The Aloineae by Riley Herbert Parkes;Majumdar Shyamal K.;

Author:Riley, Herbert Parkes;Majumdar, Shyamal K.; [K. Majundar, Ward]
Language: eng
Format: epub
Publisher: University Press of Kentucky
Published: 2021-08-15T00:00:00+00:00

9.1 Pairing of chromosomes in an individual heterozygous for a reciprocal translocation: (a) the two original chromosomes (left) and the two interchange chromosomes; (b) diplotene showing eight chromatids and one or two chiasmata in each arm; (c) the following metaphase (not showing chromatids) assuming complete terminalization of chiasmata. (Riley, 1948c)

Reciprocal translocations probably have not been an important factor in evolution. In general gross structural changes are not the kind of variation that selection uses in evolution, nor in general do they provide phenotypic differences. Rarely (as in Oenothera) position effect may occur because of chromosomal breakage, but because it is not common it is not an important factor in the evolution of the plant kingdom. Although translocations may have been of evolutionary significance in building up isolating mechanisms, generally they were not important in evolution but were important in the history and development of cytogenetics, since in the early days they provided valuable information on chromosomal behavior.

Several reciprocal translocations have been reported in the Aloineae, including some of the centric fusion type. In a plant designated Aloe gracilis var. minima, Sato (1937) found eight long, one medium, and five short chromosomes instead of the usual eight long and six short found in most Aloineae. Sato believed that the karyotype suggested a translocation had occurred in which a piece of the distal end of a long chromosome (probably chromosome L4) became attached to a short chromosome such as chromosome S3, although he admitted it might have come from a fragmentation of chromosome L4. Unfortunately no meiotic studies corroborated this diagnosis. Normally there are four satellites in this species, and the fact that this plant had only three suggests one was lost from the end of an L4 chromosome during the translocation process. Sato’s description suggests that he thought a simple translocation had taken place. Much more likely, a fairly long piece of an L4 chromosome exchanged reciprocally with a very short piece of the S3 chromosome. In Gasteria cheilophylla, plant No. 1, one of the L4 chromosomes had a satellite at each end. Sato (1937) interpreted it as the result of a translocation between the end of the long arm of an L1 chromosome with its satellite and the normally nonsatellited end of the short arm of chromosome L4, so this short arm now bore a satellite that had originally been attached at the end of the long arm of an L1 chromosome. This plant probably should have been designated G. Xcheilophylla Bak. and was probably a hybrid.

At about the same time Sato’s results were published, Resende (1937b) reported several plants with apparent translocations. He studied four seedlings of Aloe globuligemma Pole Evans and observed that two roots of one of them had an unusual chromosomal situation: there were only six instead of eight normal long chromosomes. One of the mutated chromosomes was too long and the other was too short. The segment by which the size of the long chromosome was increased was exactly the same size as that by which the other chromosome was shortened.

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