Genotype-dependent effect of B-chromosomes on chiasma frequency in Eyprepocnemis plorans (Acrididae: Orthoptera)

In order to analyze the effect of B-chromosomes on chiasma frequency, the offspring of different females of the grasshopper Eyprepocnemis plorans have been studied. From the comparison between individuals of the different families having a given number of B's and between individuals with different numbers of B-chromosomes within the same family, it can be concluded that the effect of the genetic background on chiasma frequency is greater than that produced by the presence of B's, and that there is a between-families variation in the effect of B-chromosomes which, in addition, is negatively correlated with the chiasma frequency of related individuals without B's. This genotype-dependent effect of B-chromosomes is discussed.     

The B-chromosome system of myrmeleotettix maculatus (thunb.)

It is established that in populations of Myrmeleotettix maculatus with B-chromosomes these chromosomes occur at stable frequencies and are present to the same extent in both the males and the females of the same population. It is also established that the B-chromosome content of a population is positively correlated with its chiasma frequency and that, within a population, individuals with single B-chromosomes tend to have higher chiasma frequencies than individuals lacking B-chromosomes. Since this effect is not increased by the addition of further supernumeraries it is argued that selection operates in favour of individuals with single B-chromosomes. Finally it is shown that the level of B-chromosomes in a population is related to temperature and especially to rainfall so that B-chromosomes are absent from populations in climatically stringent environments.     

The B-chromosomes of Locusta migratoria I. Detection of negative correlation between mean chiasma frequency and the rate of accumulation of the B's; a reanalysis of the available data about the transmission of these B-chromosomes

B-chromosomes were studied in two Spanish populations of Locusta migratoria. Both exhibit a high frequency of B-carrying individuals (75% and 94%). In both the B-chromosomes are mitotically unstable and they accumulate in the male germ line. The mean rate of accumulation is 28.5% in Baños de la Encina and 31.9% in Carboneras. There are no significant differences in mean cell chiasma frequency and between-cell variance of chiasmata between follicles with different number of B's within individual males. Likewise, there are no significant differences in mean cell chiasma frequency and between-cell variance between males with and without supernumeraries. However, a significant negative correlation exists between mean chiasma frequency and the rate of accumulation.     

Influence of B-chromosomes on meiosis in pearl millet

B-chromosome behaviour and the effects of B's on the endophenotype were studied in the third back-cross progeny of a cross between non-B and B-carrying parents ofPennisetum typhoides Stapf et Hubb. In the experimental material a regular increase in B-chromosome chiasma frequency and an increase in the variation of mean chiasma frequency with increased number of B's was observed. When four or more B-chromosomes were present quadrivalents occurred more frequently. In these respects the experimental material differed from the B-carrying parent. B's had no effect on the mean chiasma frequency of A-chromosomes when present in numbers of up to and including four but when present in numbers of more than four had a depressing effect. The variation of the mean chiasma frequency of the A-chromosomes increased with increasing number of B's. No correlation was observed between A-chromosome chiasma frequencies and B-chromosome chiasma frequencies. In the experimental material B's when present in larger numbers i.e. more than four had deleterious effects on A-chromosome behaviour and on fertility. In the effects of B's on the endophenotypic characters the experimental material differed from the B-carrying parent. It is suggested that the behaviour and effects of B's on the endophenotypic characters are the result of interaction between the A-chromosome genotype and the B-chromosomes.     

Presence and behaviour of B-chromosomes in Acanthophyllum laxiusculum (Caryophyllaceae)

Two karyotypic variants were recognized in populations of Acanthophyllum laxiusculum Schiman-Czeika. Variant A had 2N = 30 and variant B showed the presence of 0 to 3 B-chromosomes in addition to 2N = 30 chromosomes. Analysis of chromosome behaviour at meiosis showed that the presence of B-chromosomes increases chiasma frequency in A-chromosomes; this effect was higher for plants with odd numbers of B-chromosomes compared with plants with even numbers of B-chromosomes. Comparisons of variants A and B, suggests that B-chromosomes have an effect pollen stainability and seed production. It seems that the presence of B-chromosomes may increase pollen stainability and seed production in variant B.     

