A correlation between B chromosome frequency and sex ratio in Exochomus quadripustulatus

A survey of natural populations of the British ladybird Exochomus quadripustulatus revealed the presence of a single large, acrocentric, supernumerary (B) chromosome in all sites visited. Studies were confined to male meiosis, where more than one B was never found to accompany the six bivalents and neo-XY sex pair. The percentage of males possessing B chromosomes varied from 6.4% to 28.6% in 14 different populations. The sex ratios present in these populations also varied. In some equal numbers of males and females were present, in others there were significant excesses of females. A linear regression was found between the percentage of B chromosomes and the percentages of males and females in those populations. It is suggested that the B chromosomes are not in themselves responsible for the sex ratio differences found for similar differences in sex ratio have been found in related neo-XY species lacking B chromosomes. It is more likely that those factors affecting sex ratio are also responsible for affecting the frequencies of B chromosomes in different populations.     

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.     

Distribution of a supernumerary chromosome system and aspects of karyotypic evolution in the genus Prochilodus (Pisces,Prochilodontidae)

A cytogenetic study on eight species of the genus Prochilodus, family Prochilodontidae, from different hydrographic basins of Brazil is reported. All species had a diploid number of 2n=54 and NF=108. Supernumerary heterochromatic chromosomes of variable frequency were detected in Prochilodus scrofa and in P. cearensis. To determine the occurrence and frequency of the supernumerary chromosomes in P. scrofa, specimens from different localities belonging to the basin of the Paraná river were analysed. The high karyotypic similarity detected demonstrated that, at least in these eight species, evolution occurred without marked changes in chromosome macrostructure.     

Parallel polymorphism for supernumerary segments inChorthippus parallelus (zetterstedt)

A complex and parallel pattern of polymorphism for heterochromatic supernumerary segments in the M₇ and S₈ chromosomes has been found in 14 populations of the meadow grasshopperChorthippus parallelus. Nine distinct karyotype classes for these two chromosome pairs occur though they are not equally represented in different populations. Populations differ also with respect to the frequency of supernumerary segments they contain. In all populations the presence of supernumerary segments leads to a significant elevation of mean cell chiasma frequency compared to individuals from the same population lacking such segments. The extent of the effect appears to differ in different populations. The observed frequencies of S₈ karyotypes conform to the expectations of a Hardy-Weinberg distribution. Those of the M₇, however, do not, and in all but one of the 14 populations there is a significant excess of homokaryotypes. In the Ashurst population 26% of the individuals sampled were characterised by germ-line polysomy for the M₄ chromosome, either in the form of entire tetrasomics or more usually as mosaics ranging from tri- to hepta-somics. In all these polysomics the M₄ chromosomes in excess of two were regularly heteropycnotic at first meiotic prophase from zygotene to diakinesis. As a consequence of this multivalents are rare. Extra M₄ chromosomes do not modify the chiasma characteristics of the other chromosomes in the complement. Nor do they modify the action of the supernumerary segments in any way.     

Cytogeography and chromosome evolution of subgenus Tridentatae of Artemisia (Asteraceae)

The subgenus Tridentatae of Artemisia (Asteraceae: Anthemideae) is composed of 11 species of various taxonomic and geographic complexities. It is centered on Artemisia tridentata with its three widespread common subspecies and two more geographically confined ones. Meiotic chromosome counts on pollen mother cells and mitotic chromosome counts on root tips were made on 364 populations ( = 3.1 plants per population). These population counts are ～60% of all Tridentatae counts. Some are first records for taxa. The Tridentatae are a polyploid complex (x = 9) with ploidy levels from 2x to 8x, but mostly 2x (48%) and 4x (46%). Polyploidy occurs in nine of the 11 species and in many subspecies as well. Supernumerary or b chromosomes are present only at a low frequency. In the principal species, A. tridentata, 2x plants are larger than 4x ones, which are adapted to drier conditions, probably in consequence of their slower growth rates. Gigas diploidy is a phenomenon shared by some other woody genera, but is in contrast to the gigas polyploid nature of many herbaceous genera. Polyploidy occurs within populations and is essentially autoploid. Hybridization sometimes occurs at taxa interfaces in stable hybrid zones. Stable Tridentatae hybrid zones coupled with the group's inherent propensity for polyploidization has led to the establishment of a geographically and numerically large and successful complex of species.     

