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The following points highlight the four main hypothesis proposed for the mechanism of crossing over. The hypothesis are: 1. The Classical and Neo-Classical Hypotheses 2. Partial Chiasma Type Theory 3. Belling’s Hypothesis 4. Copy-Choice Hypothesis.
Mechanism of Crossing Over: Hypothesis # 1. The Classical and Neo-Classical Hypotheses:
The classical hypothesis of crossing over was proposed by Sax in 1932; it assumes that a “chiasma” does not represent a “crossover” event but crossing over results when chiasma is ruptured (Fig. 11.4). The neoclassical hypothesis, proposed by Matsuura in 1940 and Haga in 1944 postulates that cytologically observed chiasmata bear no relationship to genetical crossing over.
Mechanism of Crossing Over: Hypothesis # 2. Partial Chiasma Type Theory:
This theory was originally proposed by Janssens is 1909 and 1924 and was elaborated upon and supported by Darlington in 1934, 1935 and by other workers. According to this theory, first, breakage occurs which is then followed by reciprocal reunion of the segments of non-sister chromatids; this results in exchange of chromatid segments or crossing over.
At the diplotene, the homologues are associated by the chiasmata which are the direct result of crossing over and are formed exactly at the points where the exchanges of non-sister chromatids have taken place (Fig. 11.4), At any given point, crossing over occurs between two of the four chromatids, but 3- and 4-strand double crossing overs are also possible in any given region.
The partial chiasma type theory was strengthened by the work of Brown and Zohary in 1955. They used a special stock of Liliumformosanum in which one arm of a chromosome had a large deficiency, while its homologue was normal; pairing of these homologues during meiosis formed a heteromorphic bivalent.
In this bivalent, the arm having the large deficiency never formed more than one chiasma; the sister chromatids were also associated distal to the chiasma (this is against the classical theory) (Fig. 11.5). The frequency of chiasmata in the segment between the centromere and the deficiency was scored in the PMCs at diakines and MI.
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In case of crossing over in the deficient arm, the two concerned chromosomes would show one deficient and one normal chromatid in the arm; this could be observed distinctly at AI (Fig. 11.5). This feature was used to score the frequency of cytological crossing over.
There was a close correspondence between the frequencies of observed chiasmata and the frequencies of crossing over (the latter represented by the frequency of AI cells having the peculiar chromosomes in which the two chromatids in one arm were of grossly unequal length). This gave the convincing proof that chiasmata show a virtually 1: 1 correspondence with cytological crossing over.
Mechanism of Crossing Over: Hypothesis # 3. Belling’s Hypothesis:
This theory was proposed by Belling in 1931 and 1933. According to this theory, crossing over is the result of an exchange between new chromatids while they are being synthesized. He considered that the synthesis of new chromomeres occurred first alongside the preexisting chromomeres and that the inter-chromomeric regions were synthesized subsequently.
If during the synthesis of inter-chromomeric region relational coil existed between the homologues, the inter-chromomeric fibres could join non sister chromomeres (Fig. 11.6). Such a joining would produce crossing over/crossover chromatids.
This hypothesis assumes that chromosome replication takes place during pachytene, and as a result, rules out 3- and 4-strand double crossing over. But if it is assumed that sister-strand crossing over can occur between two non-sister chromatid crossovers, the 2-strand double crossover is converted into 2- and 4-strand double crossovers.
However, this theory is not completely satisfactory because:
(i) There is no evidence of chromosome replication in two phases,
(ii) The genetic material (DNA) has a continuous linear organization,
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(iii) DNA synthesis (replication) occurs in interphase. A very small amount of DNA is synthesized during zygotene-pachytene at the time of crossing over,
(iv) Chromosomes at pachytene already consist of two sister chromatids which were synthesized during the interphase.
Mechanism of Crossing Over: Hypothesis # 4. Copy-Choice Hypothesis:
This hypothesis was formulated by Lederberg in 1955; it assumes that recombination occurs during chromosome (DNA) replication. It is postulated that during DNA replication, the new strand is synthesized using one chromosome as template up to some point at which the homologous chromosome begins to be used as template.
This hypothesis is also known as the complete-copy- choice theory. Thus the entire recombinant chromatid arises from newly synthesized sections. For instance, if two linked genes, say a and b, are present in one chromosome and their alleles, A and B, are in its homologue they could be recombined as Ab and aB through complete copy choice (Fig. 11.7).
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The complete copy choice also explains some aberrant microbial recombination data, such as, “nonreciprocal recombination.”
The copy-choice hypothesis holds that crossing over occurs as synthetic error during S phase (DNA replication phase). But there are reports where recombination occurs in the absence of DNA replication, e.g., in the virus Lambda (A.) of E. coli. Similarly, it has been found that crossing over occurs more than one week after the completion of DNA replication in Schistocerca.
