Interstitial Chiasma Research Articles

Meiotic Behaviour and Dosage Effect of B-Chromosomes on Recombination inDichroplus Elongatus(Orthoptera: Acrididae)

SUMMARYIn a previous study (Vilardi 1983, 1985) a polymorphism for a mitotically and meiotically unstable B chromosome was detected in a population of Dicbroplus elongatus from Tucuman (Republica Argentina). In the present work a detailed analysis of four B carrying individuals from the same population is reported. The B's within an individual were generally telocentric, their number varying from 0 to 6. In some cells, B's were metacentric or acrocentric because of spontaneous rearrangements. At diplotene and diakinesis they could form univalents or pair, in general terminally, rendering bivalents or paseudomultivalents. At first metaphase only univalents or terminally associated B bivalents were observed. These bivalents could persist even to first anaphase migrating undivided to the same pole. Interstitial chiasma frequency and the ratio interstitial/total chiasmata per cell showed an apparent increase as the number of B's per cell were incremented, without significant changes in the total number of chi...

Meiotic behaviour, stability, transmission and recombination in four balanced tertiary trisomics of rye (Secale cereale L.)

The F3 and F4 progeny of four balanced tertiary trisomics (BTTs) of rye carrying the short translocation chromosomes of their corresponding interchanges were karyotyped and scored for the monogenic recessively inherited tigrina marker (ti) which is located terminally on the short arm of chromosome 5R. The segment carrying the dominant allele (Ti) was included in the extra translocation chromosome of each BTT, while the normal 5R pair carried the recessive allele. Recombination fractions between ti and the interchange breakpoint varied between 0.005±0.003 and 0.032±0.005 for the different BTTs. Interstitial chiasma formation could be demonstrated in only one BTT, in 1% of the first metaphase configurations without, however, resulting in recombination. Chiasma localization just proximal to the terminal C-band of 5RS was concluded to cause the low recombination observed. Less than 0.5% of the progeny consisted of plants with a deviant karyotype able to reach maturity, indicating a high level of meiotic stability of the trisomics. The results further suggest that selection towards low male and high female transmission rates of the extra chromosome is possible. The suitability of the BTTs investigated for use in hybrid breeding programs is evaluated.

Cytological Study of the Telocentric Chromosome in Seven Monotelotrisomics of Barley

The behavior of the extra telocentric chromosome in seven monotelotrisomics (Triplo 1L, 1S, 2L, 2S, 3S, 4S, 5L) of barley (Hordeum vulgare L.) was studied during meiosis. The telocentric chromosome associated with its normal homologues, forming a heteromorphic trivalent, or remained as a univalent at diakinesis and metaphase I. Tandem V-, ring-and-rod-, and rod-shaped trivalents were observed. An interstitial chiasma was observed frequently between the telocentric and its normal homologous chromosome, suggesting that the telocentric chromosome was associated fairly tightly. The frequency of sporocytes with the trivalent was significantly higher at diakinesis than at metaphase, and it also differed significantly between telotrisomic types. The telocentric chromosome lagged at AI, TI, AII, and TII in varying frequencies, depending on the telotrisomic type. The present study demonstrates that the telocentric chromosomes of barley have a rigid, complete, and stable centromere.

Seasonal variation of chiasma frequency in Phyllodactylus marmoratus (Gray) (Gekkonidae ? Reptilia)

A study of the male meiotic system in two populations of the gekko Phyllodactylus marmoratus (Gray) has shown that both total and interstitial chiasma frequencies vary cyclically throughout the year. This variation is consistent in each population and was observed over a number of years. The total chiasma frequency (an index of the number of terminal chiasmata) has a different form of cyclic variation than does the interstitial chiasma frequency, and it is argued that they are under independent genetic controls. Reproductive studies suggest that only the sperm with the lowest total chiasma frequencies and greatest range of interstitial frequencies are used for fertilization. An experimental approach has shown that prolonged exposure to low temperature produces a significant increase in total chiasma frequency. It is believed that this environmental cue is responsible for the cyclic nature of total chiasma frequency.

CYTOLOGY OF BRAZILIAN PHANEROPTERIDAE (ORTHOPTERA-TETTIGONIOIDEA): A SPECIES WITH NEO XY SEX DETERMINING MECHANISM

The chromosome number of 10 species of Phaneropteridae are reported: Euceraia insignis Piza, Topana acquillari Piza, Picnopalpa bicordata Piza, Microcenirum lanceolatum Brunner, Pararota ferrerai Piza, Parableta bicentenaria Piza, Parableta affinis Piza, Phaneroptera exigua Piza, Hyperohora angustipennis Brunner and Mendesius albosignatus Piza. Except for Mendesius albosignatus Piza, all have 2n (♀) = 31 acrocentric chromosomes with a XO (♀), XX (♂) sex determining mechanism. The chromosome number of Mendesius albosignatus is 2n (♀) = 28. The sex chromosome mechanism is of the Neo XY (♀) type. The Neo X chromosome is submetacentric. XL is at least twice the length of XR. XR pairs over its entire length with the Neo Y; they are held by an interstitial chiasma and no sign of heterochromatin could be observed in them.

