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.     

Studies on chiasma distribution and chiasma movement in Zoniopoda tarsata (Orthoptera: Romaleidae)

A sample of 27 males of Zoniopoda tarsata from Argentina was studied cytologically. The three largest autosomal pairs and the X were characterized by the presence of interstitial C-bands. Chiasma position relative to the bands was analyzed at diplotene and diakinesis. The frequency of interstitial, terminal and total chiasmata per cell was studied for the whole autosomal bivalent set, analysing the variations between stages and among individuals. The comparison of interstitial chiasma frequencies between stages and among individuals and the study of chiasma position relative to the bands in pairs 1, 2 and 3 indicated that chiasma distribution varied from diplotene to diakinesis. Therefore, terminalization does exist in this species and the movement may occur towards the centromere. The frequency of terminal associations at diplotene showed a high negative correlation (r=-0.89; p<10^-5) with the number of interstitial chiasmata. This correlation would not be expected if the two kinds of association were produced by different (independent) mechanisms. Consequently, terminal associations were considered genuine chiasmata. The correlation between interstitial and total chiasmata was very much lower then the former (r=0.39; p=0.04). This fact, besides the relatively low variation for chiasma number, observed among individuals suggests that in this species the number of interestitial chiasmata, which are the most important in controlling the genetical recombination, is mainly regulated by changes in chiasma distribution, while variations in total chiasma frequency are of much lower magnitude.Member of Carrera del Investigador del Consejo Nacional de Investigaciones Cientificas y Técnicas (CONICET)     

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.     

Cytological, genetic and evolutionary functions of chiasmata based on chiasma graph analysis.

The nature of the chiasma as a cytological parameter for analysing cross-over was reexamined quantitatively by an improved chiasma graph method. It was reconfirmed in Mus platythrix (n =13) that interstitial chiasmata at diakinesis are distributed randomly and almost uniformly along bivalents except for the centromere and telomere regions. The size of these chiasma blank regions was consistently 0.8% of the total length of haploid autosomes in all chromosomes. There was a minimum value of chiasma interference distance between two adjacent chiasmata, which was constantly 1.8% in all chromosomes. The chiasma frequency at diakinesis was 20.1+/-2. 0 by the conventional method including terminal chiasmata. However, the primed in situ labeling technique revealed that terminal chiasmata were mostly telomere-telomere associations. From these data and also from recent molecular data we concluded that the terminal chiasma is cytologically functional for ensuring the normal disjunction of bivalents at anaphase I, but genetically non-functional for shuffling genes. The chiasma frequency excluding terminal chiasmata was 14.6+/-1.8. Reexamination of the chiasma frequency of 106 animal species revealed that the chiasma frequency increased linearly in proportion to the haploid chromosome number in spite of remarkable difference in their genome size. The increase in chiasma frequency would be evolution-adaptive, because gene shuffling is expected to be accelerated in species with high chromosome numbers.     

Chiasmata and variability in Lolium and Festuca populations

There are significant differences in mean pollen mother cell chiasma frequencies between populations within Lolium perenne, L. multiflorum and Festuca pratensis. The differences are genotypically controlled. With low chiasma frequencies the chiasmata are distally located. With increasing chiasma frequency the frequency of chiasmata in interstitial segments increases. Shorter lived populations have higher chiasma frequencies than the more perennial. — The higher the chiasma frequency of a population the lower the phenotypic and genetic variance for characters under polygenic control, such as flowering time, and the less effective also is the response to selection for such characters. These observations are interpreted on the premise that high chiasma frequencies are instrumental in the breaking up of supergene sequences in interstitial chromosome segments.     

Heterosis for chiasma frequency and quantitative traits in common beans (Phaseolus vulgaris L.)

Summary Ten genotypes, including inbreds, hybrids, and advanced populations, were examined in order to elucidate the relationship between position and frequency distribution of chiasmata and quantitative traits, including yield heterosis in common beans. The hybrid and advanced population groups were determined to possess 83% and 54% increased chiasma frequency, respectively in contrast to inbred lines. The increase in chiasma frequency of these populations was further manifested in a high number of interstitial chiasmata. The regular and superior chromosome behaviour of the hybrids was found to be positively associated with quantitative measures on bean yield, harvest index and bean yield efficiency. The results were discussed from the point of view that: a) increased interstitial chiasmata may provide an effective mechanism for maintaining genetic diversity and heterosis in hybrid populations; and b) heterosis for chiasma frequency and quantitative traits may be due to dispersed genes on the chromosomes having combined intra-and interallelic interactions. The data provide evidence for the existence of positive associations between interstitially localized chiasmata with its recombination potential and regular chromosome behaviour to bean yield heterosis. The role of enhanced interstitial chiasmata to promote higher levels of genetic variation and heterozygous advantage is discussed.     

