Supernumerary chromosomes in Bandicota indica nemorivaga and a female individual with XX/XO mosaicism

Supernumerary chromosomes and an XX/XO mosaic individual of B. i. nemorivaga are described. The supernumeraries are small metacentric chromosomes and are stained all along their length in C-band preparations. They have morphology and staining characteristics similar to those observed in different populations of Rattus rattus. Extensive G-band similarity of the chromosomes of B. i. nemorivaga and R. rattus and the size, shape and staining behaviour of the supernumerary chromosomes in these genera suggest that they have acquired supernumeraries from a common ancestor.     

A mitotically unstable supernumerary chromosome with an accumulation mechanism in a grasshopper

Summary A population of the grasshopper Calliptamus palaestinensis Bdhr. from near Jerusalem, Israel was analyzed cytologically. Twenty-two percent of the males had extra heterochromatic chromosome segments or supernumerary chromosomes. The extra segments were present in either a heterozygous or a homozygous condition and in frequencies similar to those expected according to the Hardy-Weinberg law.The supernumeraries were heteropycnotic in the first meiotic division but paired and segregated regularly. In almost all the males with supernumeraries the number of supernumeraries varied between follicles in the testes but was constant within each follicle. Only 12.3% of the males had supernumeraries, but among those with supernumeraries the majority had means higher than 1.5. A calculation based on the frequency of the supernumeraries in the zygotes which are expected in the next generation indicated that of the 31 males with supernumeraries, probably not more than 3 males originated from zygotes with more than a single supernumerary. The variation in the number of supernumeraries and the high means probably resulted from preferential non-disjunction of the supernumeraries in the early mitoses prior to the differentiation of the follicles.Mitotic non-disjunction of supernumeraries as the cause of intra-individual variation is considered. It is suggested that variation due to non-disjunction would be maintained only in association with accumulation mechanisms.Postdoctoral Trainee in Biology, U. S. Public Health Service, 1962–1963.     

CYTOLOGY OF SUPERNUMERARY CHROMOSOMES IN HAPLOPAPPUS SPINULOSUS SSP. COTULA

Li , Ning , and R. C. Jackson . (U. Kansas, Lawrence.) Cytology of supernumerary chromosomes in Haplopappus spinulosus ssp. cotula. Amer. Jour. Bot. 48(5): 419–426. Illus. 1961.—The behavior of supernumerary chromosomes was investigated in 2 plants. In 1, the number of supernumeraries was highly variable in both somatic and meiotic cells, presumably as a result of somatic non-disjunction. In the second plant, the number of supernumerary chromosomes was less variable, but an end-to-end association of a pair of supernumeraries and a regular bivalent was observed in a small percentage of the microsporocytes at zygotene and pachytene. This end-to-end association apparently resulted in a permanent fusion of the supernumerary and regular chromosome ends in some instances because the union persisted throughout both meiotic divisions. There was no evidence of normal pairing between the supernumeraries and regular chromosomes.     

The distribution of repetitive DNAs between regular and supernumerary chromosomes in Species of Glossina (Tsetse): a two-step process in the origin of supernumeraries

Several species of tsetse fly within the Morsitans and Fusca subgenera of Glossina contain supernumerary (B) chromosomes. Previous studies on the meiotic behaviour of chromosomes (Southern and Pell, 1973) and the C-band patterns (Jordan et al., 1977) have indicated a close similarity between the Y chromosome and the supernumeraries. The distributions of the highly abundant families of DNA (satellite DNAs) between the autosomes, sex chromosomes and B chromosomes of G.m. morsitans, G. austeni and G. pallidipes have been examined by in situ hybridisation. In addition, the organisation and sequence homologies of satellite DNAs have been examined by restriction enzymes and heterologous hybridisations in in situ and Southern transfer conditions. The majority of satellite sequences that are homologous between species are distributed in several different arrangements between A and B chromosome telomeres with minority sequences at some centromeric and intercalary locations. There is no extensive satellite DNA similarity between the Y and B chromosomes. We suggest that the Y and B chromosome associations and synchronous allocycly during meiosis are the result of extensive heterochromatinisation of these two chromosome types, that is probably a reflection of two separate stages involved in the generation of the B chromosomes in the genus. The independent evolution of satellites and supernumeraries is discussed.     

