B-chromosomes in Iseilema laxum Hack. II. Genetics

Transmission of B-chromosomes to S₁ progenies and progenies of crosses involving parents with different numbers of B-chromosomes in Iseilema laxum Hack. is reported. The mean number of B's remained the same in S₁ progenies. In the progenies of the crosses, the mean number of B's increased and the range of their number in the offspring also is wider when they are transmitted through the male parent. The inheritance of B's is more or less normal on the female side. The phenomena of nondisjunction and precocious division of B's are more frequent in the male parent. Apart from nondisjunction and precocious divisions in the PMC's, there seems to be some post-meiotic mechanism responsible for the wide range and increase in the mean number of B's in crosses where the male parents contain B-chromosomes.     

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.     

B-chromosomes in some Himalayan hardwoods

B-chromosomes are reported for the first time for woody angiosperms. Out of six species from the eastern Himalayas in which B-chromosomes were observed, two are polyploid. The number of B-chromosomes varied from 0–5 in the different species. In Sambucus canadensis, Glerodendrum colebrookianum and Neolitsea zeylanica B's were found to pair with each other at Metaphase-I.The financial support for this work was provided by the U. S. Government from PL 480 funds in India (Grant: A7-FS-12).     

Cytogenetic studies on natural populations of grasshoppers with special reference to B-chromosomes

In Atractomorpha bedeli the frequencies of males with B-chromosomes in the males from the Tofuro-ato population were 21, 30, 32, 40, and 26%, respectively, over five years. Numbers of B's (0 to 7) in the primary spermatocytes were usually constant within the follicles but were variable among the follicles within the individuals. In males from the Okazyo-ato population, 35% contained B's in their primary spermatocytes. The cells of the gastric caeca were observed in 8 males with B's in the primary spermatocytes but no cells of the gastric caeca contained B's. Cells of ovariole walls and those of gastric caeca were analysed in 80 females from the same population; none of them contained B's. These facts indicate that B's are present in the germ line but absent in the somatic line. The difference between the germ line and the somatic one was explained on the basis of elimination of B's from the somatic cells. The inter-follicular variation in number of B's was ascribed to mitotic non-disjunction in the germ line prior to differentiation of the follicles.Contribution No. 86 from the Institute of Biology, Oita University.     

Mitotic instability leading to an accumulation of B-chromosomes in grasshoppers

The study of mitotically unstable B-chromosomes (supernumeraries) of two grasshopper species confirmed a suggestion made earlier (Nur, 1963) that the instability should always be associated with a tendency of the B's to increase in frequency. Among 780 Camnula pellucida (Scudder) males from California, 105 had B's. In the testes of these males the number of B's varied from follicle to follicle and ranged between 0 and 4. Because of this variation, the number with which each male started to develop could not be determined. However, the relatively low frequency of males with B's and the regular meiotic behavior of the latter suggested that most of the 105 males started with a single B. Cytological analysis of the cells of the gastric caeca of 31 males whose testes contained B's confirmed this suggestion by showing that only one male had two B's in these cells; all the rest had one. In the testes of the 74 other males the mean number of B's ranged from 0.89–2.50, but only two males had means higher than 2.00. The observed ratio of one male with two B's to 30 with one, suggested that only the two males with the highest means started to develop with two B's and that the other 72 males all started with one. Since the mean for the 72 males was 1.37 B's per male, it was concluded that during the development of the testes of these males the mean increased by 37%. The males with B's had fewer follicles in their testes and apparently had also a lower frequency of normal sperm. — The analysis of the testes of Locusta migratoria L. males from Japan gave results which agreed with those from C. pellucida.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D.C.     

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.     

B-Chromosomen beiChironomus

In the population “Herrenmühle” ofChironomus plumosus 11% of the individuals contain one supernumerary chromosome. This B-chromosome is present both in germ-line and somatic cells. — InChironomus melanotus 6% of the larvae of the population “Falkau” carry supernumerary chromosomes. These B-chromosomes cannot be found in all nuclei of testis and soma, their number varies between cells within the individual. In both species the B-chromosomes represent centromeric fragments of chromosome IV as can be shown by their structure and pairing behaviour. — The polytene B-chromosome ofCh. plumosus exhibits a banding pattern in the salivary gland nuclei. Furthermore it is able to form an additional nucleolus in the nuclei of the malpighian tubules. InCh. melanotus band structures can be seen only in the B-chromosome of malpighian tubules. The larvae ofCh. melanotus, carrying B-chromosomes, show heterochromatic bodies in the salivary gland nuclei, varying in number and size in the nuclei of the same gland. These bodies are interpreted to be polytenic B-chromosomes divided into subunits.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Harmful B-chromosomes in a mealy bug: additional evidence

