• Margaret Upcott

      Articles written in Journal of Genetics

    • The cytology of triploid and tetraploidLycopersicum esculentum

      Margaret Upcott

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      1. The multivalent configurations in the triploid and tetraploidLycopersicum esculentum are of the types which would be expected, if they were determined by a random distribution of chiasmata.

      2. They vary in frequency from cell to cell, but remain statistically constant at successive stages, as they would be expected to do on this assumption.

      3. The configurations (contrary to the opinion of previous authors, but as is expected on analogy with all other cases of chiasma pairing) are constant between diplotene and metaphase.

      4. The metaphase chiasma frequency is highest in the diploid and lowest in the triploid. This is attributed to a similar difference in original chiasma frequency rather than to a greater reduction in number in the triploid during terminalisation.

      5. The curve of variance is higher in the polyploids than in the diploids, as has previously been found inTulipa andHyacinthus.

      6. The formation of trivalents and univalents in the triploid gives rise to irregularities in the second division, and to the formation of restitution nuclei.

      7. The formation of quadrivalents in the tetraploid leads to numerical non-disjunction which is reflected in reduced fertility.

    • The parents and progeny of aesculus carnea

      Margaret Upcott

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      The somatic chromosomes ofAesculus Hippocastanum (2n = 40) and ofAe. Pavia (2n = 40) are exactly similar in size and shape, although each complement contains within it differences which are also distinguishable in the complement of the hybridAe. carnea (2n = 80).

      In both species, and in the hybrid, polar views of metaphase I show about half the bivalents larger than the rest, and this is due to their having chiasmata in both arms. This difference necessarily disappears at anaphase.

      Secondary pairing and the formation of an occasional quadrivalent show the parent species to be tetraploid. The hybrid must therefore be regarded as octoploid.

      4.Ae. planlierensis (2n = 60) is a hexaploid back-cross ofAe. carnea Ae. Hippocastanum, and forms varying numbers of multivalents, bivalents and univalents.

      The individuals ofAe. Hippocastanum andAe. carnea examined are heterozygous for inversions.

    • The genetic structure ofTulipa - I. A chromosome survey

      Margaret Upcott L. La Cour

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      1. The basic haploid chromosome number in the genusTulipa is 12. Polyploid series occur in both the subsections Eriostemones and Leiostemones.

      2. Although the diploid species probably reproduce sexually in the wild, the polyploids must be purely clonal since no aneuploids have been found.

      3. Two main types of change have affected the chromosome morphology of the genus. Genotypic change of size and regulated structural change altering the position of the centromere have separated the genus into three groups with large, medium and small chromosomes. Random structural change has produced complements with reduplicated fragments (T. galatica) and with clearly unmated chromosomes (T. Gesneriana var. Zomerschoon).

      4. Evidence from external and chromosome morphology, from intersterility and from the time at which meiosis occurs, points to the conclusion that the subgroupClusiana of the Leiostemones is as distinct from the other subgroups as it is from the Eriostemones.

    • The genetic structure ofTulipa - II. Structural hybridity

      Margaret Upcott

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    • The genetic structure ofTulipa - III. Meiosis in polyploids

      Margaret Upcott

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      1. The tetraploid tulip species have a lower chiasma frequency at meiosis than the diploids and triploids and fewer changes of partner at pachytene than the latter. Consequently although autotetraploid, they form few quadrivalents and are sexually fertile.

      2. The quadrivalents that they form may be classified in two ways:

      According to the distribution of chiasmata among the four chromosomes. This shows to which of the ten possible types of quadrivalent they belong. These types depend on the number of changes of partner at pachytene and on the symmetry, half-symmetry or asymmetry of the chiasma distributions per chromosome.

      According to the co-orientation of the centromeres at the first metaphase of meiosis, whether convergent, linear or indifferent. The type of co-orientation depends upon the distances apart of the centromeres in the multivalent at the time metaphase begins. Co-orientation fails altogether when the centromeres are further apart than they can be in bivalents. These principles agree with the repulsion theory of orientation.

      3. The inter- and intranuclear mean squares show no significant correlation of chiasma frequency in the diploids. A slightly positive correlation in the tetraploids and variable correlation in the triploids is presumably to be attributed to the number of changes of partner their chromosomes undergo and to variable external conditions.

    • The genetic structure ofTulipa - IV. Balance, selection and fertility

      Margaret Upcott James Philp

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    • The nature of tetraploidy inPrimula kewensis

      Margaret Upcott

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