DISTRIBUTIVE PAIRING: THE SIZE-DEPENDENT MECHANISM FOR REGULAR SEGREGATION OF THE FOURTH CHROMOSOMES IN DROSOPHILA MELANOGASTER

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Abstract
To test the possibility that the failure of a noncrossover chromosome to disturb the regular segregation of the 2 nonrecombinant 4th chromosomes at distributive pairing is due to the difference in length between the minute 4th chromosomes and the remaining chromosomes of the genome, a series of free X duplications, ranging in size from [less than or equal to]0.3 to 33 times the length of chromosme four have been introduced, one at a time, into females of the genotype y2 /y2; ciD/eyD and 4th chromosome nondisjunction frequencies have been measured among the progeny. The following conclusions have been drawn: (1) The 4th chromosomes are subject to the same rules of behavior during distributive pairing as the other chromosomes of Drosophila melanogaster. In the diploid female the fours, as noncrossovers, are virtually always members of the distributive pool and will associate nonhomologously if heterologues of similar size are also present in the pool. (2) The frequency of association between the fours and a heterologue, as measured by 4th chromosome nondisjunction frequency, is closely correlated with the similarity in length between the fours and the heterologue. (3) When the only heterologue available is a chromosome of normal size, the small size of the fours serves to ensure regular 4th chromosome segregation. (4) Distributive pairing is the normal mechansim for regular segregation of the 4th chromosomes. It is suggested that the basis for preferential segregation may be small differences in size among normal 4th chromosomes.