Chromosome polymorphism in populations of the grasshopper Trimerotropis pallidipennis from southern Argentina

Three populations of the grasshopper Trimerotropis pallidipennis from southern Argentina have been studied cytologically. A very characteristic B-chromosome was found in all three. They also showed geographical variability in respect of the presence of pericentric inversions, and the inversion system was found to influence chiasma frequency. The Laguna Blanca population, which is on the hypothetical pathway the species is believed to have followed during its migration from northern to southern latitudes, has the same karyotype composition as the N. American form, with fixed inversions in the 3 largest autosomes and the X-chromosome. Its members have a high total chiasma frequency and a great number of interstitial chiasmata. The Sierra de la Ventana population, situated at the absolute eastern border of the species distribution is highly polymorphic with respect to the presence of inversions in the medium chromosomes. Its members have the lowest total chiasma frequency and a greatly reduced number of interstitial chiasmata. Situated geographically between the other two, the Choele-Choel population has the highest frequency of inversions and many of them are homozygous. Its members have a higher total chiasma frequency than that observed in specimens from Sierra de la Ventana, and a greatly reduced number of interstitial chiasmata, similar to that observed in individuals from the latter population.     

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.     

Chiasmata and the breeding system in wild populations of diploid wheats

Seven populations of the selfer Triticum longissimum (= Aegilops longissima) and five populations of the closely related outbreeder T. speltoides (= Ae. speltoides) were scored for chiasma frequencies in pollen mother cells. The populations of the selfer have significantly higher frequencies of chiasmata than the outbreeding populations. This difference becomes even clearer when interstitial chiasmata alone are compared. It is argued that an optimal degree of effective recombination is achieved by the balance between outbreeding and interstitial chiasmata. — There are wider differences between the selfing populations than between the outbreeding populations, but the differences between families (within populations) are small in both species. Variation between plants within families seems to be lower in the selfer, but nevertheless high enough to be inexplicable on the basis of selfing alone. — Small populations subject to hardship conditions show a higher frequency of chiasmata than others.     

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)     

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.     

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.     

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.     

Chiasma frequency,distribution and interference maps of mouse autosomes

Chiasma frequencies were analysed and chiasma positions measured in diakinesis/metaphase I autosomal bivalents from oocytes and spermatocytes of F₁ hybrid C3H/HeH×101/H mice. Twenty chromosome size ranks, including the presumptive X bivalent, could be distinguished in oocytes, and nineteen autosomal ranks plus the XY pair spermatocytes. Overall, mean cell chiasma frequencies of the two sexes did not differ significantly once the contribution of the presumptive X bivalent and the XY pair were taken into account. Sex related differences in chiasma distribution patterns were evident, however. In monochiasmate bivalents, the chiasma was most commonly located interstitially in oocytes while in spermatocytes it could be either interstitial or distal. In dichiasmate bivalents, the chiasmata tended to be more centrally located in oocytes than in spermatocytes. Minimum inter-chiasma distances did not appear to show any great variation in chromosome pairs of different sizes, however, mean inter-chiasma distances did increase with the bivalent length. The minimum-inter chiasma distance data suggest that chiasma interference is complete over a chromosomal segment equating to approximately 60 Mb. Measurement of the positions of chiasmata along chromosome arms open up the possibility of producing chiasma-based genetic maps for all the autosomes of the mouse.     

Effects of centric fusions on chiasma frequency and position in Leptysma argentina (Acrididae: Orthoptera) I. Spontaneous and stable polymorphic centric fusions

