Sequential silver staining and in situ hybridization reveal a direct association between rDNA levels and the expression of homologous nucleolar organizing regions: a hypothesis for NOR structure and function.

We have developed a procedure for sequential silver staining and in situ hybridization to analyze the relationship between the amount of rDNA present in nucleolar organizer regions, as estimated by in situ hybridization, and their level of expression, as estimated by the silver signal. For simplicity we used cells from the insectivorous mole Talpa occidentalis, which have a single pair of nucleolar organizer regions in chromosome pair 3. The relative content of ribosomal cistrons was also related to the hierarchy of activation of the nucleolar organizer regions present in this chromosomal pair. Statistical analyses demonstrated that both the relative level of expression and the activation hierarchy depended mainly on the number of ribosomal cistrons in nucleolar organizer regions. We propose a functional two-step hypothesis, which is consistent with most known data concerning interchromosomal, intercellular and interindividual variation in a number of plant and animal species, including Talpa occidentalis. In step one, the first available transcription factors bind randomly to the ribosomal promoters, such that larger nucleolar organizer regions are more likely to recruit them. In the second step the remaining transcription factors are recruited in a cooperative way, thus completing activation of one nucleolar organizer region, before the next one becomes active.

Ovotestis variability in young and adult females of the mole Talpa occidentalis (insectivora, mammalia).

The age-related evolution and ontogenic origin of the ovotestes in fertile females of the Spanish mole (Talpa occidentalis) were studied. Volume of the ovotestis and its ovarian and testicular components, size of the epididymis and testicular cords, number of ovarian follicles and testicular cords, uterus weight, and age index were analyzed statistically in a large sample of young and adult individuals of this species. Comparison of means and linear correlation analyses were done. Most variables were shown to be age dependent, with a period of rapid change during puberty. In adult animals, volume of the ovarian portion and uterus weight followed a seasonal cycle of sexual activity. Interindividual variability was evident in most of the variables investigated except for the number of testicular cords per ovotestis, which remained unchanged throughout the animal's life and hence was not inversely correlated with the number of ovarian follicles. This finding ruled out an ovary-testis transdifferentiation hypothesis for the ontogenic origin of the testicular tissue in the ovotestes of female moles. An alternative hypothesis based in the absence of oocytes in a portion of the undifferentiated fetal gonad is proposed in accordance with a new general model for mammalian sex determination.