B-chromosome systems in the pocket mouse,Perognathus baileyi: Meiosis and C-band studies

The heterochromatin characteristics and meiotic behavior of the B-chromosome system of the pocket mouse, Perognathus baileyi, are described. B-chromosomes are associated both with a meiotic accumulation mechanism and with an increase in average chiasma frequency in the A-chromosome set in males. Three morphological classes of B-chromosomes are recognizable, and the mechanisms of origin of each are discussed.     

Occurrence and behaviour of B chromosomes in Artemisia frigida Willd.

In A. frigida Willd., B-chromosomes are reported for the first time. Their number was limited to 2 in all the cells studied. Analysis of chromosome behaviour at meiosis showed that B-chromosomes reduced the mean chiasma frequency and modified the phenotypic and biochemical characteristics of the plants.     

Population cytology of the genus Phaulacridium

Samples were taken from twenty three Australian mainland populations of the shorthorned grasshopper, Phaulacridium vittatum. Five of these populations were found to be polymorphic for B-chromosomes. The anaphase I segregation of the B-chromosome appeared to be random with respect to the X (contra Jackson and Cheung, 1967). The B-chromosomes did not appear to influence chiasma frequency in the population in which they were found.     

Cytogenetics of pearl millet

Summary The somatic karyotype of pearl millet Pennisetum americanum (L.) Leeke. (2n = 14) has been studied in several cultivars, but few cytological markers have been discovered which could help in the easy identification of the chromosomes. Analysis of pachytene bivalents permits such identification but is feasible only in a few cultivars. Recently, several lines having telocentric chromosomes have been produced and classified but their potentialities as cytogenetic tools have yet to be explored. Some African populations of pearl millet carry B-chromosomes in their karyotype. Cytogenetics of B-chromosomes has been reported in great detail. Bs undergo spontaneous changes to produce deficient- and iso-chromosomes. The main effect of B-chromosomes is on chiasma frequency which is exerted by the relative amounts of chiasma promoting euchromatin and the chiasma depressing heterochromatin in the Bs. Haploid plants occur occasionally and sometimes show a low degree of seed set, offering a possibility of establishing homozygous inbred lines. Cytogenetics of several spontaneous and induced autotetraploids have been reported. In general quadrivalent formation between the seven sets of four homologues was random. Seed set of the autotetraploids could be improved by selection; improved seed fertility was found to be associated with increased chiasma frequency, increased quadrivalent frequency and regular distribution of chromosomes at anaphase I. Genes controlling morphological characters of plant phenotype segregate independent of those controlling fertility and in pearl millet polyploidy per se is not limiting to plant vigour. Primary trisomics represent the best studied among the aneuploids of pearl millet. All the seven primary trisomics have been identified and described. Some were used in assigning genes to specific chromosomes but in general trisomies have poor vigour and fertility, and show low frequency of transmission. Apart from B-chromosomes, cytogenetics of interchanges has been the best studied aspect of pearl millet. The frequency of co-orientation of an interchange complex at metaphase I, which determines the fertility or sterility of the interchange heterozygote, is influenced by the genetic background and thus is theoretically amenable for selection leading to improved fertility of the heterozygote. Interchange tester-stocks have been assembled which can be used to identify the chromosomes involved in any newly obtained interchange. A complex interchange line involving all the chromosomes of the complement has also been produced, but the ring-of-fourteen produces total male and female sterility.Genotypic control of mitosis and meiosis has been reported, with reference to chromosome numerical mosaicism, multiploid sporocytes, desynapsis and chromosome fragmentation, and male sterility. Pearl millet being a largely outbreeding species, forced inbreeding was mainly found to result in loss of morphological vigour and reduction in mean chiasma frequency per PMC. Interspecific hybrids between pearl millet and several related species have been cytologically investigated and homology of the seven chromosomes of pearl millet with seven of the fourteen chromosomes of P. purpureum has been demonstrated. Cytogenetic evidence from haploids, autopolyploids and interspecific hybrids has indications to suggest that the haploid number of x = 7 is derived from x = 5, but the evidence is inconclusive and needs critical evaluation.     

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.     

