A supernumerary chromosome with an accumulation mechanism in the lecanoid genetic system

Summary The supernumerary chromosomes of a mealy bug,Pseudococcus obscurus Essig are heterochromatic but show a variable heteropycnosis. In the female, they are weakly heteropyonotic in most tissues, but in a few tissues the individual supernumeraries form striking chromocenters. At oogenesis, they remain unassociated and divide equationally during the first division; during the second, they pair and disjoin. Pairing is usually accomplished by twos so that an unpaired supernumerary is found whenever an odd number, or only one, is present; the unpaired entity is twice as likely to go to the second polar body as to the egg.The normal spermatogenesis in the mealy bugs is a highly modified meiosis in which the paternal heterochromatic set is eliminated from the genetic continuum; during this sequence the supernumeraries are fully heterochromatic up until late prophase I. They then undergo a sharp change in pycnosis and become negatively heteropycnotic. In the second meiotic division they usually segregate with the maternal euchromatic set. Their behavior during spermatogenesis thus becomes an accumulation mechanism since an unreduced number, or nearly that, is transmitted by the males.The variable behavior of the supernumeraries affords further insight into the problem of heterochromatization in the mealy bugs.The supernumeraries may have originated from fragments followed by subsequent duplications. The accumulation mechanism may have been an important factor in their establishment.In genetic systems in which the supernumeraries have an accumulation mechanism, an elimination mechanism might evolve to stabilize the number of supernumeraries. Such elimination mechanisms are known for other genetic systems but have not yet been developed in this mealy bug.The material in this paper is part of a dissertation submitted to the graduate school of the University of California in partial satisfaction of the requirements for the degree of Doctor of Philosophy. This work was supported in part by a National Science Foundation Grant (G-9772) to ProfessorSpencer W. Brown.Predoctoral Trainee in Genetics, National Institutes of Health, 1960–1961.     

Mitotically unstable B chromosome polymorphism in the grasshopper Dichroplus elongatus

Dichroplus elongatus, a widespread South American phytophagous grasshopper, exhibits polymorphisms for supernumerary chromosomes and segments (SS) in natural populations in Argentina. In this paper we review the available information on B chromosome polymorphism in D. elongatus related to geographic distribution, patterns of chromosome variation and influence on sperm formation. In D. elongatus the different forms of supernumerary variants are not independent. The proportion of B-carrying individuals (B prevalence) is negatively correlated with SS10 and positively with SS6 frequencies. The analysis of population structure considering the different supernumerary variants would suggest that the patterns of chromosome variation can not be explained only by random factors. Geographic distribution was analyzed scoring the prevalence of B chromosomes in 13 natural populations collected in three different biogeographical provinces from Northwest (Las Yungas province) and East (Espinal and Pampeana provinces) of Argentina. The detected heterogeneity may be explained by significant differentiation between Northwest and East regions and among populations within Las Yungas and Pampeana provinces. Correlation analysis suggested that B chromosome prevalence is associated with maximum temperature and with latitude. Additional information about the nature of the patterns of B chromosome variation was obtained comparing them with those obtained at the mitochondrial DNA level. The hierarchical analysis of molecular differentiation revealed discrepancy with respect to chromosome differentiation and also suggested that the pattern of B chromosomes may not be explained by historical factors. We also discussed the probable influence on fertility of carriers considering the production of abnormal sperm formation (macro and microspermatids) in relation to the number of Bs per follicle.     

Meiotic drive against an autosomal supernumerary segment promoted by the presence of a B chromosome in females of the grasshopper Eyprepocnemis plorans

Twenty-seven out of 50 progeny analyses performed with specimens of the grasshopper Eyprepocnemis plorans were informative about the transmission of a supernumerary heterochromatic chromosome segment. The simultaneous presence of a B chromosome in some of the parents involved in the crosses permitted us to test the relationship between both types of supernumerary heterochromatin with respect to their transmission. The results demonstrated that the supernumerary segment is partly eliminated through females possessing B chromosomes. The implications of this in relation to the occurrence of the extra segment in natural populations are discussed.by S.A. Gerbi     

Undertransmission of a supernumerary chromosome segment through heterozygous females possessing B chromosomes in the grasshopper Eyprepocnemis plorans.