Leptysma argentina is a highly polymorphic South-American grasshopper from the cytological point of view; all populations so far studied carry a polymorphic fusion between pairs 3 and 6. In heterozygotes, the trivalent 3-3/6-6 shows alternate orientation in 97.17% of the cells. Trivalent chiasma frequency is significantly lower than in the combined 3 and 6 bivalents of the standard homozygote; besides, there is a marked displacement of chiasmata to a distal position. In structural homozygotes the same effects, but not so marked, were observed.One individual was a double heterozygote for both the polymorphic centric fusion and a spontaneous one between pairs 5 and 7. The presence of a fragment, sometimes associated with the centromeric region of the nonfused 5 chromosome, was detected in more than 50% of the cells. The orientation of trivalent 5-5/7-7 in metaphase I was highly irregular (36% linear orientation). Neither frequency nor position of chiasmata were altered in trivalent 5-5/7-7 when compared with bivalents 5+7 of normal individuals.The results suggest that proximal and interstitial chiasma reduction observed in trivalent 3-3/6-6 of L. argentina is due to a later adaptation to the polymorphic condition or a fortuitous genetic condition present in the original mutant, rather than to a direct effect of the fusion itself on chiasma distribution.Fellow of the Consejo Nacional de Investigaciones Cientificas y Técnicas (CONICET)     

Spatiotemporal Asymmetry of the Meiotic Program Underlies the Predominantly Distal Distribution of Meiotic Crossovers in Barley

Meiosis involves reciprocal exchange of genetic information between homologous chromosomes to generate new allelic combinations. In cereals, the distribution of genetic crossovers, cytologically visible as chiasmata, is skewed toward the distal regions of the chromosomes. However, many genes are known to lie within interstitial/proximal regions of low recombination, creating a limitation for breeders. We investigated the factors underlying the pattern of chiasma formation in barley (Hordeum vulgare) and show that chiasma distribution reflects polarization in the spatiotemporal initiation of recombination, chromosome pairing, and synapsis. Consequently, meiotic progression in distal chromosomal regions occurs in coordination with the chromatin cycles that are a conserved feature of the meiotic program. Recombination initiation in interstitial and proximal regions occurs later than distal events, is not coordinated with the cycles, and rarely progresses to form chiasmata. Early recombination initiation is spatially associated with early replicating, euchromatic DNA, which is predominately found in distal regions. We demonstrate that a modest temperature shift is sufficient to alter meiotic progression in relation to the chromosome cycles. The polarization of the meiotic processes is reduced and is accompanied by a shift in chiasma distribution with an increase in interstitial and proximal chiasmata, suggesting a potential route to modify recombination in cereals.     

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.     

Orientation and segregation of Robertsonian trivalents in Dichroplus pratensis (Acrididae)

Pairing behavior, metaphase I orientation, and anaphase I segregation of centric fusion trivalents were studied in 26 single, 15 double, and 2 triple male fusion heterozygotes of the polymorphic South American melanopline grasshopper Dichroplus pratensis. They represent the seven different fusions and their combinations already described in different populations of the species. Our analysis showed the following: (1) pairing behavior is very regular in all trivalents; (2) frequencies of linear orientation was very low irrespective of the trivalent involved; (3) reorientation seems to occur frequently since frequencies of abnormal segregation and aneuploid second division cells were invariably lower than those of nonconvergent orientation; (4) aneuploidy and abnormal sperm production increases with increasing number of fusions; (5) chiasma frequency and localisation is relevant to trivalent orientation since trivalents with nonconvergent orientations showed proximal and interstitial chiasmata more frequently than convergently oriented ones. The results are in agreement with the hypothesis that these polymorphisms are old and stable, and confirm that for the maintenance of a balanced polymorphism, if this polymorphism is adaptive because of its consequences on recombination, position effects, etc., changes tending to stabilise trivalent orientation and segregation are central.     

Chiasmata and chromosome breakages are related to crossing over in Drosophila ananassae males.

A cytogenetic analysis of male crossing over in Drosophila ananassae revealed that cytological exchanges resulted in genetic crossing over, and that chiasma frequency and the genetic recombination correlated positively in chromosomes 2 and 3. Furthermore, the frequency of chromosome breakages correlated positively with chiasma frequency. Paracentric inversion heterozygosity had no detectable influence on the chromosome pairing or exchange events within the inversion loop at meiosis. Scoring of the chiasma demonstrated that males homozygous for the previously mapped enhancers of male crossing over had low frequencies of chiasmata, whereas higher frequencies of chiasmata were observed in males heterozygous for enhancers. The results presented here indicate that the genetic factors controlling male crossing over are involved in the origin of chromosome breakages and in exchange events.     