Cyclic variation of chiasma frequency and distribution in Podarcis sicula (Reptilia: Lacertidae)

The seasonal variations of chiasma frequency and distribution have been studied in the lizard: Podareis sicula. In this species, as in Phyllodactylus (King & Hayman, Chromosoma 69: 131–154, 1978), chiasma frequencies vary following a definite annual cycle, and clearly different trends are shown by interstitial and terminal chiasmata.A comparison between these seasonal chiasma frequency variations and those of environmental temperature shows the existence of a clear correlation between these two parameters. However, this correlation is different in the two types of chiasmata, and may be different within the same type of chiasma depending on the period of the year.A more significant correlation is observed between chiasma cycles and annual variations of the haematic levels of sexual steroid hormones. In particular we observe a highly significant correlation between interstitial chiasma frequencies and testosterone concentration. A less precise correlation between terminal chiasma frequencies and estradiol concentration is also observed.In Podarcis, as in Phyllodactylus, the sperm that will be used for fertilization derive from the spermatocytes showing the highest rate of interstitial chiasmata. This supports the hypothesis that the cyclic variations in interstitial chiasma frequencies represent a mechanism to ensure an adequate level of variability in a given population. The above mentioned correlation between chiasma frequencies and steroia hormone concentrations suggests that the seasonal chiasma cycles are controlled by the same environmental and hormonal factors regulating the spermatogenetic cycle.     

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.     

Simultaneous negative and positive chiasma interference across the break point in interchange heterozygotes

The impossibility to obtain real roots from equations published earlier for estimating chiasma frequencies in the two translocated segments from configuration frequencies in interchange heterozygotes, was shown to be a result of lack of independence of chiasma formation. This is interpreted as negative interference. Similarly, negative interference could be shown to operate between the two interstitial segments. In all cases where a sufficient number of bivalents was formed by the interchange complex, chiasma frequency in the interstitial segments was strikingly higher in bivalents (having no chiasmata in the translocated segments) than in multivalents (with chiasmata in one or both translocated segments). This indicates strong positive interference between the interstitial and translocated segments.Negative interference between opposite-and positive interference between adjacent segments across the break point of the interchange occurred simultaneously in the cell populations. The phenomenon was attributed to complications in effective chromosome pairing at the point of partner exchange which in interchanges is determined by the breakpoint.The material was Secale cereale where five interchanges were analysed in a total of 12000 PMC's from 14 plants.     

Male and female meiosis in grasshoppers

Male meiosis in the grasshopper Stethophyma grossum is well known as an example of proximal chiasma localisation. An investigation of female meiosis in oocytes of this species shows that both the frequency and distribution of chiasmata are quite different from the male situation. Mean chiasma frequency per cell (14.98) in considerably higher in females than in males (11.28) which agrees with the trend established in other comparative studies of male and female meiosis. More strikingly, males and females also show not only different but quite opposite patterns of chiasma distribution. In spermatocytes of males, chiasmata are strictly localised proximally in most bivalents, but in oocytes of females very few chiasmata form in proximal regions and nearly all chiasmata form either in distal or interstitial regions. The genetical significance of these findings is considered.     

Effects of supernumerary heterochromatin on chiasma condition in two species of Acrididae (Orthoptera)

Amblytropidia australis and Dichroplus elongatus were found to be polymorphic for supernumerary heterochromatin. In both, basic karyotypes are 2n=22+XO in males.Mitotically unstable extra chromosomes were detected in a population of A. australis. The Bs are telocentric and their number varies from O to 2 within individuals. Mean frequencies of interstitial and total chiasmata at diplotene were compared between individuals with and without Bs. The mean frequency of interstitial chiasmata increases with the number of Bs per cell.A supernumerary terminal segment in S₁₀ pair was observed in a heterozygous condition in several individuals of D. elongatus. The localization and frequency of chiasmata at diplotene were studied. The segment has an intrachromosomal effect since it modifies the location of chiasmata in the bivalent involved.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).     

Phase transformation and chiasma formation in locusts

The chiasmata in the eight longest of the eleven autosomes in spermatocytes of three species of locusts are greatly increased from the low basic number characteristic of the grasshopper-like or solitaria phase to higher numbers in the swarming or gregaria phase. Swarming in the field gives a greater impetus to the increase in chiasma frequency than crowding in cages. Although there seems to be some maternal effect acting on chiasma formation, the main correlation between population density and chiasma frequency seems to be effected during the last, or last two, instars of nymphal life. The effective factor is postulated to be present in the atmosphere, i.e. the air circulating in the locust breeding room and that circulating around a swarm in the field.In the species Locusía migratoria migratorioides three genetic factors have been found to control chiasma frequency. The first is a dominant factor reducing the frequency in one strain, the second is a possible polygenic complex reducing chiasma frequency in another strain selected for plasticity in hopper colour change, and the third is the recessive albino mutation which maintains chiasma frequency at the same level in crowded and solitary individuals.     

