits spindle assembly but slows progression through meiosis. Giemsa staining methods reveal two 0.5 m diameter dots at the centromeric region of each metaphase homolog; these often are more separated in maloriented homologs. This investigation was undertaken to assess whether this separation precedes the establishment of bipolar malorientation, and hence may be a cause of it, or is only a consequence of forces resulting from bipolar malorientation. Analysis showed that, in untreated cells, the average center-to-center distance between sister centromeric dots increases during the course of meiosis I. After the above-mentioned treatments, center-to-center distances similar to those normally seen in untreated half-bivalents at anaphase I were seen in bivalents, both after and before nuclear membrane breakdown. Longer exposure to temperatures that arrested meiosis increased the degree of dot separation. Based on our data, we conclude that normal orientation during the first meiotic division is aided by the close apposition of centromeric dots, and that a time-dependent maturation occurs causing centromeric dots to separate for the second meiotic division and facilitating orientation of sister kinetochores to opposite poles. If centromeric maturation occurs either prior to or during early stages of the first meiotic division, then it may contribute to persisting bipolar malorientation.     

Segregation-distortion in the B-chromosome system ofTettigidea lateralis (say) (Orthoptera: Tetrigidae)

A marginal population ofTettigidea lateralis was found to be polymorphic with respect to a large, mitotically stable supernumerary (B) chromosome. Male and female individuals may carry one or two B-chromosomes. In the male sex the frequency of individuals with one B was 33.8% whereas that of 2B-carriers was 2.9 %. A comparison with a small sample of female individuals suggests similar frequencies of B-chromosome carriers in the two sexes. The pycnocity cycle of the B's is virtually identical to that of the X chromosome which is always distinguishable by virtue of its larger size and other structural details. Persistent heterochromatic associations between the B and the X, which may last until metaphaseanaphase I, lead to a preferential migration of the B with the X to the same pole in male carriers of a single supernumerary. This distortional segregation of the B-chromosome may produce a differential transmission of the supernumerary to the two sexes if the various types of gametes are equally functional. Achiasmate, persistent B-B associations in 2B individuals can also cause segregation-distortion. The two supernumeraries segregate to the same pole in approximately 1/3 of the spermatocytes, but their poleward movement relative to that of the X is random. Both –B and +B individuals show only a single chiasma per individual bivalent. However, the presence of a single B raises very significantly the frequency at which the chiasma forms at the extreme distal ends of the L₁-L₂ and M₃-M₄ autosomes. The effect on recombination exerted by the supernumeraries and the possible implications of the segregation-distortion system ofT. lateralis are discussed in the light of recent studies on comparable B-chromosome polymorphisms.Research supported by N.R.C. of Canada.     

Pycnotic cycle of the sex chromosome of Pyrgomorpha conica (Orthoptera) and development of spermiogenesis.

We have analyzed the anomalous pycnotic cycle of the X sex chromosome of the grasshopper Pyrgomorpha conica throughout both meiotic divisions and its possible influence on spermiogenesis. During diplotene the sex chromosome shows two differentiated pycnotic regions: (i) the centromeric region, which is negatively heteropycnotic, and (ii) the noncentromeric region, which shows alternating negatively and positively heteropycnotic zones in all standard individuals. The variation in size and location of the negative heteropycnotic zones, their smooth appearance, and their lack of effect on spermiogenesis lead us to suggest that condensation differences and not euchromatinization are responsible for their presence. In two individuals the sex chromosome appeared partially isopycnotic at metaphase I, and high levels of abnormal spermatids (macrospermatids and microspermatids) were found. We suggest that the possible activity of this chromosome during the second meiotic division may promote the disruption of spermiogenesis by affecting the mechanism that maintains intercellular bridges between normal spermatids.     

The mechanism of secondary nondisjunction in Drosophila melanogaster females

Bridges (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <--> Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossing over. He referred to these XX <--> Y segregation events as "secondary nondisjunction." Cooper (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Y chromosome directs the segregation of two achiasmate X chromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <--> Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in oocytes that undergo X chromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.     

