摘要:

not available

ISSN:1923-4287    

DOI:10.1139/cjr41c-034

出版商:NRC Research Press    

年代:1941

数据来源: NRC

摘要:

Studies of the major, relic, and relational coils and of chromosome, chromatid, and chromonema length changes in meiotic, microspore, and root tip chromosomes ofTrillium erectumL. andT.grandiflorumSalisb. have led to the following conclusions:—Elongation of the chromonema occurs between early diakinesis and first anaphase and between second anaphase and microspore prophase. Contraction occurs between zygotene and early diakinesis, between first and second anaphase, and during microspore prophase. Chromonema elongation between early diakinesis and anaphase is associated with the formation of the major coil and its transition into the relic coil of the microspore prophase is accompanied by a further elongation. The tertiary split results in the half-chromatids being associated in the form of a plectonemic spiral, which persists as such to microspore prophase. The gyres of the plectonemic relic coil become partially straightened out to form a relational coil, whose twists are in the same direction as their antecedent relic coils. During microspore prophase some relational twists are apparently eliminated at intrabrachial changes of direction, others by contraction and untwisting of the chromatids.Changes of direction fall into three categories: (1) those associated with attachments, (2) those associated with chiasmata, and (3) the remainder, the frequency of which is proportional to the number of gyres. Intrabrachial changes of direction are more numerous in microspore prophase than in root tip chromosomes, presumably because Factor (2) is inoperative in the latter. There are very few data that could possibly be taken to indicate that chromosomes may have an inherent directional pattern of coiling.To the tentative hypothesis of Wilson and Huskins that the major coil of meiosis is formed by an elongation of the chromonema within a restricted space, the pellicle, the following may be added: the half-chromatids at metaphase are wound in the form of a plectonemic spiral. The straightening out of this spiral results in the relationally twisted chromatids of microspore prophase. These twists are eliminated both by cancellation at the points at which changes of direction have occurred in the major coil and also by contraction and untwisting of the chromatids.

ISSN:1923-4287    

DOI:10.1139/cjr41c-035

出版商:NRC Research Press    

年代:1941

数据来源: NRC

摘要:

Meiosis was studied in varieties ofTriticum vulgare(2n = 42),T.dicoccum(2n = 28),T.durum(2n = 28),T.Timopheevi(2n = 28), and in 16 of their pentaploid hybrids as part of a study in an attempt to establish criteria indicating relationships between 42- and 28-chromosome wheats, with particular reference to the possible relationship of the new 42-chromosome wheat, McMurachy's Selection, toT.dicoccumorT.durum.One plant each ofT.vulgarevar. Hope and Marquillo had only 41 chromosomes. One plant ofT.durumvar. Pentad had three times as many unpaired chromosomes as the other plants of this variety.A nucleus with 14 pairs and 7 univalents was not detected among the 86 pollen mother cells analysed in the cross involvingT.Timopheevi. In the remaining crosses the frequency of this association of chromosomes was lowest in the three hybrids involvingT.durumvar. Pentad, greater in the three involvingT.dicoccumvar. Khapli, still greater in the three involvingT.dicoccumvar. Vernal, and greatest in the nine hybrids involvingT.durumvar. Iumillo.Of the seven "extra chromosomes" ofT.vulgareonly six remained unpaired in some pollen mother cells of the hybrids involving Vernal or Iumillo and five in those involving Khapli or Pentad. One pollen mother cell ofF1Marquis × Pentad contained only four unpaired chromosomes.Associations of four chromosomes were rare in some, and not seen at all in others, of the hybrids involving Vernal or Iumillo, more frequent in hybrids involving Khapli, and very frequent in hybrids involving Pentad. In the latter, from 47 to 57% of the nuclei had from one to three such multiple associations, and even chains of five and six chromosomes were observed.Fragmentation of unpaired chromosomes at or in the spindle fibre attachment region was observed in a number of first anaphase figures.There were statistically significant differences in the frequencies of occurrence of micronuclei in tetrads of the 15 hybrids studied at the second reduction division.The crosses R.L. 1544 (genetically related toT.durumvar. Iumillo) × Iumillo and Hope (genetically related toT.dicoccumvar. Vernal) × Vernal were used as standards for comparison. On the basis of the results, the following criteria were used in attempting to establish relationships between the other 42- and 28-chromosome wheats: (1) the percentage of pollen mother cells with 14 pairs and 7 univalents (greatest in the hybrids between related varieties); (2) the average number of chromosomes involved in multiple associations (lowest in hybrids between related varieties); (3) fertility (greatest in hybrids between related varieties). McMurachy's Selection appeared to be most closely related toT.durumvar. Iumillo. On the basis of Criteria (1) and (2), Marquis appears to be more closely related toT.dicoccumvar. Vernal than toT.durumvar. Iumillo, but in respect of fertility it seems closer to the latter.Chromosome behaviour in the 16 hybrids cannot be neatly summarized. Even varieties within a species gave different results—results that are not in agreement with earlier published reports on chromosome behaviour in pentaploid wheat hybrids in which it has been stated that 14 bivalents and 7 univalents are most commonly found. The difficulties encountered in attempting to establish criteria indicating relationships between the 42- and 28-chromosome wheats suggest that the utmost caution must be used in drawing phylogenetic conclusions on the basis of such data.

ISSN:1923-4287    

DOI:10.1139/cjr41c-036

出版商:NRC Research Press    

年代:1941

数据来源: NRC