The meiotic behavior of tandem acrocentric compound X chromosomes in Drosophila melanogaster.

compound X chromosome consists of two X chromosomes joined to form a A single chromosome containing one centromere, two euchromatically complete (or almost complete) X chromosome elements, and heterochromatic regions whose distribution can be specified but whose content is uncertain (NOVITSKI 1954). The elements of a compound may be in reversed or tandem sequence, the centromere may be between the elements or at one end, and there may or may not be free ends. Thus there are four compounds with free ends: the attached-)(: or reversed metacentric (reversed sequence, median centromere), the reversed acrocentric (reversed sequence, subterminal centromere), the tandem metacentric (tandem sequence, median centromere), and the tandem acrocentric (tandem sequence, subterminal centromere). There are also two compound rings-the reversed compound ring X chromosome and the tandem compound ring X chromosome. All of the compounds have been synthesized and preliminary studies made by NOVITSKI ( 1954). Of the six compounds, four have been extensively analyzed. These are the attached-X, the tandem metacentric, the reversed acrocentric, and the reversed ring. The results from attached-X (e.g., BEADLE and EMERSON 1935) and tandem metacentric ( STURTEVANT and BEADLE 1936; NOVITSKI 1951 ) compound chromosomes may be summarized simply by noting that they behave, with respect to exchange, the way they would be expected to behave, based on studies of exchange in normal free X chromosomes. Reversed acrocentrics and reversed rings, on the other hand, behave alike but exhibit an exchange distribution very different from that of free X chromosomes. Here, approximately 10 percent of the tetrads have a single exchange, and the remainder have either no exchanges or two exchanges with approximately equal frequency ( SANDLER 1954,1957). NOVITSKI (1954) and NOVITSKI and SANDLER (1956) have collected data from a tandem acrocentric compound that had the long arm of the Y chromosome appended to the centromere of the compound as a second arm. The data, as will be shown below, indicate that such a compound exhibits a distribution of tetrads with respect to rank that is comparable to that of free X’s. However, it has been shown (SANDLER 1958) that while reversed acrocentrics without the long arm of the Y