Supernumerary chromosomes in the human bed bug,Cimex lectularius Linn. (Cimicidae:Hemiptera)

The chromosome complement in the human bed bug, Cimex lectularius Linn., is 26+X₁X₂Y in the male and 26+X₁X₁X₂X₂ in the female. However, a population from Cairo, Egypt has 4 supernumerary X chromosomes. In the hybrid between the Berkeley population (with no supernumeraries) and the Cairo population (with 4 supernumeraries), the behavior of supernumeraries was observed during embryogenesis and oogenesis as well as spermatogenesis.In embryogenesis the transmission of supernumeraries was quite regular. However, one chromosome may sometimes be eliminated early in the germ line. This abnormality could induce the variations in chromosome number encountered in later stages. In the first meiotic division, some of the supernumeraries were nondisjunctional. Moreover, in the second division, some supernumeraries were eliminated. These results show that there is a tendency towards a decrease in the number of supernumeraries in the hybrids.Although the supernumeraries behave like X chromosomes, they seem not to be important for sex determination and appear to be largely or entirely inert genetically. Supernumeraries in the bed bug originate from small fragments caused by structural rearrangement. They are increased by an accumulation mechanism. Supernumeraries in the bed bug appear to be of relatively recent origin. The phylogenetic sequence in their development was probably from none to a stabilized number of four in the Old World. Then the supernumeraries were lost in two specialized lines, Cimex columbarius Jenyns on domestic birds in Europe and Cimex lectularius Linn. on man in the Western Hemisphere.This study was carried out under U.S. Public Health Service Grant (GM-13197).     

Supernumerary (B-) chromosomes in the Malayan house rat,Rattus rattus diardii (Rodentia,Muridae)

Three widely separated Malayan populations of Black rat, Rattus rattus diardii (Jentink), were found to comprise individuals with supernumerary (or B) chromosomes. The number of supernumeraries varied from 0 to 4. The incidence of B's in the three populations did not differ significantly. There was also no difference between the two sexes.     

Chromosomal polymorphism caused by supernumerary chromosomes in the field mouse,Apodemus giliacus

Chromosome studies were made on 74 animals of the field mouse, Apodemus giliacus, a new form of the genus Apodemus described by Kobayashi and Hayata (1970). Extreme variations in number and morphology of chromosomes was observed. While the diploid numbers varied from 48 to 61, the number of acrocentric elements was consistently 48, except for one specimen which had 40 such elements. In contrast, the number and constitution of several biarmed elements and microchromosomes were highly variable in the complement, and, hence, responsible for the polymorphism. Karyotype analysis of meiotic chromosomes, on the basis of Giemsaand quinacrine-stained preparations, some familial studies and breeding experiments revealed that variable elements were supernumeraries of a hitherto undescribed type, whereas the 48 acrocentrics seemed to represent regular autosomes and sex elements. Most of the supernumeraries did not show pairing at metaphase I, but some did form bivalents. Usually, the supernumerary biarmed chromosomes were of regular size and fluoresced rather brightly over their entire length, suggesting that they were heterochromatic in nature. Considering the above findings and other relevant data of some allied species, the differentiation between A. giliacus and A. speciosus might have occurred rather recently, when the former species might have been involved in rapid and divergent chromosomal evolution.Contributions from the Chromosome Research Unit, Sapporo, Japan.     

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.     

The effect of B-chromosomes on chiasma frequency in wild populations of rye

The effect of supernumerary chromosomes on the chiasma characteristics of the standard A-set of rye has been studied in two wild populations from Yugoslavia carrying from one to four B-members. The mean cell chiasma frequency increases significantly with increasing number of supernumeraries.     

Parallel mosaicism of supernumerary chromosomes and sex chromosomes in Echymipera kalabu (Marsupialia)

Echymipera kalabu (Peramelidae: Marsupialia) does not have the full chromosome complement in all its adult somatic tissues. The chromosomes missing are the Y-chromosome in the male and an X-chromosome in the female. The full complement is present in the corneal epithelium and the reproductive tissue. A parallel mosaicism to this exists with respect to small supernumerary chromosomes which are found in certain animals of this species. These supernumeraries must be subject to the same control system as that which is responsible for the elimination of the sex chromosomes.     

SUPERNUMERARY CHROMOSOMES IN SAXIFRAGA VIRGINIENSIS (SAXIFRAGACEAE)

Acetocarmine squashes of root tips have demonstrated that 2n = 20 and 38 in Saxifraga virginiensis. These contrast with the earlier reported count of 2n = 28 for this species. In several populations supernumerary chromosomes were detected. Both intrapopulational and interpopulational variation in supernumerary chromosome number were detected, with the largest number of supernumerary chromosomes observed being six. Because these supernumerary chromosomes are equal in size to many of the smaller A chromosomes during mitotic metaphase, the presence of supernumerary chromosomes in this species could not be ascertained by analysis of mitotic metaphase preparations alone. During mitotic prophase, however, the supernumerary chromosomes of S. virginiensis are highly heterochromatic, appearing more densely coiled and darkly stained than the A chromosomes. This characteristic facilitated the recognition of supernumerary chromosomes in this species. The similarity in size of A and supernumerary chromosomes during mitotic metaphase and the observation of six supernumerary chromosomes in one population suggest that the count of 2n = 28 reported earlier for S. virginiensis may actually be a misinterpretation of 2n = 20 plus 8 supernumerary chromosomes. Furthermore, these findings and the observation of this same supernumerary chromosome phenomenon in other species of Saxifraga raise the possibility that some of the many disparate chromosome counts attributed to aneuploidy in the large genus Saxifraga may also be the result of misinterpretations of supernumerary chromosomes as A chromosomes.     