Comparative chiasma analysis using a computerised optical digitiser

A new computerised technique has been devised for measuring the distribution of chiasmata along diplotene bivalents. The method involves the introduction into the field of view of the microscope, of a fine light spot which can be accurately manipulated along the chromosomes of each bivalent. The data recorded include (a) the positions of the chiasmata along the bivalent in terms of their relative distances from the centromere and (b) the individual bivalent and cellular chiasma frequencies. — The method has been applied to the analysis of chiasma distribution patterns in the two known species of the genus Caledia, C. species nova 1 and C. captiva and in two chromosomal races of the latter. Statistical tests indicate that within bivalents at least 40% of the comparative distribution patterns of chiasmata between races and species are significantly different. Similar comparisons between populations within races reveal only 18% significant differences. — The observed distribution patterns of chiasmata in this genus suggest that chiasma formation is sequential from centromere to telomere. — The variation in the frequency and distribution of chiasmata between races and species suggests that the interference distances between successive chiasmata are, at least partially, independent of chiasma frequency and position. — The interracial and interspecific differences in chromosome structure are correlated with changes in chiasma pattern.     

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.     

Lampbrush Chromosomes in the Japanese Quail Coturnix coturnix japonica: Cytological Map of Macrochromosomes and Meiotic Crossing-over Frequency in Females

Cytological map of lampbrush macrobivalents of the Japanese quail (Coturnix coturnix japonica) were constructed. Investigation of chiasmata allowed to estimate the frequency of reciprocal genetic recombination (crossing over) in Japanese quail female meiosis. 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 chicken. 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 crossing over per Mb. The genetic length of one Mb (recombination rate ) 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.     

High recombination between the breakpoint of a reciprocal translocation in rye (Secale cereale L.) and an interstitially located gene

Summary The recombination fraction between the interstitially located gene an and interchange 303 of rye was found to be 0.244±0.038 in a test cross using the translocation as the male parent. In first metaphase translocation configurations in pollen mother cells of the same plant, the chiasma frequency between an and the translocation breakpoint was found to be significantly more than twice the recombination fraction. Recombination was concluded to be masked by a difference in the alternate frequency between configurations without interstitial chiasmata and configurations with interstitial chiasmata, the effect of the first type being of major importance. Random centromere orientation of translocation multivalents with interstitial chiasmata was concluded to be a realistic assumption. The exceptionally high recombination between an and translocation 303 is discussed. Consideration is also given to the use of interchanges in the establishment of a marker's chromosomal position, and to the use of translocation chromosomes in balanced systems for hybrid breeding purposes.     

Population Genetics of Drosophila Amylase. II. Geographic Patterns in D. PSEUDOOBSCURA

Morph frequencies of three related polymorphisms were determined in ten natural populations of Drosophila pseudoobscura. They are the well-known inversion polymorphism of the third chromosome and the polymorphism for alpha-amylase produced by the structural gene Amy (which resides on the third chromosome). The third polymorphism was for tissue-specific expression of Amy in adult midguts; a total of 13 different patterns of activity have been observed. The preceding paper (Powell and Lichtenfels 1979) reports evidence that the variation in Amy expression is under polygenic control. Here we show that the polymorphism for midgut patterns occurs in natural populations and is not an artifact of laboratory rearing.--From population to population, Amy allele frequencies and frequencies of inversions belonging to different phylads vary coordinately. The geographic variation in alpha-amylase midgut activity patterns is uncorrelated with that for the other two types of polymorphisms. Furthermore, no correlation was detected between activity pattern(s) and Amy genotype(s) when both were assayed in the same individual.--These results imply that whatever the evolutionary-ecological forces are that control frequencies of the structural gene variants, they are not the same factors that control the frequencies of polymorphic genetic factors responsible for the tissue-specific expression of the enzyme.     

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).