Meiotic Behavior of Tandem Me Tacentric Compound X Chromosomes in Drosophila Melanogaster.

tandem metacentric compound X chromosome is composed of two X chromoA somes attached to a medially located centromere and arranged in tandem sequence so that the synaptic configuration is a spiral (NOVITSKI 1954). An approximation to the tandem metacentric has been achieved by the construction of an attached-X heterozygous for Zn(l)y4 ( SIDOROV, SOKOLOV and TROFIMOV 1936; STURTEVANT and BEADLE 1936; NOVITSKI and SANDLER 1956). Bona fide tandem metacentrics have also been synthesized (e.g., NOVITSKI and LINDSLEY 1950) and their meiotic behavior investigated (NOVITSKI 1951, 1955; NOVITSKI and SANDLER 1956). The anaphase I1 bridges produced by exchange between the arms of a tandem metacentric are generally lethal ( STURTEVANT and BEADLE 1936; NOVITSKI 195 1 ), but NOVITSKI in 1955 postulated an exception to this generality. He noted that two structurally distinct types of second-anaphase bridges were generated by tandem metacentrics-each composed of two separating sister centromeres, one connected by a single chromatid and the other connected by two chromatids of equal length. NOVITSKI suggested that, a1 though the single bridges are lethal, the double bridges are excluded from the daughter nuclei at anaphase 11, producing nullo-X eggs that are recovered when fertilized by X-bearing sperm. The evidence favoring this hypothesis is as follows: in crosses in which the female parent carries a tandem metacentric compound X and a marked Y chromosome, one-half of the egg nuclei will receive the marked Y and thus produce, when fertilized by X-bearing sperm, regular patroclinous males that differ from their father by the presence of the marker on the Y. Nullo-X eggs that result, for example, from the exclusion of second-anaphase bridges from the presumptive egg nucleus will, when fertilized by X-bearing sperm, produce a class of exceptional patroclinous males that are distinguishable from the regular patroclinous male class by the absence of the marker carried by the maternal Y chromosome. Thus use of the marked Y enables identification of nullo-X eggs that should have received an X-chromosome centromere. NOVITSKI ( 1955) observed, in crosses of this sort, frequencies of exceptional patroclinous males of 0.044 and 0.049, which dropped to 0.005 when the tandem metacentric was made heterozygous for In(l)dl-49. He argued that since double bridges come only from the double