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.     

A re-examination of chiasma terminalization and chiasma frequency in male mice

The distribution and frequency of chiasmata have been analyzed in male BALB/c mice. Bivalents were classified in terms of the number of interstitial chiasmata (CH) and terminal associations (TA) present as follows; 1CH, 2CH, 1TA, 2TA, 1CH·1TA, 1CH·2TA, 2CH·1TA, 2CH-2TA and 0. We provide evidence that the TA frequently dissociates during 1st meiotic prophase. Consequently six of the observed bivalents may be derived from three basic bivalent types (namely 2CH·2TA, 1CH·2TA and 2TA) by dissociation of the TA according to the following schemas: (1) 2CH·2TA2CH·1TA2CH, (2) 1CH·2TA1CH·1TA1CH, and (3) 2TA1TA0. We also provide evidence that interstitial chiasmata do not move, which implies that a TA can not be formed by chiasma terminalization. The chiasma frequency estimated by assuming that terminal associations do not result from terminalized chiasmata is 17.2±2.4 compared to a value of 25.4±2.2 calculated on the assumption of chiasma terminalization.     

Cytogenetic behaviour and phosphate and potassium content in desynaptic pearl millet

Summary Continued inbreeding by self pollination resulted in a proportion of sterile plants in some families of the inbred line IP 1475 of Pennisetum americanum (L.) Leeke. Cytological examinations of the sterile plants revealed mild to extreme desynapsis and also chromosome fragmentation in some plants. Segregation ratios in the selfed families did not fit into any simple Mendelian ratio; however, in one F₂ family of the cross desynaptic x normal, segregation into 15 normal: 1 desynaptic was observed. Plants from a segregating family were classified as normals, desynaptics with 2–6 univalents, desynaptics with 2–10 univalents, desynaptics with 10–14 univalents and desynaptics with chromosome fragmentation. Estimation of the content of phosphate and potassium from the flag leaves did not reveal significant differences between the five groups of plants.     

Variation in chiasma frequency during tulip anther development

In tulip anthers meiosis begins first in the pollen mother cells next to the point of entry of the vascular bundle at the base. It then proceeds in a wave to the free end of the anther. The first cells to pass through meiosis have the lowest chiasma frequency and cells with a lower cell chiasma frequency have a lower total bivalent length. It is suggested that some factor which passes through the vascular bundle may cause the developmental variations in chiasma frequency possibly by first altering chromosome length (Summary see p. 179.)     

Phase transformation and chiasma frequency variation in locusts

Locusts exhibit two basic forms or phases, one characteristic of swarming populations, termed phase gregaria, and the other characteristic of low density populations, termed phase solitaria. It has been claimed by Nolte that locusts living at low density, both in the field and in the laboratory, have a reduced chiasma frequency compared with animals living at high density. A postulated gregarisation pheromone is held to be responsible for the stimulation of melanin biosynthesis in the swarming animals and an unknown metabolite in this pathway causes the increase in chiasma frequency, as well as other phenotypic changes associated with phase transformation. According to Nolte this represents a means of releasing stored genetic variation necessary for adaptation in the areas invaded by swarms. — This claim has been re-examined in laboratory stocks of Schistocerca gregaria using a methodology comparable to that of Nolte. No reduction in the chiasma frequency of isolated animals was observed in any of the experiments. The isolated animals did, however, develop a phenotype characteristic of phase solitaria in terms of their pigmentation and morphometrics.     

Phase transformation and chiasma frequency variation in locusts

The hypothesis, proposed by Nolte, that all locusts undergo an increase in their chiasma frequency when swarming is examined in the Migratory Locust, Locusta migratoria, using (1) animals which have been isolated in the laboratory at hatching, (2) crowded and isolated animals supplied by an independent laboratory and (3) field solitaria material. Animals isolated in the laboratory at hatching did not show a reduced chiasma frequency when compared with crowded controls despite the fact that they did develop a solitaria phenotype. Likewise, crowded and isolated animals supplied by workers at the University of Paris showed no difference in their chiasma frequency. Finally, animals living at very low density on the flood plains of the River Niger in Mali, West Africa, and exhibiting extreme phase solitaria characteristics proved to have very high chiasma frequencies which is the exact antithesis of the hypothesis. — Thus there is no evidence that there is any relationship between phase status or density and chiasma frequency in Locusta migratoria. This is in direct contradiction to Nolte's claims but in agreement with a previous study by the present author on Schistocerca gregaria.     