THE GENUS COLLINSIA. XVI. SUPERNUMERARY CHROMOSOMES

Dhillon , T. S., and E. D. Garber . (U. Chicago, Chicago, Ill.) The genus Collinsia. XVI. Supernumerary chromosomes. Amer. Jour. Bot. 49 (2) : 168–170. 1962.—The pairing and transmission of supernumerary chromosomes in Collinsia solitaria and in hybrids between C. sparsiflora subsp. arvensis and C. bruceae were studied. Two supernumerary chromosomes usually formed a bivalent with 1 chiasma at metaphase I; 4 supernumerary chromosomes occasionally yielded a trivalent and univalent at this stage. Plants with 2–4 supernumerary chromosomes were fertile and plants with 5–8, sterile. Plants with a given number of supernumerary chromosomes when used as seed or pollen parent gave gametes with a higher number of such 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.     

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.     

Supernumerary chromosomes in the second outbreeding species of the wheat group

Meiosis was examined in PMC of 138 plants of the outbreeder Triticum speltoides (=Aegilops speltoides) and 175 plants of the closely related T. longissimum (= Ae. longissima), which is a selfer. Nine of the T. speltoides plants, originating fra two of the five Israeli populations under study, contained B chromosomes in addition to the normal complements (2n = 14). No supernumerary chromosomes were found in T. longissimum. — Plants were found carrying 1, 2 or 3 B chromosomes, which seem to be stable among PMC. Their size is 2/3 of the average length of the regular chromosomes, but otherwise they are similar in appearance. Pairing between Bs is common as bivalents and trivalents, but whether this association is ehiasmatic is not clear. There is a slight reduction in chiasma frequency of the regular complement in B-carriers compared to other plants from the same population. — The only other species in the wheat group reported to have B chromosomes in natural populations is also an outbreeder, namely T. tripsacoides (=Ae. mutica). The connection between outbreeding and B chromosomes might be significant.     

Selfish DNA and breeding system in flowering plants

In many species, some individuals carry one or more B chromosomes: extra, or supernumerary chromosomes not part of the normal complement. In most well-studied cases, Bs lower the fitness of their carrier and persist in populations only because of accumulation mechanisms analogous to meiotic drive. It has been suggested that such genomic parasites are expected to persist only in outcrossed sexual species, in which uninfected lines of descent can be continuously reinfected; in inbred or asexual species, all selection is between lines of descent, and the genomic parasites are either lost or must evolve into commensals or mutualists. Here we present a simple population genetic model of the effect of outcrossing rate on the frequency of B chromosomes, and find that outcrossing facilitates the spread of parasitic Bs, but inhibits the spread of mutualists. Data compiled from the literature on breeding system and B chromosomes of British plants indicate that Bs are much more likely to be reported from obligately outcrossed species than inbred species. These results support the ideas that most B chromosomes are parasitic, and that breeding systems play a central role in the biology of selfish genes.     

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.     

Investigation on the frequency of B-chromosomes inImpatiens balsamina and the appearance of single flowers on double varieties

A survey of 17 varieties ofImpatiens balsamina for their chromosomal status and frequency of reversion of double flowering to single type was carried out on a local garden population. All the varieties are diploid with 2n=14. Two supernumerary chromosomes, which invariably form a bivalent that is indistinguishable from bivalents of the normal complement, were present in almost all the 12 double flowering varieties while single varieties had only the normal chromosomal complement. Their frequency in double varieties has been studied. No double flower was noted on any single type while single flowers were present in varying frequency on plants of double varieties. The highest frequency of this reversion as well as of supernumerary chromosomes was noted in var. “Deep pink double”. Possible origin of supernumerary chromosomes and their role in this species is discussed.     

Low-pass single-chromosome sequencing of human small supernumerary marker chromosomes (sSMCs) and <Emphasis Type="Italic">Apodemus</Emphasis> B chromosomes