The B-chromosomes (B's; supernumeraries) of the mealy bug, Pseudococcus obscurus Essig, segregate preferentially into the two functional products of male meiosis. This segregation thus serves as an accumulation mechanism. A cytological study of a population from Oakland, California, confirmed the results obtained earlier that the B's are harmful and are maintained only because of their accumulation mechanism. The wild females were studied directly. The number of B's in the males was determined by analyzing ten or more daughters of females without B's (0B females) after these were inseminated by wild males. The 0B females were exposed to the wild males in screen cages. The analysis of 4732 daughters of 231 caged females indicated that among the males which inseminated these females, there were 19.9% 0B males and the mean number of B's was 1.46 ± 0.07. Among 224 wild females which were collected at about the same time there were 12.5% 0B females, and the mean number of B's was 1.88 ± 0.09. Since the frequencies of the B's in the population changed only slightly from generation to generation, the expected zygotes of this generation were assumed to be similar to those from which both the males and the females developed. The expected zygotes were calculated from the observed frequencies of the B's among the sperm and the known rates of transmission in females. The zygotes were very similar to the females but quite different from the males. It was concluded, therefore, that the B's had little or no effect on the females carrying them, but reduced the fitness of the males. The fitness of the 0B, 1B, 2B, 3B and 4B males was calculated to be 1.00, 0.64, 0.56, 0.38 and 0.20 respectively. The rate of transmission of the B's decreased with the increase in the number of B's, from 0.84 in 1B males to 0.51 in 4B males. This decrease, and the decrease in male fitness with the increase in the number of B's are expected to help stabilize the number of B's in the population.This paper is dedicated to Professor Sally Hughes-Schrader on the occasion of her seventy-fifth birthday.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D. C.     

Structure,behaviour and repetitive DNA of B-chromosomes in Cestrum nocturnum (Solanaceae)

Abstract

The Cestrum genus is karyotypically exceptional in Solanaceae. It is characterised by a basic number x?=?8, a large chromosomal and genomic size, complex heterochromatin patterns, B-chromosomes (Bs) with particular heterochromatin and distribution of 18–5.8–26S and 5S rDNA. Cestrum nocturnum L. has a diploid number of 2n?=?16 plus a variable number of B-chromosomes. The aims of work was to analyse their numerical variation, structure and behaviour of C. nocturnum B-chromosomes by classical and molecular cytogenetics. The individuals analysed had 2n?=?16?+?0?13 B-chromosomes. All B-chromosomes were metacentric and smaller than A-chromosomes. The number of B-chromosomes showed a great variability between and within individuals, thereby denoting the occurrence of events that promote mitotic and meiotic instability. Cytogenetic techniques made it possible to observe that B-chromosomes are rich in heterochromatin, probably with AT- and GC-rich regions. In addition, molecular techniques allowed to detect homologous sequences of transposable element conserved domains of Ty1-Copia and Ty3-Gypsy superfamilies. These sequences were located by FISH in all B-chromosomes and some A-chromosomes. Our results showed that repetitive DNA could play an important role in chromosomal evolution as well as in the stability of B-chromosomes in C. nocturnum.     

Variable transmission rates of a B-chromosome in Myrmeleotettix maculatus (Thunb.) (Acrididae: Orthoptera)

Karyotype comparisons of both parents and progeny from single pair matings in the grasshopper Myrmeleotettix maculatus have shown that there is an accumulation of the large mitotically stable B-chromosome when transmitted through the female. This is presumed to result from a preferential segregation of univalent B-chromosomes at the first division of female meiosis and occurs irrespective of whether the B's are odd or even in number. In the male there is a loss of B-chromosomes. This loss does not appear to be due simply to the lagging and elimination of B-chromosomes in meiosis but probably involves sperm formation or function. When the balance of the gain and loss after one generation is calculated, it shows large overall accumulation in crosses involving individuals from a population in Wales, and a slight loss in individuals from a population in East Anglia. Such differences in transmission rates may be responsible for differences in B-frequency between populations. Since the B-chromosome frequency of these two populations has remained stable over five years, possible forces in the maintenance of the equilibria are examined. Females with B-chromosomes produce more aneuploid embryos than 0B females, but neither this cause of inviability nor general embryo mortality seem sufficient to produce an equilibrium situation. It is necessary to postulate that progeny with more than 2B chromosomes are inviable in order to approach equilibria. The presence of B-chromosomes in females has also led to the formation of polyploid embryos. The possible involvement of repetitive DNA in the formation of unreduced egg nuclei and preferential segregation is discussed.     