Looking for natural variation in chiasma frequency in Arabidopsis thaliana

Information concerning natural variation either in chiasma frequency or in the genetic basis of any such variation is a valuable tool to characterize phenotypic traits and their genetic control. Here meiotic recombination frequencies are analysed in nine geographically and ecologically diverse accessions of Arabidopsis thaliana, and a comparative study was carried out incorporating previous data from another eight accessions. Chiasma frequencies, estimated by counting rod and ring bivalents at metaphase I, varied up to 22% among accessions. However, no differences were found among plants of the same accession. There was a relationship, which does not necessarily imply direct proportionality, between the size of the chromosomes and their mean chiasma frequency. Chiasma frequency and distribution between arms and among chromosomes were not consistent over accessions. These findings indicate the existence of genetic factors controlling meiotic recombination both throughout the whole genome and at the whole chromosome level. The reliability of chiasma scoring as an indicator of reciprocal recombination events is also discussed.     

The effect of Actinomycin-D on chiasma frequency in Vicia faba

The effect of Actinomycin-D was studied on chiasma frequency in Vicia faba (var. Weibull's Akerabona). A significant decrease in chiasma frequency over control was observed in all cases irrespective of the time of fixation and chemical concentration. However, the maximum decrease was found in the material fixed after a recovery period of 24 hours. The decrease in chiasma frequency showed a linear relationship with time of recovery. Besides reduction in the chiasma frequency, cells showing complete failure of homologous pairing were also observed. These findings are discussed in the light of actinomycin-D action.     

The control of chiasma frequency in Vicia faba L.

Summary InVicia faba two groups of chromosomes can be distinguished and consequently correlations of chiasma frequency can be calculated between the groups and also within the group of small chromosomes. A negative correlation was demonstrated during some analyses while positive ones occurred in others, in fact a wide range of coefficients could be calculated by both methods. It is suggested that negative correlation of chiasma frequency occurs only when reproduction of the chromosomes is precipitated or pairing delayed, so that the degree of pairing may be affected in plants where many chromosomes are present or where the chromosomes are large in size. Such correlations can have no influence on recombination, but rather they are the occasional outcome of the mechanism controlling chiasma frequency.     

Variation in chiasma frequency among eight accessions of Arabidopsis thaliana

Natural variation in meiotic recombination frequency in Arabidopsis thaliana has been assessed by analyzing chiasma frequency variation among a range of geographically and ecologically diverse accessions. Fifty pollen mother cells at metaphase I of meiosis were analyzed from each of eight accessions and fluorescence in situ hybridization was applied to enable identification of all 10 chromosome arms. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Further analysis confirmed this finding and identified two particular accessions, Cvi and Ler, as having chiasma frequencies significantly lower than those of the other accessions. The analysis also revealed that the pattern of chiasma distribution between arms and among chromosomes is not consistent over accessions. Further detailed analyses were conducted on each individual chromosome (1-5) in turn, revealing that chromosome 4, one of the acrocentric chromosomes, is the least variable while the other acrocentric chromosome (2) is the most variable. These findings indicate the existence of recombination regulatory elements in Arabidopsis and we conclude that it may be possible in the future to identify these elements and determine their mode of action. The practical implications of such developments are considerable.     

Photosynthetic response of P-deficient Gracilaria tenuistipitata under two different phosphate treatments

Phosphorus-deficient Gracilaria tenuistipitata Zhang et Xia was cultured for 15 days at two different inorganic phosphate (P_i) concentrations: 3 μM (low P_i treatmenl) or 30 μM phosphate (high P_i treatment). The amount of ribulose-l,5-bisphosphate carboxy-lase/oxygenase (Rubisco), phycobiliproteins, Chl a and total soluble proteins were higher in the high P_i than in the low P_i treatment. The total N content of the low P_i plants was lower than in plants grown at high P_i concentrations whereas the amount of total C was highest in low P_i plants. The increase of Rubisco content in the high P_i treatment (3-fold) was parallel to the enhancement of the maximum photosynthetic rate which increased 5-fold. This correspondence was also found in the low P_i treatment in which Rubisco content and maximum photosynthesis did not change significantly from the initial values. The photosynthetic efficiency was also higher at high than at low P_i. The high P_i plants also showed higher dark respiration and growth rates. The data presented here suggest that marine macroalgae submitted to P_i deficiency exhibit a decrease in growth caused not only by P_i implication on energy transfer in photosynthesis and respiration, but also by the diminution of the amount of photosynthetic pigments and Rubisco.