Crossing over occurs in meiosis as well as mitosis. However, the process of crossing over occurring in the mitosis is rare. Homologous recombination in the meiosis involves the exchange of material between the homologous segments of two DNA molecules. Similarly, studies do mention mitotic recombination. A stage in mitotic recombination resembles the four-strand stage in meiosis. The progeny cells obtained from mitosis include a combination of genes differing from the diploid parent cell.
Mitotic recombination in Drosophila:
Curt Stern studied mitotic recombination in Drosophila strains. These strains had recessive sex-linked mutations. Two varieties of strains included bristle type and body color. One of the strains had short twisty bristles instead of normal long curves. The other type included yellow body versus normal body color. The experimenters considered a cross between grey bodied females with singed bristles and yellow bodied males with normal bristles. The locus for the singed bristles is known as sn locus. The F1 progeny had the majority of females with wild-type phenotype. However, few of them had yellow patches and singed bristles. First, the experimenters thought the reason behind this progeny to be chromosomal non-disjunction or chromosomal loss. Then they identified females with twin spots. Hence, the mitotic crossing over came into the picture.
Image 1: Mitotic recombination in Drosophila leading to different kinds of spots.
Consider a crossing over between the sn locus and centromere. The segregation of chromatids takes place. The first and the third chromatids segregate to one daughter nucleus. The second and the forth chromatids segregate to the other daughter nucleus. Upon the division of these cells, the progeny strains have twin spots. They produce a yellow patch of tissues and a singed patch of tissues. The remaining area shows wild-type phenotype. The other kind of orientation involves segregation of the first and the forth chromatids to one nucleus and second and the third chromatid to the other nucleus.
Mitotic recombination in Aspergillus nidulans:
Aspergillus nidulans is a kind of fungus. The studies of mitotic recombination in this fungus help to construct the genetic maps. The fungus does not involve controlled crosses. Hence, it is not possible to study meiotic recombination. It is a mycelial fungus. The uninucleate asexual spores of the fungus give greenish colonies. The genotype of the nucleus determines the phenotype of the asexual spores. Mixing of the two haploid strains helps them fuse together. The mitotic recombination studies require heterozygous genes. The two haploid strains differing in the phenotypes get fused together. They give rise to a mycelium having two nuclear types. Both the nuclear types divide mitotically in the same cytoplasm. These cells are known as heterokaryons since they have different nuclei.
The parasexual cycle:
It involves the genetic systems involving genetic recombination through the processes other than the meiosis and fertilization. The parasexual cycle is a typical feature of fungi such as Aspergillus. The first step of the cycle involves mycelial fusion. It gives rise to a heterokaryon (having two different nuclei). Then the two haploid nuclei fuse. They form a diploid nucleus. The next step involves a mitotic crossing over within the diploid nucleus. The diploid nucleus, later on, undergoes haploidization.
Image 2: Mitotic crossing over between the pro and the paba loci.
Consider the two haploid strains:
The first strain consists of genotype w ad+ pro paba+ y+ bi.
The second strain consists of genotype w+ ad pro+ paba y bi+.
The alleles such as ad, pro, paba, and bi show recessive phenotype. They depict adenine, proline, para-aminobenzoic acid, and biotin, respectively. The growth medium must have all the above nutrients for the survival of the mutant strains. The parental strains require the above growth supplements. However, the heterokaryon does not require them, since it consists of all the four genes capable of synthesizing them. The recessive w and y alleles control the color of the asexual spores, thereby giving rise to the colony color. The “w+y+” strain shows green coloration. The “wy+” strain shows white coloration. The “w+y” strain shows yellow coloration. The “wy” strain shows white coloration. The heterokaryon of the first and the second strains is not green. The heterokaryon shows a mixture of yellow and white spores. It has a mottled appearance. Diploidization is a relatively rare case. The two haploid nuclei fuse together. They produce a diploid nucleus in such cases. Hence, the spores in these diploid cells would also be diploid. These cells do not require growth supplements since they show genotype with wild-type alleles. Mostly, they give rise to green colonies on a growth medium or a solid medium.
Haploidization produces the haploid sectors. It leads to the formation of the haploid nuclei from the diploid nucleus. These nuclei divide by mitosis. The haploid progeny nuclei are known as haploid segregants. Consider the haploid white sectors. Half of them show a genotype of w ad+ pro paba+ y+ bi. The remaining half strains have a genotype of w+ ad pro+ paba y bi+. It involves a 50:50 segregation ratio indicating the location of genes to be on different chromosomes. The six gene loci are present on the two nonhomologous chromosomes. The white gene is present on one chromosome. The other five genes are present on the other one. The next step involves the determination of the gene order and map distances. The occurrence of the diploid segregants is rare. Hence, consider only the single-crossover segregants. The genes distal to the crossing over point become homozygous due to the crossing over in the mitosis. Thus, the recessive alleles with heterozygous traits turn to become homozygous and exhibit recessiveness.
The production diploid yellow sector in a green diploid:
Case 1: Mitotic crossing over between the pro and paba loci
This type of mitotic crossing over produces a homozygous segregant for the y allele giving yellow coloration. It also produces a twin spot having homozygosity. However, the overall colony color includes green coloration which masks the yellow coloration. The crossing over makes the genes distal to that point homozygous, thereby making the yellow segregant dependent on the para-aminobenzoic acid for the growth.
Case 2: Mitotic crossing over between the paba and y loci
The yellow sector shows heterozygosity for the para-aminobenzoic acid. The yellow sectors consist of y and paba genotypes. The possible results reveal bi phenotype (present in the diploid green sector), and y phenotype (present in the diploid yellow sector).
Mitotic Recombination and Retinoblastoma:
It is ophthalmic cancer occurring in childhood. The RB gene mutations are the culprits. Mitotic recombination in the RB gene leads to the loss of heterozygosity for the wild-type allele. The recombination occurs between the non-sister chromatids of the thirteenth chromosome. The recombination mainly occurs between the RB gene and the centromere.
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