Absence of Positive Selection on Centromeric Histones in Tetrahymena Suggests Unsuppressed Centromere-Drive in Lineages Lacking Male Meiosis

Centromere-drive is a process where centromeres compete for transmission through asymmetric "female" meiosis for inclusion into the oocyte. In symmetric "male" meiosis, all meiotic products form viable germ cells. Therefore, the primary incentive for centromere-drive, a potential transmission bias, is believed to be missing from male meiosis. In this article, we consider whether male meiosis also bears the primary cost of centromere-drive. Because different taxa carry out different combinations of meiotic programs (symmetric?+?asymmetric, symmetric only, asymmetric only), it is possible to consider the evolutionary consequences of centromere-drive in the context of these differing systems. Groups with both types of meiosis have large, rapidly evolving centromeric regions, and their centromeric histones (CenH3s) have been shown to evolve under positive selection, suggesting roles as suppressors of centromere-drive. In contrast, taxa with only symmetric male meiosis have shown no evidence of positive selection in their centromeric histones. In this article, we present the first evolutionary analysis of centromeric histones in ciliated protozoans, a group that only undergoes asymmetric "female" meiosis. We find no evidence of positive selection acting on CNA1, the CenH3 of Tetrahymena species. Cytological observations of a panel of Tetrahymena species are consistent with dynamic karyotype evolution in this lineage. Our findings suggest that defects in male meiosis, and not mitosis or female meiosis, are the primary selective force behind centromere-drive suppression. Our study raises the possibility that taxa like ciliates, with only female meiosis, may therefore undergo unsuppressed centromere drive.     

Nonrandom segregation of the mouse univalent X chromosome: evidence of spindle-mediated meiotic drive

A fundamental principle of Mendelian inheritance is random segregation of alleles to progeny; however, examples of distorted transmission either of specific alleles or of whole chromosomes have been described in a variety of species. In humans and mice, a distortion in chromosome transmission is often associated with a chromosome abnormality. One such example is the fertile XO female mouse. A transmission distortion effect that results in an excess of XX over XO daughters among the progeny of XO females has been recognized for nearly four decades. Utilizing contemporary methodology that combines immunofluorescence, FISH, and three-dimensional confocal microscopy, we have readdressed the meiotic segregation behavior of the single X chromosome in oocytes from XO females produced on two different inbred backgrounds. Our studies demonstrate that segregation of the univalent X chromosome at the first meiotic division is nonrandom, with preferential retention of the X chromosome in the oocyte in approximately 60% of cells. We propose that this deviation from Mendelian expectations is facilitated by a spindle-mediated mechanism. This mechanism, which appears to be a general feature of the female meiotic process, has implications for the frequency of nondisjunction in our species.     

A test for meiotic drive in hybrids between Australian and Timor zebra finches

Meiotic drivers have been proposed as a potent evolutionary force underlying genetic and phenotypic variation, genome structure, and also speciation. Due to their strong selective advantage, they are expected to rapidly spread through a population despite potentially detrimental effects on organismal fitness. Once fixed, autosomal drivers are cryptic within populations and only become visible in between‐population crosses lacking the driver or corresponding suppressor. However, the assumed ubiquity of meiotic drivers has rarely been assessed in crosses between populations or species. Here we test for meiotic drive in hybrid embryos and offspring of Timor and Australian zebra finches—subspecies that have evolved in isolation for about two million years—using 38,541 informative transmissions of 56 markers linked to either centromeres or distal chromosome ends. We did not find evidence for meiotic driver loci on specific chromosomes. However, we observed a weak overall transmission bias toward Timor alleles at centromeres in females (transmission probability of Australian alleles of 47%, nominal p = 6 × 10^–5). While this is in line with the centromere drive theory, it goes against the expectation that the subspecies with the larger effective population size (i.e., the Australian zebra finch) should have evolved the more potent meiotic drivers. We thus caution against interpreting our finding as definite evidence for centromeric drive. Yet, weak centromeric meiotic drivers may be more common than generally anticipated and we encourage further studies that are designed to detect also small effect meiotic drivers.     

