Sitzungsberichte der
Heidelberger Akademie der Wissenschaften

Abstracts

25

The maximum number of nucleolar necklaces (NN) corresponds to the number
of nucleolus organizing chromosomes. The tandemly arranged granules of these
NN then reflect the linear distribution of rRNA genes along the nucleolus
organizer regions. The granules stain silver (Ag)-positive and bind human autoan-
tibodies to RNA polymerase I (RPI), both properties thought to be characteristic
for an active rDNA conformation. Each granule represents an individual
transcription unit consisting of an 18S + 28S rRNA gene and the transcriptional
machinery. Adjacent granules are separated by nontranscribed spacer regions.
The number of RPI-positive granules and, thus, of transcriptionally active
rRNA genes was determined in fibroblast cells of the following vertebrate species:

Species
Number of
transcription units
Number of
chromosomal NORs
Human
115
8
Cavia cobaya
145
10
Mus musculus
106
6
Sminthopsis crassicaudata
59
2
Macropus eugenii (female)
72
2
Macropus eugenii (male)
45
1
Gallus domesticus
61
2
Bufo rubropunctatus
70
2
Gastrotheca riobambae (female)
80
2
Gastrotheca riobambae (male)
51
1

Mammals whose karyotypes are endowed with multiple NORs also exhibit a
higher number of transcriptionally active rRNA genes per diploid genome than
marsupials and lower vertebrates. Correspondingly, species with X-linked NORs
show sex specific differences in the number of transcription units.
Mouse pachytene spermatocytes contain 54 RPI-positive granules on the aver-
age, round spermatids 33. Compared to somatic fibroblasts, only a percentage of
rRNA genes is transcriptionally active during meiosis and spermiogenesis.