Cytology of inbreds and f 1 hybrids of pearl millet

Summary The meiotic behaviour of chromosomes was studied in four inbred lines and their F ₁ hybrids of P. typhoides. The inbred lines showed a decrease in mean chiasma frequency when compared with the population plants, but differed from one another in their mean chiasma frequencies. Together with the decrease in mean chiasma frequency the inbreds showed variation in mean chiasma frequencies. The inbred lines showed a number of meiotic abnormalities such as extra chromosomes, extra fragments, desynapsis, persistent nucleoli and differential condensation of chromosomes. The F ₁ hybrids of these inbreds exhibited heterosis for chiasma frequency. All the F ₁'s had mean chiasma frequencies higher than the means of the respective participating parents. The F ₁'s, however, differed in the degree of heterosis exhibited. In the F ₁ hybrids, the variation in mean chiasma frequency, both between plants and between PMC's within plants, was significantly lower than that of the inbred lines. The effect of environment was studied in the inbred lines and their F ₁ hybrids. The mean chiasma frequencies of the inbred lines were significantly lower, and the variation in mean chiasma frequencies was greater, in the stress season. The mean chiasma frequencies of F ₁'s did not show any significant differences between the two seasons. The F ₁'s exhibited less variation in mean chiasma frequency than the inbred lines, showing that F ₁'s were developmentally more stable. The F ₁'s did not show any meiotic abnormalities in either season.     

Relationships between frequency,localization and errors in chiasma formation in desynaptic rye

Four inbred lines of rye (Secale cereale) and the F₁ and F₂ from the cross between two of them have been studied. The results indicate that the genotypes used show variation in chiasma frequency, chiasma error frequency and chiasma localization. Significant correlations between these characters have been found: as chiasma frequency decreases both chiasma error frequency and distal localization increase. These correlations lead us to the assumption that these anomalies are in fact secondary effects of the failure of some preconditions for exchange. It is suggested that one such exchange precondition may be effective pairing.     

C-bands and chiasma distribution inScilla amoena,S. ingridae,andS. mischtschenkoana (Hyacinthaceae)

An effect of C-band pattern and polymorphism on chiasma distribution in pollen meiosis was recently demonstrated inScilla siberica. A further meiotic banding study has been performed in the alliesS. amoena, S. ingridae, andS. mischtschenkoana in order to analyze the effect, if any, of their specific C-band patterns and cytochemically different heterochromatin types on recombination. No clear evidence for a preferential formation of chiasmata adjacent to homozygous intercalary heterochromatin and no consistent reduction of chiasma frequency near strongly heterozygous intercalary heterochromatin blocks, as observed inS. siberica, could be found. Terminal C-band heteromorphism is suspected to cause distal chiasma defaults. The results suggest once more that there is no uniform effect of heterochromatin on crossover distribution.     

Chromosome pairing in tetraploid hybrids between Lolium perenne and L. multiflorum

Summary Chromosome association at first meiotic metaphase in tetraploid hybrids between Lolium perenne and L. multiflorum was compared with that in autotetraploid L. perenne. The hybrids were found to have significantly higher levels of bivalent frequency, and lower levels of multivalent and chiasma frequency. A significant increase in multivalent frequency with increasing chiasma formation was found in both groups, but the increase was much less in the hybrids. These differences in chromosome associations between the two groups must therefore reflect differences in chiasma distribution and it is suggested that the results indicate a significant degree of preferential bivalent pairing in the hybrids.     