The Aurora kinase Ipl1 maintains the centromeric localization of PP2A to protect cohesin during meiosis

Homologue segregation during the first meiotic division requires the proper spatial regulation of sister chromatid cohesion and its dissolution along chromosome arms, but its protection at centromeric regions. This protection requires the conserved MEI-S332/Sgo1 proteins that localize to centromeric regions and also recruit the PP2A phosphatase by binding its regulatory subunit, Rts1. Centromeric Rts1/PP2A then locally prevents cohesion dissolution possibly by dephosphorylating the protein complex cohesin. We show that Aurora B kinase in Saccharomyces cerevisiae (Ipl1) is also essential for the protection of meiotic centromeric cohesion. Coupled with a previous study in Drosophila melanogaster, this meiotic function of Aurora B kinase appears to be conserved among eukaryotes. Furthermore, we show that Sgo1 recruits Ipl1 to centromeric regions. In the absence of Ipl1, Rts1 can initially bind to centromeric regions but disappears from these regions after anaphase I onset. We suggest that centromeric Ipl1 ensures the continued centromeric presence of active Rts1/PP2A, which in turn locally protects cohesin and cohesion.     

Sexual antagonism and meiotic drive cause stable linkage disequilibrium and favour reduced recombination on the X chromosome

Sexual antagonism and meiotic drive are sex‐specific evolutionary forces with the potential to shape genomic architecture. Previous theory has found that pairing two sexually antagonistic loci or combining sexual antagonism with meiotic drive at linked autosomal loci augments genetic variation, produces stable linkage disequilibrium (LD) and favours reduced recombination. However, the influence of these two forces has not been examined on the X chromosome, which is thought to be enriched for sexual antagonism and meiotic drive. We investigate the evolution of the X chromosome under both sexual antagonism and meiotic drive with two models: in one, both loci experience sexual antagonism; in the other, we pair a meiotic drive locus with a sexually antagonistic locus. We find that LD arises between the two loci in both models, even when the two loci freely recombine in females and that driving haplotypes will be enriched for male‐beneficial alleles, further skewing sex ratios in these populations. We introduce a new measure of LD, , which accounts for population allele frequencies and is appropriate for instances where these are sex specific. Both models demonstrate that natural selection favours modifiers that reduce the recombination rate. These results inform observed patterns of congealment found on driving X chromosomes and have implications for patterns of natural variation and the evolution of recombination rates on the X chromosome.     

Genetic studies on heterochromatin in Drosophila melanogaster and their implications for the functions of satellite DNA

In Drosophila melanogaster the centromeric heterochromatin of all chromosomes consists almost entirely of several different satellite DNA sequences. In view of this we have examined by genetic means the meiotic consequences of X chromosomes with partial deletions of their heterochromatin, and have found that the amount and position of recombination on each heterochromatically deleted X is substantially different from that of a normal X. It appears that the amount of heterochromatin is important in modifying the centromere effect on recombination. — In all the deleted Xs tested, chromosome segregation is not appreciably altered from that of a nondeleted control chromosome. Thus satellite DNA does not appear to be an important factor in determining the regular segregation of sex chromosomes in Drosophila. Additionally, since X chromosomes with massive satellite DNA deficiencies are able to participate in a chromocenter within salivary gland nuclei, a major role of satellite DNA in chromocenter formation in this tissue is also quite unlikely. — In order to examine the mechanisms by which the amount of satellite DNA is increased or decreased in vivo, we have measured cytologically the frequency of spontaneous sister chromatid exchanges in a ring Y chromosome which is entirely heterochromatic and consists almost exclusively of satellite DNA. In larval neuroblast cells the frequency of spontaneous SCE in this Y is approximately 0.3% per cell division. Since there is no meiotic recombination in D. melanogaster males and since meiotic recombination in the female does not occur in heterochromatin, our results provide a minimum estimate of the in vivo frequency of SCE in C-banded heterochromatin (which is predominantly simple sequence DNA), without the usual complications of substituted base analogs, incorporated radioactive label or substantial genetic content. — We emphasise that: (a) satellite DNA is not implicated in any major way in recognition processes such as meiotic homologue recognition or chromocenter formation in salivaries, (b) there is likely to be continuous variation in the amount of satellite DNA between individuals of a species; and (c) the amount of satellite DNA can have a crucial functional role in the meiotic recombination system.     

