Metaphase Of Meiosis Research Articles

INDUCED RECIPROCAL TRANSLOCATIONS INVOLVING HOMOLOGOUS CHROMOSOMES IN MAIZE

THIS PAPER concerns the occurrence and behavior of reciprocal translocations between opposite arms of homologous chromosomes in maize plants from irradiated, dormant kernels. This type of interchange is detected at diakinesis and metaphase of meiosis I by the presence of two separate, ring-like chromosomes, often differing in size and each smaller than a normal bivalent. Such chromosomes were observed in barley plants grown from X-rayed dormant seed by Caldecott and Smith (1952), who termed them pseudo-isochromosomes. The authors did not make pachytene observations, but assumed that the breaks had occurred close to the centromeres, since in most cases the pseudo-isochromosomes showed no tendency to associate with each other or with other chromosomes. A pseudoisochromosome thus consists of a complete centromere and two arms which are identical except for a short interstitial segment. A true isochromosome on the other hand has two identical arms and a centromere which may vary in size depending on the mode of origin of the isochromosome (Darlington, 1939, 1940). The observations set forth in this paper were made during a study to determine the frequencies of interchanges induced by X rays and by thermal neutrons in a maize single-cross hybrid, L289 X 1205. The term pseudo-isochromosome will be used in the sense described by Caldecott and Smith, since pachytene observations in maize confirmed their assumption with regard to the origin of these configurations. OCCURRENCE OF INTERCHANGES BETWEEN HOMOLOGOUS CHROMOSOMES.-The frequency and distribution of interchanges between opposite arms of homologous chromosomes for various X-ray and thermal-neutron dosages are presented in table 1. Not more than one interchange of this type was observed in a given plant. The chromosome involved was positively identified in ten of the eighteen plants and tentatively identified in three other plants. The remaining five samples were too poor to make pachytene observations. In two instances the pseudo-isochromosomes were involved in a second aberration. In plant 101 pachytene configurations suggested that a pericentric inversion had occurred in one member of the chromosome 6 homologues prior to the interchange. This case

Centromeric Behaviour of the Univalents in Two Phalaris Hybrids

The hybrid between Phalaris coerulescens (2n = 14) and P. minor (2n = 28) usually forms seven bivalents and seven univalents at the first metaphase of meiosis. The univalents are derived from P. minor. The centromeres of the seven univalents do not divide at the first anaphase, but neocentromeres in distal parts of the chromosomes assume control of their movement. These neocentromeres move towards opposite poles, between which the univalents become stretched. Formation of the cell wall causes breakage of the univalent bridges. Fusions may occur between fragments or between broken ends of sister chromatids in a fragment. Neocentric activity occurs only infrequently at the second anaphase.

A cytological study on the F<sub>2</sub> plant of <i>Triticum sphaerococcum×Secale</i> <i>cereale</i>

1. In the present report, the result of a cytogenetical research on the F2 plant (TsphRF2) raised from T. sphaerococcum×S. cereale was described.2. The external characters of TsphRF2 plant, length of spike, density of spikelets and number of flowers in a spikelet resembled closely to the F1, while length of culms and of awns, and number of spikelets per spike resembled closely to T. sphaerococcum than those of the F1 (Fig. 1, and Table 1).3. The TsphRF2 plant has somatic chromosomes the number of which is less than the number of amphidiploid by 15 chromosomes.4. At the heterotypic metaphase in meiosis of PMC'S of the TsphRF2, 10_??_17 bivalents or 7_??_21 univalents were observed (Figs. 3-9). The frequency of bivalents and univalents in PMC'S was shown in Table 2.5. Both male and female gametes of the TsphRF2 plant were completely sterile.6. It seems to me that the majority of chromosomes of the F2 are due to T. sphaerococcum, considering from the external characters of the F2.

The shortening of chromosomes during mitosis and during meiosis; observations on the male germinal epithelium in Anacridium aegyptium L., Orthoptera, Acrididae

The lengths of the chromosomes during mitotic and meiotic cycles of the male germ cells ofAnacridium aegyptium L. (Orthoptera) have been measured: the shortening of the chromosomes was found to be about the same in the spermatogonial mitosis and in the 2nd meiotic division from prophase to metaphase (about 3:1); as well as from the premetaphase to the mid-metaphase of mitosis and from diplotene to the 1st metaphase of meiosis (about 2:1). In mitosis the longer chromosomes become shorter than the shorter ones; the contrary is true for zygotene to pachytene; the same in the other stages of meiosis.

The external mechanics of the chromosomes II-Meiosis in diploids

It has generally been believed that nuclear divisions, whether mitosis or meiosis, follow under average conditions in an average organism a certain course which is called “ normal ”, for which one description may lastingly suffice. In regard to the prophase of meiosis it has become evident that this ideal normality merely represents a statistical mode and that every nuclear division witnesses its own combination of chances. The individuality of organisms is indeed anticipated, and determined, by the individuality of chromosome behaviour in the germ-cells of their parents. Behaviour is unstandardized and adaptation is imperfect. In considering the behaviour of the chromosomes at the metaphase of meiosis it is possible to carry this rediscovery of the unique a stage further. It is possible to show that in the metaphase orientation and anaphase disjunction of “ normal ” paired chromosomes in “ normal ” plants a variety of abnormalities actually occur. No two cells are alike, and in the analysis of their differences lies the means of finding out the forces at work.

The External Forces Acting on Chromosomes

ACCORDING to Darlington1, in somatic mitosis or meiosis, ” the equilibrium position reached at metaphase maybe described as the result of the combination of three kinds of repulsion acting on bodies in the confined space of the spindle”, the repulsions being between chromosomes, between spindle attachments, and between poles and spindle attachments. If therefore at the first metaphase of meiosis there should exist two kinds of bivalents the different shapes or sizes of which cause the spindle attachments of the component chromosomes to be farther apart in one kind than in the other, we should expect the former type of bivalent to be pushed to the edge of the plate.

Cytological studies in cultivated pears

1. Of fifteen cultivated varieties of pears examined, eleven were diploid (2n=34) and four triploid (2n=51). 2. Secondary pairing occurred at metaphase of meiosis in diploids, but no multivalents were observed. 3. In triploid varieties, associations higher than trivalents were not observed. 4. The lower degree of secondary pairing in pears as compared with apples indicates a greater degree of differentiation of the chromosomes in pears. 5. It is considered that the evidence points to 7 being the primary basic number, and not eight as suggested bySax.