The effect of an unusual chromosome architecture on disjunction and non-disjunction in Drosophila

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Abstract
SUMMARY: Two homologous autosomes of Drosophila that were attached to form a single entire compound autosome II were found to affect the segregation of the sex chromosomes in both males and females. The compound segregated nearly always from an attachedX.Ychromosome in males with no other sex chromosome. When two sex chromosomes were present together with the compound they differed in their tendency to segregate from the compound. In males theX.Ychromosome segregated more often from the compound than did theYchromosome, and theYchromosome segregated more often from the compound than did the regularXchromosome. In females theX.Ysegregated more often from the compound than did the regularXchromosome. This preferential segregation in females was observed for exchangeXchromosomes as well as for the non-exchange chromosomes.In the presence of the compound the frequency of primary non-disjunction of the sex chromosomes was elevated in both females and males; usually both sex chromosomes segregatedfromthe compound and only rarelywithit.Flies devoid of most of the proximal heterochromatin of the sex chromosomes die. However, when the compound autosome was present some such flies survived. This indicates that a segment of the proximal heterochromatin of the sex chromosomes was intercalated into the compound when it was constructed. It was concluded that the segment intercalated into the compound carries specific sites for sex chromosome disjunction. Specific sites determine sex chromosome disjunction in males. In females they determine the disjunction of the sex chromosomes in cooperation with exchange pairing.