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     

THE GEOGRAPHICAL DISTRIBUTION OF B-CHROMOSOMES OF XANTHISMA TEXANUM DC. (ASTERACEAE). I. SURVEY OF THE RANGE

The distribution of B-chromosomes throughout the range of Xanthisma texanum has been determined from approximately 750 individuals from 80 wild populations in Texas, Oklahoma, and New Mexico. Counts of meiotic chromosome figures of n = 4_II + 1 B_I and n = 5_II (4_II + IB_II) are reported for the first time for subsp. texanum var. texanum and for subsp. texanum var. orientalis. The wide occurrence of B-chromosomes in natural populations of subsp. drummondii is also documented. B-chromosomes are present in 22 (27.2%) randomly distributed populations whose characteristics are in no way distinct from populations without B-chromosomes. In populations with B-chromosomes present, 19.8% of the plants contained one or two B-chromosomes, but in total only 5.8 % of all plants contained one or two B-chromosomes. All populations in one isolated population group of subsp. drummondii lack B-chromosomes.     

B-CHROMOSOMES,THEIR ORIGIN AND RELATION TO MEIOSIS IN INTERSPECIFIC LOLIUM HYBRIDS

The following Lolium species were shown to have 14 chromosomes: L. canariensis Steud. (= L. gracile Parl.), L. loliaceum (Bory and Chaub.) Hand.-Mazz., L. multiflorum Lam., L. perenne L., and L. temulentum L. B-chromosomes in addition to the normal complement were observed during metaphase I and anaphase I for L. persicum Boiss. and Hohen., L. remotum Schrank, L. rigidum Gaud., L. strictum Presl. (= L. rigidum?), and for several synthesized interspecific hybrids. General absence of B-chromosomes in diakinesis suggested that they originated by misdivision of A-chromosomes during prometaphase I. The B-chromosomes reached a maximum of eight but the number varied for microsporocytes of the same plant. B-chromosomes appeared spontaneously in progenies from sibbing of hybrid plants without supernumeraries but were also eliminated in the F₂ from other sibbings. Degree of chromosome pairing during meiosis was unrelated to the subsequent presence of B-chromosomes. Normal pairing of seven bivalents was typical for L. perenne × L. multiflorum, L. perenne × L. rigidum, L. multiflorum × L. loliaceum, and L. rigidum × L. loliaceum. Between five and seven bivalents were recorded for L. multiflorum × L. persicum, L. multflorum × L. remotum, L. multiflorum × L. strictum, L. rigidum × L. persicum, L. rigidum × L. remotum, L. rigidum × L. strictum, and L. rigidum × L. temulentum. The results indicated that these Lolium species have a common and generally undifferentiated genome suggesting relatively recent speciation.     

疏花长柄山蚂蝗复合体(豆科)的修订

通过文献、标本研究,结合野外考察及数据统计分析,确认疏花长柄山蚂蝗复合体包括4种1亚种,讨论了各分类群的重要性状,并给出了新的检索表。将湘西长柄山蚂蝗(Hylodesmum laxum subsp.falfolium)处理为原亚种的新异名;订正了《中国植物志》和《Flora of China》等著作中该类群的几个错误的地理分布,记载了我国几个省区的分布新记录,并给出了凭证标本。     

Inter-population sex chromosome polymorphism in the grasshopper Podisma pedestris

Populations of the alpine grasshopper Podisma pedestris are either completely XO/XX or else completely neo-XY/neo-XX in character. The neo-XY form is limited in its occurrence to near the extreme southwest edge of the present distribution of the species. Both kinds of population also contain B-chromosomes. These are of two types differing with regard to size, stability and origin. Both types can occur in the same population but have not so far been found in the same individual. The pattern of distribution of the larger, mitotically-stable supernumerary suggests that it might be a remnant X-chromosome which became converted into a B in the initial phase of neo-XY evolution. The smaller, partly unstable supernumerary, on the other hand, may well have originated from a heteromorphic S₁₁ pair also found in some of the populations with small B's. The available evidence points to the neo-XY system having evolved following a secondary movement of the species into the higher ground it now occupies after the climatic amelioration at the end of the last ice age.     