The transmission ratio (ks) for a supernumerary chromosome segment was studied in a total of 54 heterozygous females collected from two Spanish natural populations of the grasshopper Eyprepocnemis plorans. Our analysis clearly demonstrated that ks is negatively dependent on the number of B chromosomes in the female. The possible mechanisms by which B chromosomes may cause undertransmission of the supernumerary segment, and the implications of this phenomenon for the maintenance of this extra chromosome segment, are discussed.     

Distortional meiotic segregation of a supernumerary chromosome producing differential frequencies in the sexes in the short-horned grasshopper Phaulacridium vittatum

Samples of males reveal the presence of a large, mitotically stable supernumerary chromosome in five populations examined. A stable frequency (11.22±0.3%) of male carriers has been observed over nine years in one population. The supernumerary follows closely the condensation cycle and behaviour of the sex chromosome in spermatogonial mitosis and meiosis. A structure simulating a terminalized chiasma frequently joins the precocious sex and supernumerary chromosomes during meiotic prophase; these two chromosomes move preferentially (70%) to opposite poles during the subsequent stages producing a differential transmission of the supernumerary to the two sexes. It is possible that the stable frequency in the population is maintained by a conserved balance between the two sexes without the need of an elimination system. The evidence supports the interpretation that the supernumerary chromosome is partially homologous with the sex chromosome. The possibility that the supernumerary might evolve into a neo-Y chromosome is suggested.     

Is the Y chromosome of Drosophila an evolved supernumerary chromosome?

The Y chromosomes of most Drosophila species are necessary for male fertility but they are not involved in sex determination. They have many puzzling properties that resemble the effects caused by B chromosomes. Classical genetic and molecular studies reveal substantial affinities between Y and B chromosomes and suggest that the Y chromosomes of Drosophila are not degenerated homologues of the X chromosomes, but rather that their Y chromosomes evolved as specialized supernumeraries similar to classical B chromosomes.     

The odd-even effect in mitotically unstable B chromosomes in grasshoppers

The odd-even effect, by which B chromosomes are more detrimental in odd numbers, has been reported in plants and animals. In grasshoppers, there are only a few reports of this effect and all were referred to as traits related to the formation of aberrant meiotic products (AMPs). Here we review the existing information about B chromosome effects on AMPs, chiasma frequency and the number of active nucleolus organizer regions (NORs) per cell. Polysomy for A chromosomes and B chromosomes are two kinds of chromosome polymorphism frequently found in grasshoppers. In some aspects, e.g. meiotic behaviour and mitotic instability leading to individual mosaicism (in the case of mitotically unstable Bs), polysomic As show similar characteristics to B chromosomes. In fact, polysomy is regarded as one of the main mechanisms for B chromosome origin. Here we review some features of meiotic behaviour in known cases of polysomy and mitotically unstable Bs in grasshoppers, in looking for possible causes for the odd-even effect. In all these traits, the odd-even effect was apparent, although its appearance was not universal in any case, with variation among species or populations within the same species. The equational division and lagging of the extra chromosomes, when univalents, could favour the appearance of abnormal meiotic products, and the formation of bivalents, when there are two or more extra chromosomes, inhibits this process. Therefore, the odd-even effect might be a consequence of the concomitant operation of both aspects of extra chromosome meiotic behaviour. The possibility that the odd-even effect might result from an increase in cell stress generated by odd numbers is suggested.     

A supernumerary chromosome segment in Locusta migratoria.

A single female of Locusta migratoria was found to be heterozygous for a supernumerary heterochromatic segment distally located on the M6 autosome close to its nucleolus organiser region (NOR). Reactions to several chromosome banding techniques revealed its heterochromatic nature and its composition of GC-rich DNA sequences and likewise the NORs in this species. This suggests an origin for the extra segment by amplification of GC-rich DNA sequences contained in the distal NOR of the M6 chromosome, which is reinforced by the observation that the NOR of segmented M6 chromosomes produced the larger nucleolus in embryo prophase cells, such as would be expected from the presence of rRNA genes in the extra segment. No accumulation mechanism was detected in this female after analyzing the 213 embryo offspring produced, but an increase in the number of nucleoli per interphase nucleus was noted in heterozygous embryos in respect to standard homozygous ones.     

