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Previous studies of reversed acrocentric compound-X chromosomes suggested peculiar influences of heterochromatin on both the synthesis and meiotic behavior of such compounds. It seemed, with respect to synthesis, that the long arm of the Y chromosome on an X.YL chromosome was necessary in order for the heterochromatic exchange giving rise to reversed acrocentrics to occur, even though YL itself did not participate in the compound-generating event. With respect to behavior, the resulting compounds appeared, presumably as a consequence of their singular generation, to contain an interstitial heterochromatic region that caused the distribution of exchanges between the elements of the compound to be abnormal (many zero and two-exchange tetrads with few, if any, single-exchange tetrads). Removing the interstitial heterochromatin (or, curiously, appending YL as a second arm of the compound) eliminated the recombinational anomalies and resulted in typical tetrad distributions.—We provide evidence that these peculiarities, while presumably real, were likely the consequence of a special X.YL chromosome that was used to synthesize the reversed acrocentrics examined in the early studies and are not general properties of either reversed acrocentric compounds or of interstitial heterochromatin. However, we show that specific heterochromatic regions do, in fact, profoundly influence the behavior of (apparently all) reversed acrocentric compound-X chromosomes. In particular, we demonstrate that specific portions of the Y chromosome and of the basal X-chromosome heterochromatin, when present as homologs for reversed acrocentric compounds, markedly and coordinately increase bath the frequency of exchange between the elements of the compound and the fertility (egg production) of compound-bearing females. It is, we suppose, some aspect of this heterochromatic effect, produced by the% special X.YL chromosome, that caused the earlier-analyzed compounds to exhibit the observed anomalies.

HETEROCHROMATIC EFFECTS ON THE BEHAVIOR OF REVERSED ACROCENTRIC COMPOUND-X CHROMOSOMES IN DROSOPHILA MELANOGASTER