Chromosome Alterations Caused by Hybridization and Their Bearing upon Certain General Genetic Problems

Abstract

1. An account of alterations occurring in chromosomes due to hybridisation is made and the observed Gases of these alterations are analyzed.
2. All the phenomena of hybrid changes, termed “amphiplasty, ” are subdivided as follows: a) sporadic amphiplasty distinguished by its Chance occurrence and including the various chromosome reorganisations (dislocations) which seem to be stimulated by the hybrid conditions; b) regular amphiplasty which invariably occurs in a given interspecific combination. This in turn includes differential amphiplasty which affects only individual chromosomes, and neutral amphiplasty which is manifested in all the chromosomes of a given set.
3. Differential amphiplasty is so far established in thirteen interspecific Crepis hybrids (out of twenty-one investigated). In all known Gases it consists in the “disappearance” of the satellite of the satellited chromosome of one and the same parental species. The disappearance of the satellite is due to its fusion with the proximal end of the satellited chromosome.
4. The change, if any, occurs in all individuals of a given interspecific combination, no matter in which direction the Cross is made. It thus appears evident that not the eventual cytoplasmic differences between the species crossed are responsible for the phenomenon but that the latter is brought about by interaction between foreign chromosome.
5. Polyploid condition does not change the Situation at all. Thus, for instance, even when as many as three haploid sets of Crepis tectorum are opposed to a single haploid set of C. capillaris all the three satellited chromosomes of the former species appear without their satellites while the satellited chromosome of C. capiliaris remains wholly intact as regards its satellite.
6. Fusion of the satallite with the proximal end of the chromosome is in no way connected with eventual general shortening of the ehromosomes. Thus, in the hybrid C. capillaris×C. parviflora fusion of the satellite occurs in C. capillaris while all the ehromosomes in C. parviflora become very much shortened the satellited chromosome of the latter species remaining nevertheless wholly intact. The differential alteration of the satellited chromosome must be therefore considered a specific response to the peculiar conditions produced in the hybrid cell.
7. The change is reversible and the satellited chromosome recover their normal shape as soon as the pure chromosome complement is extracted from the hybrid by means of segregation.
8. It remains to be found out whether any individual chromosome of one species can induce alteration in the satellited chromosome of the other or, an the contrary, the action is strongly specific. However, it was established that the presence of an incomplete haploid set of the one species is sufiicient to cause changes in two satellited ehromosomes of the other.
9. Neutral amphiplasty is expressed in general shortening or lengthening of all the ehromosomes of a species entering the given Cross. It was found by means of extensive measurements in three interspecific hybrids of C. capillaris that changes, if any, affect all the ehromosomes of a given species and, moreover, the changes within the specific set always go in the Same direction. Thus, in the Cross C. capillaris×C. neglecta the total length of the haploid chromosome set of the former species is increased by 16 per cent each individual chromosome increasing approximately in the Same degree; an the other hand, the total length of the ehromosomes of C. neglecta decreases by 14 per cent and the same degree of shortening is observed in every individual chromosome of this species.