Extrachromosomal rDNA and polarity of pro-oocytes during ovary development in Creophilus maxillosus (Coleoptera, Staphylinidae)

In telotrophic ovary of Creophilus maxillosus, the differentiation of the oocyte and nurse cells takes place within the linear clusters of sister oogonial cells. The amplification of rDNA occurs in the nuclei of pro- oocytes which are the most posterior cells of the clusters. During the consecutive oogonial divisions extrachromosomal rDNA segregates preferentially to the pro-oocyte of the next generation. We analyzed the ultrastructure of pro-oocytes and pro-nurse cells in the early and late phase of rDNA amplification in pupal ovary of Creophilus maxillosus. We found that pro-oocytes of the same generation contain variable amounts of extrachromosomal rDNA and that the presence of extra DNA is not limited to the nuclei of pro-oocytes; extra DNA is also present in the nuclei of some pro-nurse cells. Pro-oocytes can experience partial loss of extrachromosomal DNA during early oogonial divisions which is caused by the imprecise segregation of this material to the posterior pole. We believe that this imperfect segregation is a source of extrachromosomal DNA present in the nuclei of pro-nurse cells. Ultrastructural analysis showed that multiple nucleoli do not disperse in oogonial mitoses but remain associated with extrachromosomal chromatin and segregate with it to the posterior pole of the pro-oocyte. We also analyzed the ultrastructure of the germ plasm - a cytoplasmic structure present at the posterior pole of pro-oocytes. We have found that this structure contains spectrin and at the ultrastructural level is strikingly similar to the spectrosome which is present in germline cells of Drosophila. We also found spectrin in the intercellular bridges which connect oogonial cells and are known to contain fusomes.