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Title: Studies in nuclear cytology
Author: Callan, H. G.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1950
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My work falls broadly into three groups of subjects: 1. Studies on the determination of secondary sex characters in invertebrates : papers 2, 3 and 4. These papers describe research on one of the first problems which engaged my attention : the causal determination of the differentiation of secondary sex characters in invertebrates. While I was a student at Naples in 1938/9 I was impressed by the problem of parasitic castration and sex reversal in crustacea. The problem has great physiological and genetic interest it has also great complexity. This became apparent to me when I found that the effects of epicarid isopods on their prawn hosts is quite unlike the action of similar isopods on Upogebia, this latter animal showing all the familiar features of sex reversal when parasitised. The castration of unparasitised prawns by means of X -rays gave evidence that the cyclically developed female secondary sex characters are dependent on the presence of a maturing ovary for their differentiation : the same situation was found, thong of published, for the adult female characters of the crab Pachygrapsus, though this animal undergoes sex reversal, male to female, on being parasitised. It was hoped that castration of males might be accomplished by means of X -rays since this would supply crucial evidence for the resolution of the whole problem. However, too high a level of irradiation. is necessary for castration of male crabs, which die before completing a further moult. Surgical castration likewise proved impracticable and the problem was left in this state. 2. Studies in chromosome cytology : papers 5,6,70,9,10, 11,14,(16,17),19,20 and 21. My interest in chromosome cytology, originally aroused by Dr J.R.Baker and Dr E.B.Ford,F.R.S., at Oxford 2. and further stimulated by contact with Dr C.D.Darlington, P.R.S., then of Merton,London, led me to study a number of problems in this field. It is with these problems that the main bulk of my papers are concerned. Paper 6 demonstrated that a mmltiple chromosome mechanism determines sex in the earwig. The situation is complicated by the fact that males may have two alternative chromosome constitutions and that the Y chromosome in this species is dicentric :the mechanism is considered to be related to the abnormal sex ratios in earwigs which are found in the wild. Paper 8 describes the behaviour of a supernumerary autosome at meiosis in the grasshopper Mecostethus. Paper 9 describes how a fertile plant hybrid has arisen by the hybridisation of two autopolyploid species : this mechanism is an alternative means of arriving at an alloployploid constitution. Paper 10 describes the discovery of low temperature sensitive heterochromatic segments of the chromosomes of newts both at mitosis and meiosis and leads to the inference that desoxyribose nucleic acid ix plays an important role in chromosome spiralisation. Paper 11 describes _the upsets to the spindle mechanism at mitosis which can arise as a result of exposure of newt cells to low temperature or to the action of colchicine. Paper 14 demonstrates that the centromere is not necessarily a barrier to chiasma interferenee at meiosis, as was previously held to be the case. This was first shown for the mosquito Culex and has subsequently proved true for many other animal. Paper 19 shows how chiasma interference across the centromere is suppressed waen multivalent chromosome associations are formed at meiosis. Papers 20 and 21 describe the reduction in chiasma frequency and the localisation of chiasmata in hybrids between geographical races of newts. Evidence is also given of the origin of a chromosomal translocation which distinguishes the chromosome complement of one race from the other two and which can act as a barrier to genetic leakage between these races. Papers 16 and 17, which do not rightly fall under the heading of chromosome cytology, are nevertheless_ related to this theme since they are concerned with the possibility of .a connection between heterorshromatic chromosome segments, ribose nucleic acid synthesis and growth rate determination. 3. Experimental work with giant nuclei : papers (12, 13), 15, 18, 22, 23. Partly as a result of a controversy over the distribution of nucleic acid in the cell (papers 12 and 13) and more generally because, having worked for some years in the field of descriptive cytology and cytogenetics, 1 was impressed with the need for an experimental attack on the many outstanding and general problems concerning the cell nucleus, i turned my attention three years ago to the exploitation of giant nuclei. A great deal of this work is not yet ready for publication : it is the general line of research in which I an most actively engaged at the present time. Since, however, a number of observations have already been made, these are summarised in a final manuscript entitled " Experimental studies on amphibian oocyte nuclei ". This should be read as an introduction to my published papers in this field.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Sc.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available