A deficiency screen of the major autosomes identifies a gene (matrimony) that is haplo-insufficient for achiasmate segregation in Drosophila oocytes

In Drosophila oocytes, euchromatic homolog-homolog associations are released at the end of pachytene, while heterochromatic pairings persist until metaphase I. A screen of 123 autosomal deficiencies for dominant effects on achiasmate chromosome segregation has identified a single gene that is haplo-insufficient for homologous achiasmate segregation and whose product may be required for the maintenance of such heterochromatic pairings. Of the deficiencies tested, only one exhibited a strong dominant effect on achiasmate segregation, inducing both X and fourth chromosome nondisjunction in FM7/X females. Five overlapping deficiencies showed a similar dominant effect on achiasmate chromosome disjunction and mapped the haplo-insufficient meiotic gene to a small interval within 66C7-12. A P-element insertion mutation in this interval exhibits a similar dominant effect on achiasmate segregation, inducing both high levels of X and fourth chromosome nondisjunction in FM7/X females and high levels of fourth chromosome nondisjunction in X/X females. The insertion site for this P element lies immediately upstream of CG18543, and germline expression of a UAS-CG18543 cDNA construct driven by nanos-GAL4 fully rescues the dominant meiotic defect. We conclude that CG18543 is the haplo-insufficient gene and have renamed this gene matrimony (mtrm). Cytological studies of prometaphase and metaphase I in mtrm hemizygotes demonstrate that achiasmate chromosomes are not properly positioned with respect to their homolog on the meiotic spindle. One possible, albeit speculative, interpretation of these data is that the presence of only a single copy of mtrm disrupts the function of whatever "glue" holds heterochromatically paired homologs together from the end of pachytene until metaphase I.     

Variable transmission rates of a B-chromosome in Myrmeleotettix maculatus (Thunb.) (Acrididae: Orthoptera)

Karyotype comparisons of both parents and progeny from single pair matings in the grasshopper Myrmeleotettix maculatus have shown that there is an accumulation of the large mitotically stable B-chromosome when transmitted through the female. This is presumed to result from a preferential segregation of univalent B-chromosomes at the first division of female meiosis and occurs irrespective of whether the B's are odd or even in number. In the male there is a loss of B-chromosomes. This loss does not appear to be due simply to the lagging and elimination of B-chromosomes in meiosis but probably involves sperm formation or function. When the balance of the gain and loss after one generation is calculated, it shows large overall accumulation in crosses involving individuals from a population in Wales, and a slight loss in individuals from a population in East Anglia. Such differences in transmission rates may be responsible for differences in B-frequency between populations. Since the B-chromosome frequency of these two populations has remained stable over five years, possible forces in the maintenance of the equilibria are examined. Females with B-chromosomes produce more aneuploid embryos than 0B females, but neither this cause of inviability nor general embryo mortality seem sufficient to produce an equilibrium situation. It is necessary to postulate that progeny with more than 2B chromosomes are inviable in order to approach equilibria. The presence of B-chromosomes in females has also led to the formation of polyploid embryos. The possible involvement of repetitive DNA in the formation of unreduced egg nuclei and preferential segregation is discussed.