Chiasma frequency variation with altitude in Cepaea hortensis (Müll.)

Populations of Cepaea hortensis have been sampled from an area of 121 sq km on the north slopes of the Brendon Hills, Somerset, and analysed for chiasma frequency. Variation in chiasma frequency is due to variation in a single large bivalent--the A group bivalent. In 12 populations studied during 1973 no association between chiasma frequency and ecological factors could be detected. In particular, there is no evidence of a negative correlation of population density with chiasma frequency which is known to exist in populations of C. nemoralis within the area. In 1974, 17 populations, four of which were utilised in the 1973 analysis, were studied. Negative correlations of interstitial (but not total or terminal) chiasma frequency and between-cell variance with altitude were found. It is likely that the correlation of between-cell variance with altitude is due to the strong correlation of variance with chiasma frequency which exists in these populations. There is evidence from several populations that chiasma frequency is stable between years. The implications of the negative correlation of chiasma frequency with altitude and the lack of a correlation with population density are discussed.

Duplication deficiency heterozygotes in Aedes aegypti.

Two types of duplication-deficiency heterozygotes have been discovered from a translocation stock of Aedes aegypti. The structural interchange involved one arm of chromosome 2 with chromosome 1 and the other arm with chromosome 3. Both types of dp-df heterozygotes were deficient for a part of chromosome 2. But in one case a part of chromosome 1 and, in the other, a part of chromosome 3 was duplicated. During meiosis, chromosomes with a duplication usually formed homomorphic ring bivalents in a variable frequency. The remaining four chromosomes formed chain multiples. Ring multiples were never observed. In one heterozygote where chromosome 3 was involved in duplication, approximately 91 per cent of the cells showed a chiasma in one of the interstitial regions. The frequency of the interstitial chiasma corresponded with the expected equational segregation during first anaphase of meiosis.

Mating systems and population structure in two closely related species of the wheat group

Correlations between chiasma frequencies and population means and variances of 36 quantitative characters were calculated for seven populations of the predominantly selfing species Triticum longissimum and five populations of the outcrossing species Triticum speltoides. Significant negative correlations between the interstitial chiasma frequencies and both means and variances are found in the selfing species. This coincides with predictions from recent theoretical work concerning selection for degree of recombination. The positive correlations between some of the means and variances and the chiasma frequencies for the outcrosser are explained by the existence of supernumerary chromosomes which have been shown elsewhere to increase interstitial chiasma frequency.

THE MEIOTIC BEHAVIOUR OF A NULLI-HAPLOID PLANT IN AVENA SATIVA L.

A nulli-haploid plant (2n = 20) was found in the progeny of an induced monosomic line of Avena sativa L. var. Rodney (2n = 6x = 42). At metaphase 1, the number of associations ranged from 1 to 6 with a mean frequency of 3.5. The pairing frequency can best be explained by the absence of a genetic system, similar to that of chromosome 5B in Triticum aestivum, which would control the diploidisation of hexaploid oats.Chromosomes were associated terminally with an occasional interstitial chiasma. Closed bivalents were rare, and the frequency of trivalent associations was low. This meiotic behaviour confirms the genetic evidence as to the homoeology of the three genomes of the hexaploid oats, but seems to indicate a great deal of structural divergence between them.It is suggested that the long sub-terminal chromosome 7 might carry the gene for diploidisation.

Some theoretical aspects of selective segregation in interchange complexes

1. A population of Allium triquetrum heterozygous for an interchange has been discovered. The meiotic complex of a chain of four is oriented in an alternate fashion in about 75% of cells. 2. The suggestion is put forward that adjacent centromeres of a complex, if linked by at least one chiasma, behave as in a bivalent. Because there are four centromeres in a complex of four chromosomes, the basic orientation frequency is 50% alternate, 50% adjacent. Adjacent orientations are not basically the result of rigidity caused by chiasmata, and an interaction between homologous centromeres or other chromosome parts is not involved in the orientation mechanism. 3. Chiasmata may impose rigidity on the complex and so affect orientation frequencies. 4. Two (necessarily homologous) centromeres linked by an interstitial chiasma will be co-oriented as in a bivalent and adjacent 2 orientations are eliminated. 5. In chain forming configurations without interstitial chiasmata. one type of adjacent orientation is eliminated, depending on whether the centromeres are situated on one or the other pachytene axis; the configuration eliminated is replaced by alternate orientations, the basic frequency of which is thus increased from 50% to 75%. Interstitial chiasmata in a chain configuration alter the orientation frequencies. 6. Ring complexes may orientate in an alternate fashion in 75% of cells. 7. Relative lenghts of pachytene axes have no effect on orientation frequencies. 8. Multiple interchange heterozygotes such as Oenothera have probably evolved a special feature that, together with the typical orientation mechanism found in most organisms, enables a very high frequency of alternate segregation to be attained. 9. Apparent genotypic control of segregation may be the result of genotypic control of chiasma position.

