The following points highlight the fourteen main factors affecting crossing over and chiasma formation. The factors are: 1. Temperature 2. Age 3. Water Content 4. Sex 5. Nutritional Effects 6. Chemicals 7. Radiations 8. Shift in Population 9. Variation within the Organism 10. Chromosome Structure 11. Interference 12. Heterochromatin 13. Centromere 14. Genetic Effects.
Crossing Over and Chiasma Formation: Factor # 1. Temperature:
Chiasma frequency is depressed under high and low temperatures. Typically, the response of chiasma frequency to temperature is represented by an inverted U- shaped curve. Chiasma formation declines at high and low temperatures and sometimes such temperatures may lead to asynapsis.
Crossing Over and Chiasma Formation: Factor # 2. Age:
In general, crossing over has been found to decrease with the increasing maternal age in Drosophila, and in some other organisms.
Crossing Over and Chiasma Formation: Factor # 3. Water Content:
Chiasma frequency may be reduced due to low water content, and even complete failure of pairing and crossing over may occur in extreme cases. The degree of de-synapsis in plants has been shown to be greater in dry than in normal seasons.
Crossing Over and Chiasma Formation: Factor # 4. Sex:
Sex differences in chiasma frequency are known in many plants and animals. In some cases, the frequency of crossing over is greater in the homogametic sex as compared to that in the heterogametic sex. In Diptera, females are homogametic, while males are heterogametic, and crossing over is absent in most of the Dipteran males.
Although maize is a monoecious plant, crossing over in chromosome 5 close to the heterochromatic segment has been found to be higher in the PMCs than in the megaspore mother cells. In certain plant species, e.g., Fritillaria spp. meiosis is achiasmate in PMCs, while chiasmate in MMCs (Section 11.1.1).
Crossing Over and Chiasma Formation: Factor # 5. Nutritional Effects:
Concentrations of calcium and potassium in the diet have been reported to affect the recombination and chiasma frequencies. For example, Drosophila females fed on a high calcium diet show lower recombination frequencies between sex-linked genes, while crossing over increases when they are fed on a medium containing chelating agents that remove the metallic ions.
Crossing Over and Chiasma Formation: Factor # 6. Chemicals:
Certain chemicals have pronounced effect on chiasma formation. The inhibitors of RNA synthesis (actinomycin D) and those of DNA synthesis (mitomycin C), both affect crossing over in the female Drosophila. Recombination was found to be increased by these chemicals in the regions near the centromere.
Mitomycin C has also been found to increase mitotic crossing over in Saccharomyces cerevisiae. Another inhibitor of DNA synthesis, 5- fluorodeoxyuridine, has been found to increase genetic recombination in Ustilagomaydis. Recombination is reduced due to the prevention of chromosome pairing by colchicine.
When protein synthesis is blocked chemically during zygotene-pachytene stages, chiasma formation is prevented. Alkylating agents have been found to depress recombination in Chlamydomonasreinhardtii.
Crossing Over and Chiasma Formation: Factor # 7. Radiation:
Effects of radiations on crossing over and chiasma formation have been studied in different organisms.
Two important conclusions have emerged from these studies:
(a) The frequency of crossing over decreases if irradiation is done during the pre-meiotic DNA synthesis phase, e.g., in Lilium and Tradescantia, etc.
(b) The frequency of crossing over and chiasma formation increases due to irradiation during the zygotene stage of these species.
In Chlamydomonas, recombination is dependent on irradiation dose-rate. The frequency and position of chiasmata are affected by infra-red radiation, and the increase in chiasma frequency due to infra-red treatment involves the production of interstitial chiasmata.
Crossing Over and Chiasma Formation: Factor # 8. Shift in Populations:
In some cases, differences in chiasma frequency may occur due to a shift in the phase/physiological state. An increase in chiasma frequency is observed with the transition of the solitaria phase to gregaria phase in South African locusts.
Crossing Over and Chiasma Formation: Factor # 9. Variation within the Organism:
Chiasma frequency in plants has been found to vary in different flowers.
Crossing Over and Chiasma Formation: Factor # 10. Chromosome Structure:
Structural rearrangements in the chromosomes affect the frequency of crossing over. Short inversions may act as “crossover suppressors” within the inverted region because the inversion interferes with chromosome pairing.
However, such chromosomal aberrations in one pair of homologous chromosomes may increase the frequency of crossing over in remaining normal non-homologous chromosomes; this phenomenon is also called “Schultz Redfield Effect” after its discoverers.
This effect is both intra- and inter-chromosomal; it could be the result of a genetic homoeostatic effect which keeps the crossing over frequency close to an optimal value within the population.
Crossing Over and Chiasma Formation: Factor # 11. Interference:
The occurrence of one crossing over event at a given site ordinarily reduces the probability of occurrence of a second crossing over events in its vicinity in either direction; this is called positive interference. In some cases, however, there is an increase in the probability of a second crossover event in the neighbourhood; this is called negative interference.
Both these phenomena together constitute chromosome interference of chiasma interference or position interference, and it results in either increased or decreased frequencies of double crossovers.
The first chiasma appears freely, usually close to the pairing initiation point at a distance called differential distance. The average distance at which successive crossovers and chiasmata are formed after the formation of the first chiasma is known as the interference distance.
It may, however, be noted that the distances between neighbouring chiasmata are variable, and the interference distance represents the average of these distances for a species. Another type of interference, known as chromatid interference refers to the non-random participation in successive cross-overs of the four chromatids belonging to the two homologous chromosomes.
In general, a chromatid involved in a crossing over has a reduced chance to be involved in a second chiasma as compared to the one that has so far not been involved in crossing over. The expected ratio of the frequencies of 2-strand, 3-strand and 4-strand double crossovers is 1 : 2 : 1 based on random chromatid, i.e., in the absence of chromatid interference.
A significant deviation occurs from this expectation due to chromatid interference. This type of interference also influences the recombination frequency.
Crossing Over and Chiasma Formation: Factor # 12. Heterochromatin:
It is well known that the frequency of crossing over is lower in the heterochromatic regions, but sometimes it increases in the regions adjacent to the heterochromatin. The presence of heterochromatic B chromosomes also affects the frequency of crossing over.
In maize, B chromosomes are almost heterochromatic; their presence enhances crossing over in the normal chromosomes, however, the euchromatic B chromosomes do not have such an effect.
Crossing Over and Chiasma Formation: Factor # 13. Centromere:
When a chromosome segment carrying genetic markers is placed close to the centromere, there is a decline in the frequency of crossing over between these genes, although the physical distance between them remains the same.
Crossing Over and Chiasma Formation: Factor # 14. Genetic Effects:
Chromosome pairing, crossing over and chiasma formation are affected by both, oligo- and polygenes. In several cases, asynapsis and de-synapsis (section 11.2) are controlled by single genes. Polygenic control of chiasma frequency has been shown in rye (S. cereale) which is a crossbreeding species.
Positive heterosis for chiasma frequency is often observed in F1 hybrids mainly as a result of non-allelic interactions. On the other hand, negative heterosis for chiasma frequency has been observed in the self-pollinated species wheat (T. aestivum)-, again this is largely due to non-allelic interactions.