Transmission analysis of mitotically unstable B chromosomes in Locusta migratoria.

Seventeen controlled crosses in which the mitotically unstable B chromosome of Locusta migratoria was carried by one parent only have provided evidence that B chromosomes are significantly eliminated during sexual transmission in males, at a mean rate that almost counteracts the premeiotic accumulation derived from mitotic instability during germ line development. On the other hand, B chromosomes are significantly accumulated in females, presumably by their preferential migration to the secondary oocyte during the first meiotic division. These results substantially change the current knowledge about this B chromosome system, because the main B accumulation occurs in females and not in males, as was hitherto thought. Furthermore, this case shows that the maintenance of a single B system in natural populations may be the result of many different forces and mechanisms acting for and against B chromosomes.     

A boy with small supernumerary marker chromosome X identified by FISH

Marker or ring X chromosomes are frequently seen in Ullrich-Turner Syndrome with 46,X,r(X) karyotype, but only 8 children were reported with an extra marker X chromosome in at least some of their cell lines, we describe a 5 years old male patient who is mosaic (17%) for a cell line with an extra ring shaped marker X chromosome in addition to a normal 46,XY cell line. He had mild motor mental retardation, a dysmorphic face, dysplastic ears, high arched palate, cryptorchidism and brachydactyly. G-banding showed 46,XY[83]/47,XY,+r?[17] karyotype. NOR banding revealed no satellite region but its centromere was intact in C-banding. By fluorescent in situ hybridization (FISH) technique, dual X/Y alpha-satellite probes were used to detect the origin of ring shaped marker chromosome and 17% of his cells had two X chromosome signals due to marker X; hybridization with X chromosome inactivation center (XIST) specific probe revealed the absence of the locus on the ring chromosome. In this report, clinical features of our patient are compared with previously reported cases and the cytogenetic and molecular cytogenetic techniques used to detect origin of marker chromosome are discussed.     

Small supernumerary marker chromosome (sSMC) derived from chromosome 22 in an infertile man with hypogonadotropic hypogonadism

We describe the first case of a supernumerary inv dup(22)(q11.1) in an infertile male with hypogonadotropic hypogonadism. This case supports the opinion that supernumerary inv dup(22)(q11.1) could play a role in male infertility. We suggest that the breakpoint in the region 22q11.1 and/or fourfold dosage of centromeric/pericentromeric sequences of the chromosome 22 may be the cause of hypogonadotropic hypogonadism resulting in impaired spermatogenesis and infertility in our patient.     

Male and female segregation distortion for heterochromatic supernumerary segments on the S8 chromosome of the grasshopper Chorthippus jacobsi

The mode of inheritance of supernumerary segments located on three different chromosome pairs was investigated in controlled crosses with specimens of the grasshopper Chorthippus jacobsi. While extra segments located on chromosomes M₅ and M₆ showed Mendelian inheritance, that on S₈ did not. Thus, the two supernumerary heterochromatic chromosome segments located distally on the S₈ chromosome accumulated through non-Mendelian transmission through both sexes. The observed transmission patterns may be explained by gametic selection for spermatozoa carrying segmented S₈ chromosomes, in addition to meiotic drive for segmented S₈ chromosomes in heterozygous females. The significance of these findings for the maintenance of these polymorphisms in natural populations is discussed.by S.A. Gerbi     

Severe psychomotor retardation in a boy with a small supernumerary marker chromosome 19p

We describe the clinical case of a nine-year-old boy with psychomotor retardation and a small supernumerary marker chromosome (sSMC) present in mosaic form. Fluorescence in situ hybridization (FISH) using centromere cross-hybridizing probes D1/5/19Z (pZ5.1), the whole chromosome paint probe 19, pool YACs19p (839B1, 872G3, 728C8), and pool YACs19q (767C4, 761C1, 786G6) demonstrated that the sSMC was derived from chromosome 19p. Based on GTG-banding and FISH analyses, the patient's karyotype was interpreted as: 47,XY,+mar.ish der(19) (:p13.3-->p11:)(839B1+, 872G3+,728C8+, D1/5/19Z+) de novo[52]/46,XY[48]. To our knowledge, only two other similar cases have been reported. This case helps to better delineate karyotype-phenotype correlations between sSMC 19p and associated clinical phenomena.     