THE CYTOGENETICS OF XANTHISMA TEXANUM DC. (ASTERACEAE) AND ITS B-CHROMOSOMES

The karyotype of A-set and B-chromosomes of Xanthisma texanum DC. are described in detail and measurements are given. The distribution of B-chromosomes in stem tissue is described and the loss of B-chromosomes from primary roots confirmed. The number of B-chromosomes in pollen mother cells was constant within single inflorescence buds and in all buds throughout the flowering period. The behavior of one and two B-chromosomes during meiosis is described in detail. During meiosis, B₁'s divided in 71% of the cases during anaphase II and in 29% of the cases during anaphase I. First anaphase division resulted in some lagging and elimination of B-chromatids during second division. During anaphase I, B₁₁'s divided into chromosomes and in anaphase II these divided into chromatids in 95% of the cases. In the other 5 % non-disjunction of the B-chromosomes or chromatids and A-set chromosome abnormalities occurred. Post meiotic preferential distribution during pollen mitosis resulting in a doubling of the number of B-chromosomes passed to progeny via pollen occurred, but no doubling occurred in the female line. Crosses involving plants with two B-chromosomes each yielded fewer plants with three B-chromosomes than expected. Polyploidy is extremely rare in the species. The evolutionary significance of B-chromosome behavior is discussed in relation to its origin and survival.     

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.     

Pollination and seed set in tropical wetland grasses

This study concerns 39 different grass species occurring in the wetland regions of Godavari delta in the west Godavari district (16°15′–17°30′N and 80°50′–81°55′E) of Andhra Pradesh, southern India. In each species, the sexual status of lemmas, the period of stigma receptivity, pollen/ovule ratio, pollen longevity and daily pollen release were examined. The functional sexual systems were determined after performing controlled pollinations and testing seed quality by weight and germination potential. The florets of 35 species are morphologically hermaphroditic. Among the other four species, Chionachne koenigii and Iseilema laxum are monoecious; Chrysopogon aciculatus and Pennisetum pedicellatum are andromonoecious. Examination of the sequential opening of florets and controlled pollinations revealed that C. koenigii and I. laxum are obligate outcrossers with incompatibility to geitonogamous pollen; C. aciculatus and P.pedicellatum have a high level of compatibility to xenopollen. These and another nine species were treated as predominantly outcrossing species. Yet another five species were predominantly self‐fertilized, while the remaining 21 species displayed a mixed breeding system. In a majority of the species the P/O ratios are not high and thus do not conform to the expected large P/O ratios in wind‐pollinated plants and postulated breeding systems except in a few species. The majority of these grasses shed pollen over a short period spreading from 2 am to 8 am, when usually low turbulent conditions exist, thus allowing short distance transport and restricted pollen shadows with higher concentration of pollen near the source. This appears to be the appropriate strategy for maximization of pollination in view of the short lifespan of pollen that extends mostly over 1–3 h period, and the patchy distribution of the grasses under study. The restricted pollen shadows facilitate the low levels of pollen production in the grass species under study to fertilize most or all the available ovules.     

B-Chromosomes and E-1 isozyme activity in mosaic bulbs of Scilla autumnalis (Liliaceae)

The electrophoretic patterns of esterase (E-1), alcohol dehydrogenase (ADH), and glutamate oxaloacetate transaminase (GOT) isozymes were studied in two Spanish populations of the lily Scilla autumnalis with B-chromosome carrying individuals. The E-1 isozyme activity appears only in those individuals with B-chromosomes. None of the bulbs free of B's show it. Five bulbs, mosaic for B-content, were identified. Electrophoretic analysis shows that these bulbs are characterised by mosaicism for E-1 isozyme activity. An analysis of individual roots by both electrophoretic and cytological methods shows that tissue mosaicism for B-content correlates with tissue mosaicism for E-1 isozyme activity. The electrophoretic analysis of different roots from bulbs heterozygous for the Est-1 locus indicates that the structural gene for E-1 is not located on the B-chromosome itself. Rather there is a derepressor effect of Bs on E-1 isozyme activity. Since ADH and GOT patterns are unaffected by the presence of B-chromosomes it is clear that they do not exhibit a generalised derepressor effect.