Chromosome number and genetic system in several Trifolium species related to T. alexandrinum

Chromosome counts have been done in 12 species of Trifolium, closely related to T. alexandrinum L., and the chiasma frequency in meiosis, pollen fertility, and seedset upon selfing was estimated. In all species, the basic number of chromosomes was 2n=16 arranged in eight bivalents. Two species, T. salmoneum Mout, and T. berytheum Boiss., have 0–7 B-chromosomes in eight out of nine plants examined. The frequency of B-chromosomes was somewhat correlated with pollen fertility. Chiasma frequency ranged between 10.8 and 16.1 per cell with specific and ecotypic differentiation. Except for T. alexandrinum, the species analyzed were completely (or, in a few cases, highly) self-incompatible.Contribution from the Volcani Institute of Agricultural Research, Bet Dagan, Israel. 1970 Series, No. 1677 E. This research was financed by a grant from the U.S.D.A. under P.L. 480, Contract A10-CR-56.     

The effect of B-chromosomes of rye on the chromosome association in F1 hybrids Triticum aestivum x Secale cereale in the absence of chromosomes 5B or 5D

Summary T. aestivum var. Chinese Spring (monosomic 5B and 5D, respectively) was crossed with S. cereale (with and without B-chromosomes). The resulting nullisomic 5B hybrids exhibited a high degree of chromosome association both at 20°C and 10°C. The presence of B-chromosomes reduced association slightly whether 5B was present or not.In nullisomic 5D hybrids B-chromosomes of rye raise chromosome association at 20°C when compared to hybrids with 5D, with as well as without, B's. At 10°C, due to the absence of the Ltp gene on 5D, chromosome association in nullisomic 5D hybrids is low, and no effects of rye B-chromosomes is detectable.The hypothesis that B-chromosomes of rye carry (an) asynaptic gene(s) decreasing effective pairing, and (an) independent post-synaptic gene(s) increasing chiasma frequency on effective pairing sites, is presented.The work was supported by a fellowship of the Gulbenkian Foundation and partly carried out while the author was at the Department of Genetics, Agricultural University, Wageningen, the Netherlands     

Effects of centric-shift polymorphisms on chiasma conditions inTrimerotropis pallidipennis (Oedipodinae:Acrididae)

Two populations ofTrimerotropis pallidipennis from Mendoza (MA) and Uspallata (UA) (Argentina), polymorphic with respect to centric shifts and B-chromosomes, were cytologically analysed. The medium-sized chromosomes from both populations were polymorphic for 7 (MA) and 5 (UA) inversions respectively. A clear tendency towards chiasma localisation was evident in both populations although they differ significantly with respect to mean frequencies of both total and interstitial chiasmata. The analysis of these frequencies in relation to the polymorphisms showed that in UA, the inversions do not affect chiasma formation while in MA, total chiasma frequency is negatively correlated in a significant fashion with the number of heteromorphic bivalents. This decrease is related, in non-B-carriers, to a significant increase of interstitial chiasmata. Therefore a redistribution of chiasmata, which could increase genetic recombination, is evident. The successful maintenance of polymorphisms in each population might depend, in part, on their effects on recombination, which could be related with the local requirements of genetic variability.     

The relationship of abnormal chromosome 10 to B-chromosomes in maize

Abnormal chromosome 10 (K10) is known to increase recombination in maize and to induce preferential segregation in knobbed heterozygotes during megasporogenesis. In spite of the considerable interest generated by these findings, the origin of the K10 chromosome is unknown. It has been postulated that the extra segment of K10 arose by simple translocation between normal 10 and a B-chromosome. This hypothesis was tested by comparing meiosis in haploids with either K10 or the normal 10 and carrying a single B-chromosome. The frequency of bivalent configurations was found to be similar in the two types of haploids suggesting that the K10 and B-chromosomes do not share homologies that lead to chiasma formation. These results lend no support to the hypothesis that the K10 chromosome came from a B type. The implications of these results to the action of K10 at meiosis are also discussed.     

A propos de l'article ‘The B-chromosomes of Locusta migratoria I. Detection of negative correlation between mean chiasma frequency and the rate of accumulation of the B's; a reanalysis of the available data about the transmission of these B-chromosomes’, par. J. Cabrero,E. Viseras et J. P. M. Camacho

Comment on the article ‘The B-chromosomes of Locusta migratoria I. Detection of negative correlation between mean chiasma frequency and the rate of accumulation of the B's; a reanalysis of the available data about the transmission of these B-chromosomes’ — Some critical remarks on the said article (published in Genetica 64: 155–164, 1984) are presented.     