A CYTOGENETIC STUDY OF CLARKIA UNGUICULATA II. SUPERNUMERARY CHROMOSOMES

Mooring , John S. (Washington State U., Pullman.) A cytogenetic study of Clarkia unguiculata. II. Supernumerary chromosomes. Amer. Jour. Bot. 47(10): 847–854. Illus. 1960.—Supernumerary chromosomes morphologically comparable to those of the basic complement occur in high frequency in wild populations of this onagraceous annual plant. This paper evaluates the role that these supernumerary chromosomes play in the population dynamics of this species. The results of crossing plants with various numbers of supernumeraries showed that these chromosomes often increased in number in the seed parent. Examination of microsporocytes showed that almost 30% of the 491 plants determined had 1 or more supernumerary chromosomes. Studies of natural populations revealed that the frequency of individuals with supernumeraries varied from 0 to 79% in different populations, and also that in 1 population their frequency varied significantly in different years. It is concluded that supernumerary chromosomes in this species are adaptive under at least certain conditions. It is speculated that they are components of polygenic systems which increase tolerance to environmental extremes.     

The cytogenetic structure of Tasmanian populations of Phaulacridium vittatum

In 8 out of 20 Tasmanian populations of Phaulacridium vittatum from 0.3-11.0 percent of the males carried a single supernumerary chromosome. In such males the X and B univalents are both heteropycnotic at first prophase of male meiosis and associate with another in a non-homolgous manner in about two-thirds of the diplotene cells examined. In all 56 B-containing individuals studied, however, these associations lapse by first metaphase and the X and the B move at random with respect to one another at first anaphase. The supernumerary in this species is stable and only 5 of the 56 individuals with supernumeraries showed evidence of B-chromosome non-disjunction in the pre-meiotic mitoses. Since there was no other evidence for the loss or gain of supernumeraries in the male line it is clear that B-transmission is regular in the males of this species. There were significant differences between population with respect to mean cell chiasma frequency but there was no significant effect of the B-chromosome on this metric. Additionally a comparison of mean cell chiasma frequency in follicles with and without supernumeraries from a mosaic individual shows no significant difference.     

A stably transmitted pair of translocated supernumerary chromosomes in maize.

A pair of stably transmitted supernumerary chromosomes of unknown source has been found in a maize stock carrying a desynaptic mutant. The presence of the supernumerary chromosome appears to be unrelated to the meiotic mutant, but is believed to have been derived from a translocated B chromosome contaminant. The supernumerary chromosomes carry a segment of a A chromosome in this stock where there appear to be two normal copies of each of the 10 A chromosomes. Thus, this A chromosome segment is present in quadruplicate. Surprisingly, a quadrivalent configuration is formed in most microsporocytes, which involves not only synapsis but also chiasma formation in the A chromosome segments involved in the quadrivalent. This represents a strong preferential pairing of supernumeraries with the normal A chromosome segments. Such nonrandom association and crossing over might provide information on the nature of early homologue alignment at meiosis.     

Persistent meiotic association of a rare B chromosome and an autosomal segment inAtractomorpha similis (Orthoptera: Acridoidea)

Supernumerary chromosomes have been reported in many species particularly in the Orthoptera. Although it is assumed that these chromosomes are ultimately derived from elements of the standard karyotype, in no case recorded is there convincing evidence of even partial homology between true supernumeraries and other chromosomes. In the present paper, a very rare B chromosome in the grasshopperAtractomorpha similis is reported. It associates specifically and persistently with a large segment on Autosome 5 (A5). Associations were seen in 29 out of 288 meiocytes scored, and persisted to Anaphase I. At least four of the 29 associations appear to be of a genuinely chiasmate nature, implying some degree of homology. The A5 segment is itself supernumerary, occurring in 8% of individuals scored. Supernumerary segment polymorphism is extremely common in this species, as is germ line polysomy for Autosome 9. In the light of these pecularities, a possible mode of derivation, involving Autosome 9, is proposed for the B chromosome in question. Supernumeraries of the closely related speciesA. australis are also discussed insofar as they suggest the occurrence of seemingly diverse modes of B chromosome derivation within the genusAtractomorpha.     

