CONTENTS INTRODUCTION ... ..... ... .. 54 Cross-Specific Comparisons 55 LOWER EUCARYOTES .... 57 Criteria for Mutant Isolation 57 Fertility 57 Effects on specific meiotic events 58 Mitotic disturbances 58 Yeasts 59 Fertility mutants ........ ... ... 60 Mutants with altered meiotic exchange .. ...... 62 Defective mating-type control of sporulation 63

The genetic control of meiosis.

Publisher Summary This chapter analyzes the behavior of the XY pair in small number of selected species, and discusses the essential problems concerning the XY pair on that basis. The leptotene stage of mammalian spermatocytes is poorly understood, and the limited available evidence suggests that at least in human and mouse spermatocytes the X and Y chromosomes are not fully heterochromatic at this stage. Although zygotene nuclei should show the pairing process, this process has seldom been observed in mammalian spermatocytes. Examination of thick and thin sections of each stage in the cycle of the seminiferous epithelium of the mouse show that after leptonema the chromosomes become strongly condensed during zygonema, and that zygotene nuclei are relatively small and strongly stained. The beginning of pachynema may be conventionally set at stage 12 in the cycle of the seminiferous epithelium of the mouse. At this stage the sex pair is already forming a single body and has no nucleolar region. The sex pair undergoes several changes during pachynema. In spreads corresponding to early pachynema, the XY body of the mouse shows a smoothly curved side facing the inner nuclear space, and a rough side corresponding to the attachment of the X and Y chromosomes to the nuclear envelope.

The behavior of the XY pair in mammals.

Morphology of chromosomes replicating in the presence of 5-bromodeoxyuridine was studied using long-term cultures of Chinese hamster cells (line Blld-ii-FAF28). The cytological effect of the analog administered in various concentrations, at different stages of the S period, and during one and two successive mitotic cycles was studied. — The main cytological manifestation of the BUdR action consisted in spiralization delay of certain chromosome regions. The degree of the delay was dependent on the time interval between the introduction of the agent and mitosis, as well as on the agent's concentration. With prolongation of the interval, the spiralization delay diminished and disappeared being therefore always observable only in late replicating chromosome regions. Increased concentration of BUdR (in the range of 25 to 400 μg/ml) produced enhancement of the delay of chromosome spiralization. — After two successive reproduction cycles in the presence of BUdR, a great number of metaphases contained chromosomes the sister chromatids of which showed unequal spiralization delay. Autoradiography of 3H-BUdR distribution showed that the sister chromatid with a more pronounced underspiralization corresponds to the chromatid incorporating BUdR into both strands of the DNA molecule. — Mechanisms of the effect observed, as well as chemical influence on chromosome spiralization as a usefull tool of displaying linear chromosome differentiation, are discussed.

Differential spiralization along mammalian mitotic chromosomes. I. BUdR-revealed differentiation in Chinese hamster chromosomes.

Chinese hamster ovary cells (line CHO) have been used extensively for metabolic, genetic, and radiobiological studies with only a superficial appreciation for the degree of aneuploidy characteristic of the line. A thorough karyologic analysis of CHO chromosomes using autoradiographic replication patterns, as well as centromere band (C-band) and Giemsa band (G-band) analysis, is presented. Our results demonstrate that only 8 of the 21 CHO chromosomes are normal when compared with euploid Chinese hamster chromosomes. In the 13 altered chromosomes, we found evidence of translocations, deletions, and pericentric inversions. These altered chromosomes have been characterized with respect to both origin and destination of translocated material. With the exception of the X2 chromosome, essentially all of the euploid chromatin is present in CHO cells. Autoradiographic replication patterns show that the normal sequence of chromosomal DNA synthesis is altered. Some sites which replicate late in euploid cells replicate early in CHO, and several late-replicating chromosomes in CHO cells replicate in early- or mid-S in euploid material. These studies may serve to elucidate the observed differences in mutagenic behavior between euploid fibroblasts and CHO cells.

The chromosomes of CHO, an aneuploid Chinese hamster cell line: G-band, C-band, and autoradiographic analyses.

The established cell line derived many years ago from Chinese hamster ovary (CHO cells) has been studied for the extent of chromosomal variation. Because this cell line is used extensively for genetic studies, the contribution of chromosome variability to genetic variability has also been examined. The quasidiploid CHO cells were found to have a banded karyotype somewhat altered from that of the Chinese hamster from which the line was derived. However, most of the genome could be accounted for among the rearranged marker chromosomes. In addition, the CHO line was found to have a relatively stable karyotype, the same basic karyotype being found in a majority of the uncloned cells, as well as in most cells of several but not all independent clones. Many, but not all, mutant cell lines derived from CHO also showed the same basic karyotype. Quasitetraploid cells, derived either spontaneously or by Sendai-virus-induced fusion, showed considerably more variation resulting in loss or gain of whole chromosomes, rearrangement of chromosomes, and appearance of new “marker” chromosomes.

Chromosome Stability in CHO Cells

A study of male meiosis has been carried out on air-dried testicular preparations from 10 adult Djungarian hamsters (Phodopus sungorus campbelli Th.). The general course of meiosis in this species seems to be similar to that in most of the mammals so far studied. A short description of the main meiotic stages from pachytene to metaphase II has been presented. The sex-chromosomes (X and Y) show end-to-end pairing and prereduction. Between pachytene and diplotene there probably exists a diffuse stage. The large size of nuclei, the presence of double, occasionally crossed thin chromatin threads with a small number of double chromatin knobs are typical for this stage. The sex-vesicle is still present here but it is not so distinct as at pachytene, probably due to the beginning of straightening of the sex-bivalent. The differential arm of the X chromosome shows allocycly. During diplotene-diakinesis this arm looks like a thin double thread which is several times longer than the pairing arm. At metaphase I the differential arm is more condensed but usually the X does not become metacentric as at metaphase of mitosis. The allocycly of the X has been observed already at pachytene. The light zone of the sex-vesicle is formed by a smooth coiled thread or band which in some of the nuclei looks like a loop or “tail” protruding from the dark zone. A possible relationship between the allocycly of the X chromosome arm and its active functional state during meiotic prophase is suggested.

Meiosis in the Djungarian hamster. I. General pattern of male meiosis.

The Djungarian hamster, bred under usual laboratory conditions, developed different spontaneous neoplasms, most often mammary and skin cancers. Some mammary tumors were serially transplanted to noninbred animals. This hamster was susceptible to some chemical carcinogens and oncogenic viruses but resistant to others. It had comparatively few chromosomes (2n equal to 28), most of which could be recognized even in conventionally stained preparations. The chromosome-breaking effect of gamma-rays, chemical carcinogens, and viruses was studied with primary cultures of cells. Several transformed cell lines were developed. Karyotype abnormalities were generally not seen in primary and induced neoplasms. This hamster is a new suitable tool for cancer research and cytogenetic studies.

Djungarian Hamster—A Suitable Tool for Cancer Research and Cytogenetic Studies

Chromosomal Variability in Clonal Populations of a Chinese Hamster Cell Strain

Polyploid spermatocytes were found in air-dried meiotic preparations of 8 of 10 adult males of the Djungarian hamster (Phodopus sungorus campbelli Thomas). Most of them were tetrapl

Helen E. Pogosianz

Papers

Meiosis in the Djungarian hamster. I. General pattern of male meiosis.

Djungarian Hamster—A Suitable Tool for Cancer Research and Cytogenetic Studies

Chromosomal Variability in Clonal Populations of a Chinese Hamster Cell Strain

Meiosis in the Djungarian hamster