Absence of amplification of ribosomal DNA in the polytrophic meroistic ovary of the giant silkworm moth,Antheraea pernyi (Lepidoptera: Saturniidae).

In the meroistic insect ovary, the oocyte synthesizes little if any RNA. Most of the RNA which accumulates in the oocyte is synthesized by trophocytes. In the polytrophic meroistic ovary each oocyte is associated with a cyst containing 1,3,7 or 15 trophocytes. The trophocytes are derived from the same cell as the oocyte. The trophocyte cysts and the oocytes of the giant silkworm moth,Antheraea pernyi (Lepidoptera: Saturniidae), are large enough to enable their isolation by microdissection. The nucleus of each trophocyte is highly polyploid, containing hundreds of nucleoli. In order to determine whether DNA coding for rRNA (rDNA) is amplified in trophocytes ofA. pernyi, the percentage of the genome hybridizing with rRNA in somatic tissues was compared to that percentage in gametogenic tissues. RNA-DNA hybridization analysis indicates that approximately the same proportion (0.018%) of the DNA extracted from male and female gemetogenic tissues (testis, isolated trophocytes, and isolated oocytes) and somatic tissues (brain, Malpighian tubules) hybridizes with rRNA. The fact that DNA hybridizing with rRNA comprises the same proportion of the total DNA extracted from trophocytes, spermatogenic cells, and male and female somatic cells indicates that rDNA is not amplified in the trophocytes ofA. pernyi. In the polytrophic ovary, polyploidization of the entire trophocyte genome rather than amplification of a small part of it accounts for the increase of rDNA available for transcription.