Double insertion of homogeneously staining regions in chromosome 1 of wild Mus musculus musculus: effects on chromosome pairing and recombination

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously staining regions (HSRs) was carried out. The region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal breakpoint of Is(HSR;1E3)2Icg is desynapsed during the early pachytene stage and heterosynapsed at the midpachytene, as shown by electron microscopic analysis of synaptonemal complexes. The HSRs have no effect on the segregation of chromosome 1 in heterozygous mice. The lack of homosynapsis in the region under study causes chiasmata redistribution in heteromorphic bivalents. In normal males, single chiasmata are located in the medial part of the chromosome. In heterozygotes, this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing single chiasmata. The total number of chiasmata per bivalent is much higher in heterozygous males than in normal ones. The recombination frequency between proximal markers fz and In also is higher in heterozygous animals. The increase in the total chiasma number in the heteromorphic bivalent is due to the addition of double chiasmata located mostly at precentromeric and pretelomeric regions of the chromosome.     

Recombination and chiasmata: few but intriguing discrepancies.

The paradigm that meiotic recombination and chiasmata have the same basis has been challenged, primarily for plants. High resolution genetic mapping frequently results in maps with lengths far exceeding those based on chiasma counts. In addition, recombination between specific homoeologous chromosomes derived from interspecific hybrids is sometimes much higher than can be explained by meiotic chiasma frequencies. However, almost the entire discrepancy disappears when proper care is taken of map inflation resulting from the shortcomings of the mapping algorithm and classification errors, the use of dissimilar material, and the difficulty of accurately counting chiasmata. Still, some exchanges, especially of short interstitial segments, cannot readily be explained by normal meiotic behaviour. Aberrant meiotic processes involving segment replacement or insertion can probably be excluded. Some cases of unusual recombination are somatic, possibly premeiotic exchange. For other cases, local relaxation of chiasma interference caused by small interruptions of homology disturbing synaptonemal complex formation is proposed as the cause. It would be accompanied by a preference for compensating exchanges (negative chromatid interference) resulting from asymmetry of the pairing chromatid pairs, so that one side of each pair preferentially participates in pairing. Over longer distances, the pairing face may switch, causing the normal random chromatid participation in double exchanges and the relatively low frequency of short interstitial exchanges. Key words : recombination frequency, map length, chiasmata, discrepancy, chromatid interference.     

The effect of colchicine on meiosis in Lilium speciosum cv. “Rosemede”

Chromosome pairing and chiasma frequency were studied in meiocytes at diakinesis of Lilium speciosum cv. Rosemede fixed up to 21 days after the start of either continuous or 3 day pulse colchicine treatment. The two treatments gave similar results. In pulse treated pollen mother cells (PMCs) the mean chiasma frequency per cell fell from 26.4 in controls to 8.5 after fourteen days while the mean number of univalents per cell increased from 0.05 to 17.58. There was a negative correlation between mean chiasma frequency per bivalent and per PMC in colchicine treated buds; univalents were preferentially induced in bivalents with one chiasma, and preferentially excluded in bivalents with 4 chiasmata. Some chiasmata were redistributed to surviving bivalents despite the concurrent reduction in chiasma frequency per meiocyte. — Colchicine sensitivity began in premeiotic interphase and extended to mid or late zygotene in PMCs; ongoing synapsis was unaffected. However, susceptibility to univalency was asynchronous between bivalents occurring at zygotene in short chromosomes but at late premeiotic interphase in the longest chromosomes. The number of chiasmata per bivalent could be altered by colchicine without inducing univalents, but the ultimate effect was to reduce the number of chiasmata per bivalent (or per chromosome arm) directly to zero. The major factors determining the order and extent of reduced pairing and chiasma number were total chromosome length and arm length. Pairing and chiasma formation in embryo sac mother cells were less sensitive to colchicine than in PMCs, but their behavior was otherwise similar.     

Chiasma distribution in Truxaline grasshoppers

Similar patterns of chiasma distribution are found within the individual arms of the chromosome complement in four species of Truxaline grasshopper. There is a linear relationship between chiasma frequency and chromosome arm length although the telocentric elements have a consistently higher mean number of chiasmata per unit of arm length. The positions of successive chiasmata can be defined in terms of residual (r.c. and r.t.) and interference (T) distances which vary in value according to both arm length and chiasma frequency. There is a tendency for one chiasma to lie in a distal position which is accentuated when additional chiasmata form. Supernumerary B chromosomes do not appear to influence the overall control mechanism of chiasma distribution. There is no indication that bivalents within a nucleus compete for chiasmata nor does the chiasma distribution in one arm of the metacentric members influence that in the other. It is suggested that the control of chiasma formation is determined mainly by interference factors.     