Cytogenetics of pearl millet

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

Genetic and environmental components of chiasma control

Variation in chiasma frequency within and between individuals has been investigated in Schistocerca gregaria and Stethophyma grossum. By taking sequential samples of the same testis in S. gregaria it has been demonstrated that there is considerable variation in chiasma frequency between times, within individuals, and that this variation is not evenly distributed between individuals of the same population. The response and recovery patterns after exposure to successive temperature treatments also indicate a differential sensitivity of individuals within the experimental population. — An analysis of interfollicular variation in chiasma frequency has revealed significant differences between follicles within individuals both in S. gregaria and a Spanish population of S. grossum. A comparable analysis on individuals of S. grossum carrying supernumerary segments shows that the presence of such segments increases the amount of variation between follicles within individuals and also between individuals within the population. — The distribution of cell chiasma frequencies in S. gregaria is normal whereas in Stethophyma cell frequencies approximate to a Poisson distribution. The structurally different supernumerary segments, present in Austrian and Spanish populations of S. grossum, both increase mean chiasma frequency and they modify the between cell variance in different ways — the former interchromosomally and the latter intrachromosomally. — The differences in chiasma frequency between follicles and between times within individuals and the differential reaction to heat shock reflect differences in the pattern of genotype-environment interaction. Similarly it can be argued that the presence of polymorphisms involving supernumerary material may play an equivalent role by increasing the between cell or between bivalent variance within individuals, a unique form of genotype-environment interaction at the endophenotypic level. — Such variation in genotype sensitivity to environmental modification has important adaptive value especially in organisms which are subject to recurrent, often random, changes in their environment.     

Genetic and environmental components of chiasma control

Two selected lines of Schistocerca gregaria with mean chiasma frequencies of 17.68 and 21.66 were crossed in all possible combinations up to the F₂ generation. A genetic analysis of both generation means and within family variances reveals that the control of chiasma frequency in these lines is governed by a large additive genetic component (d) and a small dominance component (h) directed towards high frequency types. 40% of the total variance can be explained in terms of environmentally induced variation. It is argued that such significant and readily induced changes in the chiasma pattern, associated with a mean cellular difference of 4 chiasmata, can be expected to lead to an alteration in the linkage relationships within the genomes of each line. Unfortunately the intensity of the effect upon linked non-allelic associations can only be assumed since, at the present time, there is no evidence to correlate chiasma frequency variation in the parent with ensuing exophenotypic variation among the progeny.     

Variation in chiasma frequency in male Trichosurus vulpecula (Marsupialia: Mammalia)

Chiasma frequency in spermatocytes was found to show extensive variation between individual males from a natural population of the marsupial Trichosurus vulpecula. Two factors which might contribute to the observed variation were investigated; season and age. However, neither of these had any significant effect on chiasma frequency or chiasma frequency variation.     

Unusual chromosome pairing and B chromosomes inBriza spicata (Poaceae)

One plant from a population ofBriza spicata (Poaceae) was found to have highly irregular meiotic behaviour. It is characterized by having a reduced chiasma frequency, a large between cell variance in chiasma frequency and the formation of multivalents involving pairs of A chromosomes. The B chromosome present in this plant also forms multivalents with a pair of A chromosomes. It is suggested that the normal control of strict bivalent pairing has broken down and homoeologous chromosomes are associating as multivalents. Furthermore, the partial homology of the B chromosome with a pair of A chromosomes is revealed.     

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.     

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.     

An effect of potassium on chiasma frequency and recombination

High and low concentrations of potassium and calcium were combined factorially and applied to the plant,Lolium temulentum. An effect on mean plant chiasma frequency was detected when such plants underwent meiosis at 30°C. Potassium rather than calcium was shown to be the mineral responsible for the observed effects. High concentration of potassium resulted in an increase in mean chiasma frequency. At 20°C little effect of either mineral could be demonstrated, although potassium again was responsible for most of the variation produced.Potassium has also an effect on stability. At 30°C high concentrations of potassium reduce plant variation, whereas at 20°C the converse occurs and an increase in plant variation results. It is suggested that such a difference between the two temperatures supports the conclusion that different genetic systems control the plant means and plant variation.The effect of potassium on chiasma frequency was confirmed inDrosophila melanogaster by studying its effects on recombination in the X chromosome. A striking influence on body size, possibly correlated with recombination, was also observed.The conflict of such results with previous work in which calcium has been shown to be the most important mineral is discussed. The difficulties of arguing a direct role at meiosis for external treatments is emphasised.     

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.     

The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S₁₁ chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S₁₀ and S₁₁ chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.On educational leave from the Forest Research Laboratory, Fredericton, N. B. Canada.     