Chromosome Structure in <i>Tradescantiae</i> VIII

Observations in T. reflexa an rod-shaped gemini at first metaphase favor the two-plane theory of crossing-over. Rod-shaped gemini at first metaphase commonly originate from gemini with one interstitial chiasma at diakinesis (diag. 1, figs. 1 and 6). Terminalisation of chiasmata thus commonly occurs after the major coil is established. On the basis of the one-plane theory as the result of this terminalisation of chiasmata in rod-shaped gemini, partial antitropy (diag. 1, fig. 7) should be found to occur frequently but was found rarely.On the basis of the two-plane theory as the result of terminalisation without crossing-over partial antitropy (diag. 1, fig. 5) should be found rarely and as the result of terminalisation after crossingover but showing no interstitiat reversal in direction of coiling (the types shown in diag. 1 figs. 2 and 3) should be found frequently, both of which premises accord with the actual observations.

Structure of Meiotic Chromosomes in Microsporogenesis of Tradescantia

1. The chromatids of Tradescantia chromosomes are coiled together in a single coiled chromonema at early metaphase of the first meiotic division. At late metaphase the two coiled chromatids in each homologue separate, forming a bivalent with four separate coiled chromatids. The primary coils disappear during interphase, and at the second meiotic division only the secondary coils are found. 2. The primary coils of meiotic chromosomes are assumed to be caused by the contraction or compression of the two flexible chromatids within the chromosome pellicle without rotation of the chromatids. 3. The direction of the primary coils is apparently at random for the homologous chromosomes of each bivalent. Within each homologue the direction of coiling may change at the fiber constriction, but there is a tendency for the coiling to be in the same direction on both sides of the fiber. The direction of coiling may also change at an interstitial chiasma. 4. We have found no evidence that the meiotic chromatids are spli...

Segmental Interchange in <i>Pisum sativum</i>

1. The relations between three different lines of Pisum sativum, the K line, the T line and the N lines, are discussed, with regard to chromosome structure. All three lines have seven pairs of chromosomes at meiosis. The K line when crossed with N lines was found by HAKANSSON to give a ring of four chromosomes at meiosis. Similarly the T line gives a ring of four chromosomes with N lines. The K line when crossed with the T line gives an association of 6 chromosomes at meiosis.2. This shows that the K line and the T line are related to N lines by a single interchange of chromosome segments and to each other by a double interchange, involving three chromosomes of the haploid set.3. Metaphase pairing is found to be due to chiasma formation, interstitial as well as terminal chiasmata being present at metaphase as in previously described Pisum plants.4. About 78% of the configurations of six chromosomes examined have an interstitial chiasma, the “median chiasma, ” between nonadjacent chromosomes of opposite parental complements. When this chiasma is present, the configurations assume the shape of a “figure-of-eight, ” similar to that described in Oenothera by DARLINGTON (1931).5. The presence of the “median chiasma” shows that an homologous interstitial segment is present in the otherwise non-homologous chromosomes taking part in it. A hypothesis is put forward to explain the origin of this segment.6. The “figure-of-eight” is shown to be a cytological demonstration of previous genetical crossing-over, on the single assumption that only homologous parts of chromosomes pair. This crossing-over gives segmental interchange between chromosomes only homologous in an interstitial region.7. As a result of this segmental interchange, there are four viable gametic types containing n-chromosomes possible, namely the two parental types and two new types containing the interchanged chromosomes. The different possible zygotic combinations to be obtained on selfing this plant are predicted. Instead of the progeny being restricted to plants with simple pairing and plants with associations of six chromosomes at meiosis, plants with rings of four chromosomes are expected, as a result of the new combinations resulting from segmental interchange.8. A re-examination of the ring of four chromosomes formed when the T line is crossed with N lines showed the presence of chiasmata on both sides of the attachment constriction in one pair of segments, in some cases. This demonstrates that the point of attachment is at some distance from the place where the change of segment occurs. It is suggested that such configurations occur also in Zea Mays and in Datura.The author desires to thank Miss C. PELLEW for supplying the material for cytological study, Dr. C. D. DARLINGTON and Dr. F. W. SANSOME for criticism and advice, and Mr. H. C. OSTERSTOCK for taking the photomicrographs and for assistance with the diagrams.

Chromosome Studies in <i>Anemone</i> I

(1) Most of the species of Anemone examined had a basic number of eight. Anemone Hepatica, a diploid species, had a basic number of seven (2n=14). Somatic doubling was found in several root-tips of A. Hepatica and A. vernalis.(2) Eight paired chromosomes can be distinguished at pachytene. The chiasma frequency at diplotene is 6.1 per bivalent. There is a fairly high percentage of terminal chiasmata per bivalent: a parallel reduction occurs in the total number of chiasmata per bivalent and the number of terminal chiasmata per bivalent between diplotene and metaphase. Most of the bivalents have a single interstitial chiasma at metaphase.(3) The reduction in the number of total chiasmata per bivalent and terminal chiasmata per bivalent, indicates that terminalisation takes place and that the chiasmata are resolved on reaching the ends of the chromosomes. This can be interpreted as due to lack of terminal affinity.(4) If this lack of terminal affinity were combined with strong terminalisation, univalent formation would result at metaphase although crossing-over had occurred.