Genetic and environmental components of chiasma control

A disruptive selection programme for high and low chiasma frequency over four generations has produced two lines of Schistocerca gregaria with mean values of 21.6 and 18.0, respectively. The mean of the base population was 20.4. Realised heritability estimates for both the high and low lines were 27.4% and 48.8% respectively; only the latter value is significant. The non-significant value in the high line has resulted either from unidirectional dominance or because the genes responsible for high chiasma values themselves undergo a higher recombination rate in high frequency individuals and thus reduce the response to selection. — It is suggested that the genes governing chiasma frequency can be regarded as mainly neutral due to the large additive genetic component, at least in low chiasma frequency types. Accordingly, chiasma frequency variation and its adaptive significance may not be such an important component of fitness as has been proposed in the past.     

The effects of derived B-chromosomes on meiosis in pearl millet Pennisetum typhoides

The cytological behaviour of Standard, Deficient and Iso-Bs and their effects on the endophenotype were studied in Pennisetum typhoides. The Standard and Deficient Bs were acrocentric with the short arms consisting entirely of heterochromatin. The Deficient B was derived from the Standard B by spontaneous loss of short segment extending over two chromomeres in the euchromatic long arm. The Iso-B was derived from the Deficient by centromere misdivision, the long arm forming the Iso-B. The Iso-Bs were, therefore, less heterochromatic than either the Standard or the Deficient Bs. These three types of Bs had different effects on the mean cell A-chiasma frequency. The Deficients had a depressing effect, the Iso-Bs had an enhancing effect, while the Standard Bs had no effect on the mean A-chiasma frequency but did affect its variance. When the Standard and Deficient Bs were present in combination with Iso-Bs in different numbers, the values of mean A-chiasma frequencies were intermediate between the values of Standard, Deficient and Iso-Bs present separately in corresponding numbers. It is suggested that a balance between the amount of heterochromatin and euchromatin present in the B-chromosomes is responsible for the differential effects of these three types of Bs on mean A-chiasma frequency. In this species, the extra euchromatin present in the form of Iso-Bs seems to have an enhancing effect on the mean A-chromosome chiasma frequency whereas the extra heterochromatin has a repressing effect.     

Chromosome organisation in the Australian plague locust Chortoicetes terminifera

In Chortoicetes terminifera 45 independently-occurring B-chromosomes were tested and 23 distinct banding variants were detected with either G- or C-banding; six types were found more than once. In particular the Type I banding morph was detected 12 times indicating that individuals carrying this type may be under a different regime of selection compared with individuals bearing other types of banding morph; or the Type I may be subjected to a higher rate of meiotic drive in either or both sexes than other types. Also the Type I appeared to be obviously related to four other banding morphs whereas most types were not obviously related to any other banding morphs, but a few were similar in banding pattern to one or two other types. Three types of B-chromosomes were found in three or more different populations. A relatively high frequency of the Type I banding morph was found in one population, which was probably mainly composed of non-migratory individuals, and also in a laboratory-raised population. The most likely mechanisms for small changes in the banding sequence of the B-chromosomes are three-break insertions which are often indistinguishable from inversions. Rearrangements which add or delete bands, or sequences of bands, to or from B-chromosomes are probably the result of exchanges which are now known to take place in rare individuals with two B-chromosomes. The most distal region of all the banding morphs of the B-chromosome in C. terminifera, plus a short interstitial region in some types, is not late-replicating and has the banding characteristics of euchromatin. The rest of the chromatin of the B-chromosomes is heterochromatic and is the latest replicating heterochromatin in the whole genome. It consists of G-bands, which are also deeply stained with C-banding, and alternating G-interbands, which in turn are stained grey with C-banding. Both of these staining combinations are seen in heterochromatin of the normal complement. The heterochromatin of the B-chromosomes is condensed throughout 1st meiotic prophase in the male and in all somatic interphase nuclei where it can be quickly detected using the G-banding technique. The B-chromosome has a relatively constant, acrocentric morphology with a tendency to increase of length of the long arm as band numbers increase. Isochromosomes of the long arm have been seen only in laboratoryraised embryos. From egg pods with significantly fewer than expected B-chromosomes it is strongly suggested that more than one male may fertilize the eggs in a single pod.     

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

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