Genetic interactions between wheat and rye genomes in triticale

Summary Six primary triticale lines were produced from two advanced breeding lines of Triticum durum and three inbred genotypes of Secale cereale. The wheat and rye parents and the triticale derivatives were crossed in all possible combinations within each species group. Chiasma and univalent frequency of parents and hybrids were determined. The primary triticale lines had more univalents and less chiasmata per pollen mother cell than the corresponding wheat and rye parents together. The parental wheat F₁ exhibited negative heterosis for chiasma frequency whereas all rye hybrids had much higher chiasma frequencies than their inbred parents. Triticale F₁s generally showed lower chiasma frequencies and more univalents than their parents, but the degree of pairing failure was dependent upon which of the parental species within the triticale, wheat or rye, was in the heterozygous state. F₁s with heterozygous wheat genome only showed the least reduction in chiasma number (presumably caused by gene actions within the wheat genome), while F₁s with heterozygous rye genome showed high reduction in chiasma frequency and an increase in pairing failure (induced by negative interactions between the heterozygous rye and the wheat genome in triticale). A high correlation was found between the frequency of undisturbed pollen mother cells and the frequency of aneuploids in the subsequent generation. A higher number of aneuploids occurred in those populations which were heterozygous for the rye genome.     

Metaphase I bound arms and crossing over frequency in rye

Using a Giemsa C-banding procedure it has been possible to identify at meiosis three chromosome pairs of a local Spanish rye cultivar. Two of these chromosomes (3 and 5) were heterozygous for an interstitial C-band in the long arm and the other (chromosome 7) was heterozygous for a telomeric C-band, also in the long arm. From the frequency of being bound at metaphase I and the frequency of recombined chromatids at anaphase I in the arms considered, estimates of actual chiasma frequencies have been derived. The results have been compared with those obtained in a Fl between two inbred lines. It is concluded that: (i) Although the frequency of bound arms analyzed was similar in all cases, the chiasma frequency was higher in the cultivar than in the Fl plants. Cultivar plants showed a variation in chiasma frequency for the bivalent arms studied which was correlated with the frequency of bound arms per cell, indicating that the estimation for chiasma frequency by means of bound arm frequency has an error that increases with increasing number of bound arms per cell, (ii) Evidence of chiasma terminalization has not been found, (iii) It is suggested that the different rye chromosomes have different chiasma localization patterns, which, in turn, are related with the chiasma frequency.     

The control of chiasma distribution in the locust,Schistocerca gregaria (Forskål)

Chiasma distribution at diplotene in Schistocerca gregaria males can be taken to indicate the positions at which crossing over occurred prior to diplotene since chiasma terminalisation is entirely absent. Analysis of chiasma frequency and position leads to a model for the mechanism controlling distribution which has three main components. — i) The bivalents vary in length between cells due to unknown factors and increase in bivalent length leads to an increase in chiasma frequency. — ii) Within bivalents chiasma initiation is sequential from telomere to centromere with the first chiasma usually forming close to the telomere. — iii) Interference operates with the same polarity and determines a distance within which crossing over is precluded.     

Asynapsis in Lolium perenne

Eighteen plants displaying varying degrees of asynapsis ranging from weak to very strong were found among four out of six populations of Lolium perenne L. (2n=14) which had been subjected to three cycles of directional phenotypic selection for productivity of green material. No plants were found displaying univalents in the original generation but the incidence increased with cycles of selection, indicating the genetic control and differential distribution of asynaptic genes among these populations. — The analysis of univalents and chiasma frequency of pollen mother cells (PMC) of six partially asynaptic plants chosen for detailed study revealed that univalents occurred throughout all PMC chiasma classes irrespective of chiasma frequency, but the higher the chiasma frequency of any PMC the less the likelihood of univalents occurring. The relationship between chiasma frequency and univalent frequency per PMC per plant was negative. — Mean chiasma frequency per bivalent increased for the asynaptic cells in comparison with the normal in both the weak and medium asynaptic groups which was explained by the availability of additional chiasmata for redistribution.     

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)     

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.