Somatic pairing refers to close or loose association of homologous chromosomes during mitotic divisions. Somatic pairing and somatic crossing over are genetically controlled events and have been reported in a wide array of animals and plants. These events assume special evolutionary significance in sexual fungi like Aspergillus, in Drosophila and in maize.
Indirect evidence of premeiotic pairing in maize has been reported by Rhoades (The Cell 2, 1961) and Maguire (Genetics 53:1071-1077, 1966). We report here somatic pairing and somatic crossing over at metaphase in root tip meristems of a Colombian popcorn race Pira Naranja and Xochimilco teosinte (K68-3), seeds of which were received from CIMMYT. The conventional procedures described by Sachan and Tanaka (Chromosome Inf. Serv. 20:3-4, 1976) for Feulgen staining and by Sachan and Tanaka (Jpn. J. Genet. 51:139-141, 1976) for C-banding in the root meristem of Zea chromosomes were used.
The illustrations (Figs. 1 and 2) are self-explanatory of the perfect pairing of homologous chromosomes in maize at mitotic metaphase arrested in root tips by pretreatment with 0.002 M 8-hydroxyquinoline at 14±18 C for 3.5 hours. The event of somatic pairing occurs in the root tips of Zea roughly with a frequency of one in 500 metaphase plates. The photographs can not be confused with C-mitosis as there are strong evidences in support of somatic pairing and somatic crossing over. Firstly, both conventional and C-banded karyotype preparations of maize and teosinte had a chromosome count of 2n=20. Secondly, in haploid C-mitosis, one expects complete correspondence in morphometry of both the chromatids. We know that chromosome 6 of Zea is highly heteromorphic. Figure 2 gives clear identity of heteromorphic pairing of chromosome 6. One can also note chiasmata formation and probable exchange of genetic material in some of the paired homologues both in maize and teosinte.
Figure 1. Somatic pairing and crossing over in teosinte (K68-3).
2. C-banded somatic karyotype of Pira showing pairing and crossing
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