Amplification of ribosomal DNA in Acheta

Ribosomal cistrons for 28S and 18S RNA have been located in the mitotic chromosomes of several species at the nucleolus organizer region, but when amplification occurs at meiosis the number of chromomeres per site and the number of sites involved in this process have not been determined. In Acheta domesticus where amplification occurs for these genes every chromosome of the complement has been identified at pachytene by its size and characteristic chromomere markers both in oocytes and spermatocytes. In oocytes there are 5 amplification sites. Of these, two are major chromomeres, or DNA bodies, which are localized in the autosomes Nos. 6 and 11. Since these major chromomeres are absent from the chromosomes of spermatocytes, a comparison of the chromosomes of the two tissues at pachytene makes it possible to determine the size of the amplicons (the regions of amplification) in this species. The amplicon of chromosome 6 consists of 2 chromomere pairs and that of chromosome 11 of one chromomere pair. The total chromomere number of the autosomes is 185.3 in the oocytes and 178.4 in the spermatocytes. The X chromosome is an exception since it has 41.7 chromomeres in the oocytes, whereas in the spermatocytes it has none. The significance of these data for the understanding of gene amplification is discussed.