Supernumerary chromosomes sporadically arise in many eukaryotic species as a result of genomic rearrangements. If present in a substantial part of species population, those are called B chromosomes, or Bs. This is the case for 70 mammalian species, most of which are rodents. In humans, the most common types of extra chromosomes, sSMCs (small supernumerary marker chromosomes), are diagnosed in approximately 1 of 2000 postnatal cases. Due to low frequency in population, human sSMCs are not considered B chromosomes. Genetic content of both B-chromosomes and sSMCs in most cases remains understudied. Here, we apply microdissection of single chromosomes with subsequent low-pass sequencing on Ion Torrent PGM and Illumina MiSeq to identify unique and repetitive DNA sequences present in a single human sSMC and several B chromosomes in mice Apodemus flavicollis and Apodemus peninsulae. The pipeline for sequencing data analysis was made available in Galaxy interface as an addition to previously published command-line version. Human sSMC was attributed to the proximal part of chromosome 15 long arm, and breakpoints leading to its formation were located into satellite DNA arrays. Genetic content of Apodemus B chromosomes was species-specific, and minor alterations were observed in both species. Common features of Bs in these Apodemus species were satellite DNA and ERV enrichment, as well as the presence of the vaccinia-related kinase gene Vrk1. Understanding of the non-essential genome elements content provides important insights into genome evolution in general.     

CHROMOSOMAL DIFFERENTIATION IN CLARKIA DUDLEYANA

Snow , Richard . (U. California, Davis.) Chromosomal differentiation in Clarkia dudleyana. Amer. Jour. Bot. 47 (4) : 302—309. Illus. 1960.—Clarkia dudleyana (n=9) is a common, colonial annual of the early-summer California flora. Of 275 individuals, derived from 9 natural populations and their garden-grown representatives, 17.1% were heterozygous for reciprocal translocations. Supernumerary chromosomes were also found in about 2% of the plants examined. The translocation heterozygotes are not distributed regularly over the species range but are concentrated near the geographical center of distribution. Most of the populations contained none or only a few heterozygotes, but in one colony 69% of 42 plants sampled were heterozygous. Judging from the meiotic metaphase associations observed, at least 5 different chromosome arrangements are present at this locality. Hybrids between colonies have invariably been translocation heterozygotes, the largest association found in such hybrids being a chain of all 18 chromosomes (a potential ring of 18). No correlation is evident between geographical separation and degree of cytological differentiation. Heterozygotes with smaller rings of 4 or 6 chromosomes, whether from natural populations or resulting from interpopulation hybridization, are highly fertile owing to the regular alternate disjunction of the chromosomes of the rings. In the larger rings of 12 to 18 chromosomes, derived from interpopulation crosses, segregation is much more irregular and leads to high sterility. It is possible that at least in some localities the heterozygotes enjoy a selective advantage over their homozygous sibs. It is also postulated that homozygosity for a particular chromosome arrangement may be selectively favored in a certain habitat, as a result of a position effect attendant upon placing formerly non-linked genes in the same linkage group through reciprocal translocation. The high degree of chromosomal differentiation between some populations of this species suggests that the complex heterozygotes of Oenothera have arisen as a result of hybridization of cytologically differentiated races.     

Intercalation and the cellular origin of supernumerary limbs in Xenopus

The hypothesis that a specialized polarizing zone controls the pattern of the anterior-posterior axis during limb development in Xenopus has been tested by analysing the cellular contribution to supernumerary limbs. Supernumerary limbs were generated by grafting hindlimb buds contralaterally between X. borealis and X. laevis to appose anterior and posterior limb tissues. Cells derived from these two species of Xenopus are readily identified by staining with quinacrine. The analysis of cellular contribution showed that supernumerary limbs consist of approximately half anterior-derived (57%) and half posterior-derived (43%) cells. These data are not consistent with the polarizing zone theory but are consistent with the hypothesis that both supernumerary limbs and normally developing limbs arise from intercalary interactions between limb bud cells with different positional values.     

The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S₁₁ chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S₁₀ and S₁₁ chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.On educational leave from the Forest Research Laboratory, Fredericton, N. B. Canada.     

Interaction among different heterocromatic variants in the grasshopper Dichroplus elongatus

Simultaneous chromosome polymorphisms for supernumerary elements allow us to analyse the relationships among different forms of heterochromatic variation in nature. We report simultaneous variation patterns for supernumerary segments in chromosomes S10 (SS10), S9 (SS9) and S6 (SS6) and B chromosomes in nine populations of the grasshopper Dichroplus elongatus from two biogeographic provinces from east Argentina. Our results show spatial chromosome differentiation for three out of four supernumerary heterochromatic variants (B chromosomes, SS6 and SS10). The incidence of B chromosomes was negatively correlated with the SS10 frequency. The distribution pattern analysis shows different degree of differentiation among populations for each supernumerary heterochromatic variant suggesting that the detected chromosome variation cannot be explained by interaction between migration and genetic drift. Moreover, the observed population chromosome differentiation was not in agreement with the hierarchical analysis of molecular of heterogeneity at mitochondrial DNA level (mtDNA). The present results point out the importance of the interaction among heterochromatic variants in the chromosome intraspecific variation in east Argentina natural populations of the grasshopper D. elongatus.     