A set of duplicons on human chromosome 9 is involved in the origin of a supernumerary marker chromosome

Human chromosome 9 is involved in a number of recurrent structural rearrangements; moreover, its pericentromeric region exhibits a remarkable evolutionary plasticity. In this study we present the molecular characterization of a constitutional rearrangement, involving the 9p21.1q13 region, which led to the formation of a supernumerary marker chromosome (SMC). We defined the sequence of the breakpoints and identified a new set of duplicons on human chromosome 9, named LCR9s (chromosome 9 low-copy repeats). Two of these duplicons were shown to be involved in a somatic exchange leading to the formation of the SMC. High-resolution FISH coupled to database search demonstrated that a total number of 35 LCR9 paralogs are present in the human genome. These newly described chromosome 9 duplicons have features that may be crucial in driving structural chromosome rearrangements in germinal and somatic cells.     

Prenatal detection of an unstable ring 21 chromosome

Summary An unstable ring chromosome 21 detected through prenatal studies was associated at birth with an apparently normal male phenotype. At 14 months of age, examination indicated only minor developmental delay. The majority of cells examined from amniocyte, fibroblast, and lymphocyte cultures contained an asymmetrical dicentric ring 21 chromosome which was larger than a normal chromosome 21. This ring is presumed to be a duplication for most of chromosome 21 and a deletion of part of the terminal regions. The karyotype is described as mos45, XY,-21/46,XY,r(21)(p13q22.3). The child is monosomic for part of the sub-band 21q22.3 in every cell and trisomic for the remainder of the chromosome in most of his cells. The terminal deletion does not appear to have been severely detrimental to the phenotype and the effective trisomy present in many cells studies was insufficient to cause the Down syndrome.     

Characterization of a familial small supernumerary marker chromosome in a patient with adult-onset tongue cancer

Cytogenetic analysis in peripheral blood lymphocytes of a 50-year-old female with tongue cancer showed the presence of one to three copies of a small supernumerary marker chromosome (sSMC) in a mosaic state. Family studies also revealed the marker in mosaic form in four (age <29 years) of eleven clinically normal individuals studied from her family of 16 individuals spanning three generations. Due to the extremely small size of the marker chromosome, identification by classical cytogenetics was not informative. Multicolor FISH followed by whole chromosome painting identified the marker as a derivative of chromosome 21. This is the first report of sSMC21 in an adult-onset tongue cancer patient and some of her family members with no clinical symptoms.     

De novo supernumerary marker chromosome originating from chromosome 17 resulting in a normal pregnancy outcome

We report here a prenatal case with de novo supernumerary marker chromosome originating from chromosome 17 in non-mosaic form resulting in normal pregnancy outcome. In this case, a 26-year-old pregnant woman was referred for amniocenthesis and microdeletion Fluorescence In Situ Hybridization (FISH) testing at 18 weeks of gestation due to history of a previous child with Angelman Syndrome. PWS/AS region deletion was excluded by FISH. A de novo supernumerary, non-satellited, monocentric marker chromosome was detected during conventional cytogenetic analysis. With the use of FISH testing, it was found that the marker chromosome originated from chromosome 17. Additionally, the marker chromosome was found not to contain the Smith-Magenis and Miller Dieker syndrome regions. After detailed review of the literature, genetic counseling was given to the family, and the family decided to continue the pregnancy to term. A female child was born at term without any phenotypical abnormalities and clinical complications. Follow-up at 15 months-of-age revealed no developmental abnormalities. To our knowledge, our patient is the first reported prenatal case with a de novo monocentric, supernumerary marker chromosome derived from chromosome 17 in a non-mosaic form that resulting in normal pregnancy outcome.