Amphibians employ a genetic mechanism of sex determination, according to all available information on sex chromosomes or breeding tests. Sex reversal allows breeding tests to establish which sex is heterogametic and provides an indication of the mechanism of sex determination. Cases of spontaneous and experimental sex reversal (by temperature, hormones or surgery) are reviewed and illustrated by previously unpublished studies on crested newts. These newts respond conventionally to temperature and hormone treatment but provide anomalous results from breeding tests. It is suggested that both the evolution from temperature dependency to a genetic switch and from ZZ/ZW to XX/XY are superimposed on a generally uniform mechanism of sex determination in all vertebrates.
Meiotic cells of zebrafish have been prepared to show synaptonemal complexes (SCs) by light and electron microscopy. Completely paired SCs from both spermatocytes and oocytes were measured to produce an SC karyotype. The SC karyotype resembles the somatic karyotype of zebrafish and has no recognisable sex bivalent.Measurements of total SC length indicate that SCs grow longer and develop centromeres during pachytene. Oocytes consistently have longer SCs than spermatocytes, presumably correlated with the reported higher recombination frequency in females than in males.
Triturus cristatus carnifex provides a particularly clear example of sexual dimorphism for chiasma frequency and localisation. Oocytes from normal XX females routinely carry one proximal chiasma on each arm of their lampbrush bivalents. Spermatocytes from normal XY males have more numerous and relatively distal chiasmata. Lampbrush chromosomes from the oocytes of sex-reversed XY neofemales are found to resemble those from normal oocytes in having one proximal chiasma on each bivalent arm. A comparison of particular markers on the heteromorphic long arm of chromosome 1 provides evidence to equate the lampbrush 1A to somatic 1A, and confirms previous reports that lampbrush chromosome 1A is slightly longer than 1B. The XY sex bivalent of neofemales does not show any obvious heteromorphy of recognised marker loops.
The discovery in human leukemic cells of particulate elements encapsulating 70S RNA and RNAdirected DNA polymerase made possible the synthesis of a [3IJDNA probe that could detect leukemia-specific sequences in the DNA of normal and leukemic individuals. In an earlier study of a series of unrelated leukemic patients, we established that the nuclear DNA of their leukemic cells contain particle-related sequences that cannot be detected in leukocytes of normal individuals. This result is inconsistent with the virogene concept that demands the inclusion of one complete copy of oncogenic information in the genome of every normal cell.The present study carries this analysis one step further by showing, with two sets of identical twins, that the leukemic member contains particle-related sequences in the DNA of his leukocytes that cannot be detected in the leukocytes of his healthy identical sibling. This finding implies that the additional leukemia-specific information found in the DNA of the leukemic individuals must have been inserted subsequent to fertilization. This outcome argues against the virogene hypothesis or any other etiologic concept that invokes vertical transmission through the germ line of the particle-related information found uniquely in the DNA of leukemic cells.We have shown by molecular hybridization that human adenocarcinomas of the breast (1), leukemias (2), sarcomas (3), and lymphomas (4) contain RNA molecules possessing a small but significant homology to RNAs of tumor viruses that cause the corresponding malignancies in mice. More telling for a viral involvement was the demonstration with the simultaneous detection test (5) that the RNA detected in these human cancers was 70 S in size and encapsulated with RNA-directed DNA polymerase in a particle possessing a density between 1.16 and 1.19 g/ml (6-9). Furthermore, DNA synthesized endogenously by these RNA-enzyme complexes exhibited evidence of complementarity to RNAs of the analogous murine viruses (6-9).Taken together, these experiments documented the existence in human neoplasias of particulate elements possessing four features diagnostic of animal RNA tumor viruses. The data did not of course establish that the particles identified were causative, but the evidence for their involvement was sufficiently convincing to encourage further exploration of their significance.In our studies of human leukemias we developed the methodology required to separate the particles containing the RNA-dependent DNA polymerase and its 70S RNA template. These particulate elements were then used to generate 3H-labeled DNA probes suitable for detecting corresponding 2629 sequences in genomes of any cells of interest. We found (9) that RNA of human leukemic particles shared sequences with the DNA of normal cells, a feature observed (10) with animal RNA tumor viruses and the normal DNA of their indigenous hosts. Sequences common to both normal DNA and the [3H]-DNA synthesized by leukemic particles were removed by exhaustive hybridization to a vast excess of ...
The limbs of salamanders can regenerate even if their nerves are irradiated, provided that some other non-irradiated tissue reaches the site of amputation. This conclusion is reached by repetition of an earlier experiment, which yielded contradictory results. The experimental design does not demonstrate any radiosensitivity of nerves, but indicates that very few non-irradiated cells suffice to initiate regeneration. On the basis of this conclusion and other unconfirmed experiments, the possibility is considered that irradiated cells can recover their ability to regenerate.
The duration of the cell cycle in the blastema of regenerating limbs of axolotls has been measured by means of [3H]thymidine pulse labelling and autoradiography. A chase was required to define the pulse period. An average cell cycle at 20 degrees C takes 53 h, S-phase takes 38 h; including parts of mitosis, G1 is 10 h and G2 is 5 h long. The protracted cycle and S-phase are consonant with the large genome in axolotis and other urodeles. The rapidly growing blastema probably contains a steady population of about 5000 proliferating cells, as there is a regular withdrawal of differentiating cells from the population. The kinds of determination which exist in this population of cells, or are exerted on it, are briefly considered.
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