The evolution of hypervariable sex and supernumerary (B) chromosomes in the relict New Zealand frog,Leiopelma hochstetteri

Chromosomes exhibiting elevated levels of differentiation are termed hypervariable but no proposed mechanisms are sufficient to account for such enhanced evolutionary divergence. Both hypervariable sex and supernumerary (B) chromosomes were investigated in the endemic New Zealand frog, Leiopelma hochstetteri, which is chromosomally polymorphic both within and between populations and has sufficiently elevated variation that different populations can be identified solely by their C-banded karyotypes. This frog is further distinguished by the univalent, female-specific W-chromosome (0W/00 sex determination) uniquely possessed by North Island populations. This sex chromosome exhibited variation in morphology, size, and heterochromatin distribution, sufficient to resolve 11 different types, including isochromosomes. Five of the 12 populations examined also had supernumerary chromosomes that varied in number (up to 15 per individual) and morphology. Specific variations seen among the hypervariable chromosomes could have resulted from heterochromatinisation, chromosome fusions, loss-of-function mutations, deletions, and/or duplications. Frogs of the same species from Great Barrier Island, however, had neither supernumeraries nor the female-specific chromosome. The 0W/00 sex chromosome system must have been derived after the isolation of Great Barrier Island from North Island populations by raised sea levels between 14 000 and 8000 years ago. Furthermore, biochemical divergence between populations is minor and therefore the chromosomal variation seen is comparatively recent in origin. The one characteristic common to all known hypervariable chromosomes is curtailment or lack of recombination. Their accelerated evolution therefore is possible via the mechanism of Muller's ratchet, either alone or in concert with other factors.     

The B-chromosome system of the grasshopper Eyprepocnemis plorans subsp. plorans (Charpentier)

Supernumerary chromosomes of two types have been observed in the grasshopper Eyprepocnemis plorans subsp. plorans. One of these (the B-type) is similar in size to an S autosome; the other is smaller (B-type). Both are telocentric and mitotically stable. The frequencies of individuals with the B-type supernumeraries in five natural populations were 56, 56, 70, 71 and 30 per cent respectively. The equivalent levels of the B-type supernumerary were 0, 0, 13, 3 and 0 per cent respectively. Because of the relative infrequency of the B-type only the B-type has been studied in detail. In males with 1B, anaphase I segregation of X and B was random in four populations but non-random in that from Otivar. Here the B was distinctive in having a secondary constriction near the centromere. A study of chiasma frequency among A-chromosomes revealed that the B-type supernumerary increases significantly both the mean chiasma frequency and the between-cell variance. A comparison of body morphometrics failed to reveal any effect of these B-chromosomes on the exophenotype.     

SUPERNUMERARY CHROMOSOMES IN TRADESCANTIA II. EFFECT OF COLCHICINE ON MEIOTIC ISOCHROMOSOME CONFIGURATIONS

The effect of colchicine on meiotic pairing and configuration frequencies of three homologous isosupernumerary chromosomes was investigated. In the absence of colchicine, the three isochromosomes displayed a high degree of interchromosomal pairing and chiasma formation. As a consequence, a high frequency of bivalents and trivalents were observed at diakinesis-metaphase I. The unique structure of isochromosomes enables them to pair intrachromosomally (i.e., foldback pairing) yet the preferential occurrence of interchromosomal pairing suggests that all six arms of the three isosupernumerary chromosomes were in close association prior to or upon initiation of synapsis. Supernumerary chromosomes in microsporocytes treated during presynapsis or early synapsis with colchicine exhibited a significant reduction (P < 0.001) in the number of bivalents and trivalents at diakinesis. However, there was no reduction in overall chiasma frequency among supernumeraries due to the induction of increased intrachromosomal pairing and chiasma formation. A colchicine-sensitive association or alignment of homologues preceding effective pairing has been demonstrated in standard chromosomes of a number of plant species. This study provides the first evidence to indicate that at least certain supernumerary chromosomes may display presynaptic association as well. The results also support the strongly held contention that colchicine is not directly preventing or inhibiting the actual formation of chiasmata, since no reduction in chiasma frequency was observed in the isochromosomes.     

The supernumerary B chromosome of maize: drive and genomic conflict

The supernumerary B chromosome of maize is dispensable, containing no vital genes, and thus is variable in number and presence in lines of maize. In order to be maintained in populations, it has a drive mechanism consisting of nondisjunction at the pollen mitosis that produces the two sperm cells, and then the sperm with the two B chromosomes has a preference for fertilizing the egg as opposed to the central cell in the process of double fertilization. The sequence of the B chromosome coupled with B chromosomal aberrations has localized features involved with nondisjunction and preferential fertilization, which are present at the centromeric region. The predicted genes from the sequence have paralogues dispersed across all A chromosomes and have widely different divergence times suggesting that they have transposed to the B chromosome over evolutionary time followed by degradation or have been co-opted for the selfish functions of the supernumerary chromosome.