THE TRANSMISSION OF A HETEROCHROMATIC ISOCHROMOSOME IN LYCOPERSICON ESCULENTUM

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Abstract
The inheritance of a genetically inert isochromosome from the short arm of chromosome 2 in the tomato is reported. The isochromosome has unusual properties in that it does not pair with the normal chromosome 2 and that it does not affect plant growth and fertility. From test crosses it was determined that transmission was 30% through both male and female gametes. More than 2 complete isochromosomes could not be accumulated in any one plant. Reduced isochromosomes occurred regularly. These were found to be less detrimental and could be accumulated up to three per plant. Also the average number of extra chromosomes/plant was found to be increased for the reduced isochromosomes. It is argued that extra chromosomes or other chromosomal abnormalities which have a selective disadvantage when they arise, but which are still present after several or many generations, have acquired characteristics favorable to their perpetuation.

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