Extrachromosomal amplification of rDNA in oocytes of Hemerobius spp. (Insecta, Neuroptera)

In previtellogenic oocytes of the neuropteran, Hemerobius spp., two distinct, DNA-positive intranuclear structures have been observed. Chromosomes of meiotic prophase assemble in the center of the oocyte nucleus forming a highly polymorphic karyosphere, which persists in this position until the very late stages of vitellogenesis. The extrachromosomal DNA body, containing amplified ribosomal genes, undergoes fragmentation and dispersion in the nucleoplasm. At the onset of previtellogenic growth, transcription of extra rDNA starts, which is accompanied by the appearance of dense, granular material (multiple nucleoli). Arising nucleoli gradually fill the nucleoplasm. At the electron microscopic (EM) level two electron dense structural forms of the granular material have been described. Together with general histological and ultrastructural analysis the amplification of rDNA genes in Hemerobius spp. oocytes has been demonstrated by means of the spreading technique, which has shown that extra rDNA is organized in rings containing various numbers of active ribosomal genes. The transcription activity of amplified genes is manifested in the form of typical "Christmas tree" structures.