Chiasma frequency variation inCepaea hortensis (Müll.) and a comparison withC. nemoralis (L.)

Chiasma frequency variation has been studied in 12 populations ofCepaea hortensis and the results compared with those previously obtained forC. nemoralis from the same populations. Variation in chiasma frequency is effectively restricted to a single large bivalent. In both species there are significant differences between populations in chiasma frequency. Although mean population mean chiasma frequencies are similar inC. hortensis andC. nemoralis, C. hortensis is less variable thanC. nemoralis. This can be seen, to a greater or lesser extent, at several levels-(i) within cells, (ii) between cells, (iii) between snails, and (iv) between populations-and is most noticeable when interstitial chiasmata are considered separately from terminal chiasmata. Data from 6 populations suggest that chiasma frequency remains stable between successive years. It is suggested that, since sympatric populations were studied, the most likely explanation for differences in chiasma frequency statistics between the two species is thatC. hortensis andC. nemoralis are responding differently to similar selective pressures.     

Chiasma frequency and position in male and female mice of chromosomes involved in homozygous and heterozygous translocations and tertiary trisomy

Chiasma frequency and position were analyzed at a predominantly late diplotene-diakinesis stage of the first meiotic division in oocytes and spermatocytes from T(1;13)70H homozygotes and heterozygotes, T(2;8)26H heterozygotes and from Ts(I¹³)70H tertiary trisomics of the mouse, Mus musculus. For T70H/T70H, the 13¹ long marker bivalent was studied and for the other karyotypes, analysis was confined to the multivalent configurations adopted by the rearranged chromosomes and their homologues. For the 13¹ bivalent, the chiasma frequency tended to be increased in the female. For the T26H and the T70H multivalents, the chiasma frequencies were higher in the female, largely due to the much higher values in the short interstitial segments. This observed enhancement has been attributed to pairing differences rather than to differences in chiasma forming capability. Both in the telomeric region of the 13¹ bivalent and in the short translocated segments of the reciprocal translocation and tertiary trisomic multivalents, females showed fewer chiasmata than males. The determinations of chiasma position in the 13¹ bivalent indicated chiasma interference with respect to position, leading to clustering of chiasmata somewhat beyond the centromere and towards the telomere of this chromosome.     

Lampbrush chromosomes in the japanese quail Coturnix coturnix japonica: cytological maps of macro chromosomes and meiotic crossover frequency in females

Cytological maps of lampbrush macrobivalents of the Japanese quail (Coturnix coturnix japonica) were constructed. Investigation of chiasmata allowed determination of the meiotic frequency of reciprocal genetic recombination (crossing over) in Japanese quail females. The total chiasma number in bivalents of Japanese quail oocyte nuclei was determined to be 53-58. Macrobivalents 1-5 and Z of the Japanese quail had on average 3.3 chiasmata per bivalent, and microbivalents, 1.0-1.1 chiasmata per bivalent. The chiasmata (crossover) frequency in Japanese quail females was lower than in chicks. In macrochromosomes of Japanese quail females, one crossover occurred per 43.9 Mb, and in chicken, per 30.0 Mb. Judging from chiasma frequency, the genetic length of the Japanese quail genome is likely to be 2650-2900 cM. Crossover frequency in the species was 0.023 per Mb in macrobivalents and 0.07-0.08 Mb in microbivalents and for the total genome, 0.041 crossovers per Mb. The genetic length of one Mb (theta) in female Japanese quails was 1.14 cM in macrochromosomes, 3.60-4.12 cM in microchromosomes, and about 1.96-2.15 cM averaged over the genome.     

Metaphase I orientation of Robertsonian trivalents in the water-hyacinth grasshopper, Cornops aquaticum (Acrididae, Orthoptera)

Trivalents resulting from polymorphic Robertsonian rearrangements must have a regular orientation in metaphase I if the polymorphisms are to be maintained. It has been argued that redistribution of proximal and interstitial chiasmata to more distal positions is necessary for a convergent orientation, the only one that produces viable gametes. Cornops aquaticum is a South-American grasshopper that lives and feeds on water-hyacinths, and has three polymorphic Robertsonian rearrangements in its southernmost distribution area in Central Argentina and Uruguay. The orientation of trivalents in metaphase I, the formation of abnormal spermatids and the frequency and position of chiasmata in the trivalents, was analysed in a polymorphic population of C. aquaticus. In this study we observed a correlation between the number of trivalents with the frequency of abnormal spermatids; additionally, the number of chiasmata, especially proximal and interstitial ones, was strongly correlated with the frequency of the linear orientation. Therefore we confirmed our previous assumption, based on other evidence, that the chiasmata redistribution in fusion carriers is essential to the maintenance of the polymorphisms.