CYTOGENETICAL ANALYSIS OF CHROMOSOMAL ABERRATION DUE TO TRANSLOCATIONBETWEEN THE 23RD AND 25TH LINKAGE GROUPS IN THE SILKWORM, BOMBYX-MORI

The chromosome aberration ''T(23;25)Nd'', which has been identified by genetical studies as an attachment or translocation between chromosom es of the 23rd and 25th linkage groups, was investigated cytogenetical ly. In the cross of T(23;25)Nd/tub;oy x tub;oy, only the parental comb ination of chromosomes was produced. The + + larvae segregating in thi s cross formed primary spermatocytes with 27 chromosome pairs. which i s one pair less than normal. Primary spermatocytes of tub oy larvae ha d 28 chromosome pairs, which is the standard number for this animal. T he chromosome number of all the embryos formed from this cross was 2n = 55 and 2n = 56 (normal chromosome number) in a ratio of one to one. In 2n = 55 cells one large chromosome was observed, which appears to h ave been the result of fusion between two chromosomes. Moreover, in th e + + larvae at the diakinesis stage of the oocyte, two chromosomes of different sizes were arranged tandemly along the large chromosome. We conclude that the present chromosomal aberration has originated from a reciprocal translocation with break points close to the telomeres in both chromosomes of the 23rd and the 25th linkage groups.