A sequence specific to B chromosomes of Brachycome dichromosomatica

Supernumerary B chromosomes represent one of many causes of numerical chromosome variation that exist in higher plants and animals. Sequences of DNA unique to B chromosomes of Brachycome dichromosomatica were enriched prior to cloning and resultant clones hybridizing only to plants containing B chromosomes were further investigated. Sequences of DNA that were characterised include members of a family of 176-bp tandem repeats that are specific to the B chromosomes of B. dichromosomatica, an annual Australian native plant species with only two pairs of A chromosomes and up to three dispensable B chromosomes. Sequence analysis of these six related clones indicated that some regions of the sequence are more highly conserved than others or, alternatively, that some adenine residues at the NdeII site are methylated. The repeat is homologous to DNA from Brachycome ciliaris var. languinosa but not to DNA from other related taxa growing in the vicinity of the B. dichromosomatica populations.     

The Genome of Nectria haematococca: Contribution of Supernumerary Chromosomes to Gene Expansion

The ascomycetous fungus Nectria haematococca, (asexual name Fusarium solani), is a member of a group of >50 species known as the “Fusarium solani species complex”. Members of this complex have diverse biological properties including the ability to cause disease on >100 genera of plants and opportunistic infections in humans. The current research analyzed the most extensively studied member of this complex, N. haematococca mating population VI (MPVI). Several genes controlling the ability of individual isolates of this species to colonize specific habitats are located on supernumerary chromosomes. Optical mapping revealed that the sequenced isolate has 17 chromosomes ranging from 530 kb to 6.52 Mb and that the physical size of the genome, 54.43 Mb, and the number of predicted genes, 15,707, are among the largest reported for ascomycetes. Two classes of genes have contributed to gene expansion: specific genes that are not found in other fungi including its closest sequenced relative, Fusarium graminearum; and genes that commonly occur as single copies in other fungi but are present as multiple copies in N. haematococca MPVI. Some of these additional genes appear to have resulted from gene duplication events, while others may have been acquired through horizontal gene transfer. The supernumerary nature of three chromosomes, 14, 15, and 17, was confirmed by their absence in pulsed field gel electrophoresis experiments of some isolates and by demonstrating that these isolates lacked chromosome-specific sequences found on the ends of these chromosomes. These supernumerary chromosomes contain more repeat sequences, are enriched in unique and duplicated genes, and have a lower G+C content in comparison to the other chromosomes. Although the origin(s) of the extra genes and the supernumerary chromosomes is not known, the gene expansion and its large genome size are consistent with this species'' diverse range of habitats. Furthermore, the presence of unique genes on supernumerary chromosomes might account for individual isolates having different environmental niches.     

Is the aneuploid chromosome in an apomictic Boechera holboellii a genuine B chromosome?

The Boechera holboellii complex comprises B. holboellii and B. drummondii, both of which can reproduce through sex or apomixis. Sexuality is associated with diploidy, whereas apomictic individuals can either be diploid, aneuploid or triploid. Aneuploid individuals are found in geographically and genetically distinct populations and contain a single extra chromosome. It is unknown whether the supernumerary chromosomes are shared by common descent (single origin) or have originated via introgressive hybridizations associated with the repeated transition from diploidy to triploidy. Diploid plants containing the extra chromosome(s) reproduce apomictically, suggesting that the supernumerary elements are associated with apomixis. In this study we compared flow cytometry data, chromosome morphology, and DNA sequences of sexual diploid and apomictic aneuploids in order to establish whether the extra chromosome fits the classical concept of a B chromosome. Karyotype analyses revealed that the supernumerary chromosome in the metaphase complement is heterochromatic and often smaller than the A chromosomes, and differs in length between apomictic plants from different populations. DNA sequence analyses furthermore demonstrated elevated levels of non-synonymous substitutions in one of the alleles, likely that on the aneuploid chromosome. Although the extra chromosome in apomictic Boechera does not go through normal reductional meiosis, in which it may get eliminated or accumulated by a B-chromosome-specific process, its variable size and heterochromatic nature does meet the remaining criteria for a genuine B chromosome in other species. Its prevalence and conserved genetic composition nonetheless implies that this chromosome, if truly a B, may be atypical with respect to its influence on its carriers.