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.     

Effects of B chromosomes and supernumerary segments on morphometric traits and adult fitness components in the grasshopper, Dichroplus elongatus (Acrididae)

The South American species, Dichroplus elongatus, is polymorphic for B chromosomes and supernumerary segments in chromosome pairs M6 (SS6), S9 (SS9) and S10 (SS10). Both forms of supernumerary heterochromatin shape chiasma frequency and distribution and B chromosomes also affect male fertility. Here, we analysed the effects of these polymorphisms on morphometric traits (total, 3rd femur, 3rd tibia, thorax and tegmen lengths) and several adult fitness components, including male and female mating success, and female reproductive potential. B chromosomes tend to decrease, and SS6 segments to increase the body size of carriers. The analysis of reproductive potential suggested that B chromosome carrying females have higher numbers of embryos per clutch and ovarioles per ovary. The uni- and multivariate analysis of mating success revealed that sexual selection favours larger individuals of both sexes and males with standard karyotype. B chromosomes may have accumulation mechanisms, which involve preferential transmission of B chromosomes to germ cells or functional gametes. The maintenance of Bs might be explained by interactions among accumulation mechanisms and trade-offs between detrimental and favourable effects on different fitness components.     

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.     

The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: III. mutation rate of B chromosomes

B chromosome variation in nine Moroccan populations of the grasshopper Eyprepocnemis plorans was analysed for 3 consecutive years. In addition to B1, which was the predominant B chromosome in all nine populations, we found 15 other B variants, albeit at very low frequency. Eight variants were found in adults caught in the wild, four appeared in adults reared in the laboratory and seven were found in embryo progeny of controlled crosses between a 0B male and a B-carrying female. Some variants were found in more than one kind of material. At least the seven B variants that appeared in embryo progeny of females carrying a different B type arose de novo through mutation of the maternal B chromosome. The mutation rate of B chromosomes was 0.73%, on average, which explains the high variety of morphs and banding patterns found. The most frequent de novo mutations observed in these chromosomes were centromere misdivision with or without chromatid nondisjunction, which generates iso-B-chromosomes or telocentric Bs, respectively, as well as translocations with A and B chromosomes and deletions. But the whole variation observed, including that found in adult individuals, suggests that other mutations such as duplications, inversions and centric fusions do usually affect B chromosomes. Finally, B chromosome mutation rate was remarkably similar in both Moroccan and Spanish populations, which suggests that it might be dependent on B chromosome intrinsic factors.     

B chromosome morphotypes of Apodemus peninsulae (Rodentia) from the Russian Far East

Polymorphism of B chromosome morphotypes in the natural populations of the Korean field mouse Apodemus peninsulae (n = 367) has been studied in five regions of the Russian Far East: Magadan and Amur regions, the Jewish Autonomous region, Khabarovsk and Primorskii regions. Six groups of B chromosome morphotypes have been described on the size and morphological parameters. On the basis of their combinations 78 cell clones have been revealed, 23 of which are found in the animals with a stable karyotype and 76--in mosaics. The mosaics have also more diverse and unique combinations of B chromosome morphotypes. No differences between the geographic and local populations of mice on the studied characteristics have been found. Homogeneous structure of DNA in the B chromosomes of this species at the territory of the Russian Far East, as shown previously, allowed us to combine the numerical and dimensional data for each clone by introduction conditional "mass quantity" of B chromosomes (mB index). The leading role of natural selection in production of "critical" mass of supernumerary chromosomes in individuals with a stable karyotype and the weakening of its role in mosaics has been suggested.     

B chromosomes of maize (Zea mays L.) are positioned nonrandomly within sperm nuclei

We used fluorescence in situ hybridization to identify and map the position of B chromosomes (supernumerary chromosomes) within maize sperm cells. Observations on over 1,000 sperm cells from several genotypes show that, on average, the B chromosomes are positioned in the tip one-fourth of the sperm nucleus two-thirds of the time. In contrast, the centromeres and knobs of the A chromosomes (the normal set) are not restricted to the tip portion of the nucleus. To our knowledge, this is the first example of specific chromosome positioning within a plant gamete. Studies on nuclear architecture of somatic cells in both plants and animals suggest that chromosome behavior and gene expression may correlate with chromosome position within the nucleus. The functional significance of nonrandom positioning of the B chromosomes within maize sperm is as yet unclear. Received: 10 May 2000 / Revision accepted: 6 September 2000     

Latitudinal clines in the grasshopper Dichroplus elongatus: Coevolution of the A genome and B chromosomes?

Argentine populations of Dichroplus elongatus (Orthoptera: Acrididae) are polymorphic for B chromosomes. Previous studies showed that B chromosomes affect body size and some fitness components in Northwestern populations. We studied phenotype and B′s variation patterns along a latitudinal cline as well as the relationship between karyotype and body size related traits in 17 populations from East. Body size related traits showed a ‘saw tooth’ pattern of variation being small at low and high latitudes and large at intermediate latitudes in most of the analysed populations. Analyses of variance and principal components demonstrated that in most analysed populations B carrier males are associated with a decrease in body size related traits with respect to individuals with standard karyotype. Accordingly with the relationship between karyotype and body size, an opposite pattern of latitudinal variation in the frequencies of B′s with respect to body size variation was observed in this area. i.e. smaller individuals tend to have a higher frequency of B chromosomes. The comparison of the differentiation of both karyotype and body size traits with molecular neutral markers demonstrated the relative importance of selection moulding chromosome and phenotype variation. The observed pattern of phenotypic variation is likely to be the result of local adaptation to season length along the latitudinal gradient. The observed contrary pattern of B′s clinal variation may reflect the population ability to maintain this chromosome in relation to the local adaptation. The available evidence indicates that the distribution of B chromosome frequency was shaped by selective factors.     

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.     

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     

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.     

Double fertilization in maize: the two male gametes from a pollen grain have the ability to fuse with egg cells

In flowering plants, two male gametes from a single pollen grain fuse with two female gametes, the egg and central cells, to form the embryo and endosperm, respectively. The question then arises whether the two male gametes fuse randomly with the egg and central cells. We investigated this question using two nearly isogenic maize lines with supernumerary B chromosomes (TB10L18) or without (r-tester). B chromosomes regularly undergo non-disjunction at the second pollen mitosis, producing one sperm cell with zero B chromosomes and one with two. We first confirmed earlier studies showing an excess of transmission of the B chromosomes to the embryo rather than to the endosperm. We then tested the possibility of a directed fertilization. For TB10L18 pollen, we could demonstrate the existence of a size dimorphism between the two sperm cells, correlated to the content in B chromosomes, as detected by fluorescence in situ hybridization (FISH). However, no directed fusion of B chromosome containing sperm to egg cells could be detected when using in vitro fertilization. The absence of directed fusion in vitro could also be demonstrated for control lines. We conclude that both male gametes have the capacity to fuse with the egg cell in maize, although sexual reproduction results in a preferential transmission of supernumerary B 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 complex B chromosome system in the Korean field mouse, Apodemus peninsulae

Information on B chromosomes of six subspecies of A. peninsulae Thomas, 1906, from 79 local populations of Russia (Siberia, Altai, Buryatia and the Far East), Mongolia, China, Korea and Japan (Hokkaido) is reviewed. The frequency of animals with B chromosomes is higher in this taxon than in other mammals and ranges from 0.4 up to 1.0, excluding two insular populations (Sakhalin Island and Stenin Island, Primorye) where Bs were not found. The B chromosome polymorphism shows four levels of variation in number (intraindividual mosaicism, intrapopulational and interpopulational), as well as variability in size, morphology and differential staining. Geographic variation was found among populations for these cytogenetic characteristics and, in some cases, it coincided with subspecies distribution. Comparative chromosome banding of micro and macro Bs illuminates possible pathways for their origin.     

Transmission analysis of B chromosomes in Rattus rattus from Northern Africa

Traditionally, B chromosomes have been classified as parasitic or heterotic, depending of whether or not they show selfish behaviour. Nevertheless, experimental evidence has been found supporting the idea that supernumerary chromosomes may evolve from parasitism to neutrality. In this work, B chromosome transmission in Rattus rattus has been analysed by performing several crosses between individuals carrying different numbers of supernumerary chromosomes. Our results demonstrated a Mendelian transmission rate through males, but slight accumulation of the Bs through females. This parasitic behaviour is shared in populations as distant as Asia and Africa, and even in a related species in Australia, suggesting the possibility of an ancient origin of these supernumerary chromosomes.     

A single, recent origin of the accessory B chromosome of the grasshopper Eyprepocnemis plorans

B chromosomes are dispensable chromosomes found in >2000 eukaryotic species, usually behaving as genomic parasites. Most B chromosomes seem to be made up of the same kind of DNA sequences present in the A chromosomes. This sequence similarity makes it difficult to obtain specific molecular probes that may permit B-presence diagnosis without cytogenetic analysis. We have developed a sequence-characterized amplified region (SCAR) marker for B chromosomes in the grasshopper Eyprepocnemis plorans, which specifically amplifies a 1510-bp DNA fragment exclusively in B-carrying individuals. Fluorescent in situ hybridization and fiber FISH analyses showed that this marker is a tandemly repeated DNA sequence closely intermingled with 45S rDNA. PCR reactions showed the presence of SCAR-like sequences in the A chromosomes, but in two separate fragments, supporting the intraspecific origin of B chromosomes in this species. SCAR marker DNA sequence showed to be identical in B chromosome variants from several localities from Spain and Morocco, and it was very similar to those found in B chromosome variants from Greece and Armenia. This strongly suggests that this sequence was already present in the ancestral B chromosome of this species. In addition, the scarce sequence variation observed among several B variants from very distant populations suggests either a functional constraint or, more likely, a recent and unique origin for B chromosomes in this species.     

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.     

Cytogenetics of chromosome pairing in wheat

Meiotic chromosome pairing in Triticum aestivum is controlled by genetic systems promoting and reducing pairing. The pairing of homoeologous chromosomes is prevented principally by the activity of a single locus (Ph) distally located on the long arm of chromosome 5B. In certain hybrids, supernumerary chromosomes (B chromosomes) from Aegilops species can compensate for the absence of chromosome 5B preventing or reducing homoeologous pairing. Temperature-dependent variants and colchicine sensitivity have been used to show that there are at least two stages in the G1 of meiosis at which the occurrence of meiotic pairing is determined. Wheat may differ from lily in the detailed organization of meiosis.     

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     

The B chromosomes of the grasshopper Eyprepocnemis plorans and the intragenomic conflict

The grasshopper Eyprepocnemis plorans harbours an extremely widespread polymorphism for supernumerary (B) chromosomes, which is found in almost all circum-Mediterranean and Caucasian populations hitherto analysed. B chromosomes in this species have been shown to evolve through several stages of parasitic and near-neutral nature, presumably because of an arms race between the standard (A) and B chromosomes. This intragenomic conflict can either be solved with the extinction of the neutralised B chromosome or, more interestingly, with the replacement of the neutralised B by a mutant version being parasitic again and thus prolonging B chromosome life. This species thus provides a complete view of the long-term life-cycle of parasitic B chromosomes.