Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.593822
Title: Chemotaxonomy and cytology of birches in Scotland
Author: Williams, Dorothy A.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1981
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Abstract:
The two arborescent birch species recognised within the morphologically variable Betula alba complex in Britain are B.pendula Roth (2n=28) and B.pubescens Ehrh. (2n=56). Reports of the occurrence of an intermediate cytotype (2n=42) within a mixed B.pendula/B.pubescens population in Glen Gairn, Deeside (Brown and Al-Dawoody, 1977) raise interest in view of the extremely rare occurrence of natural hybrids in Europe (Vaarama,1969). An integrated cytological and biochemical approach was taken with the aims of developing a reliable method of separating the two variable species of the Balba complex, identifying intermediate-cytotypes and determining their genetic origin. A programme of artificial crosses and self-pollinations was undertaken to provide material of known genetic background for comparison with natural populations and information about the crossing behaviour of the three Glen Gairn cytotypes. The results of the crossing programme showed that hybridisation between B.pendula and B.pubescens is possible under artificial conditions, that interspecific crosses between parents of different ploidies are more successful when the lower ploidy tree is used as female parent, and that individuals vary in their success as parent trees. Betula species show high levels of self-compatibility but this is more variable in intermediate-cytotype and 56-cytotype (B.pubescens) individuals and some self-pollinations produced a small proportion of germinable seed. The performance of intermediate-cytotype trees in inter-cytotype crosses, selfing and open-pollination indicated a closer relationship with B.pubescens than with B.pendula. The most useful method for routine counting of somatic chromosome numbers was found to be squash preparation of leaf-bases stained by a modified Feulgen technique. Somatic counts were made of adult trees and seedling progenies from open-pollination and controlled crosses. Despite some experimental error in counting small and numerous chromosomes, real variation was recognised within and between cytotypes. The occurrence of an intermediate cytotype in the Glen Gairn population was confirmed but only recognisable in progeny somatic counts. Higher-cytotype adult trees, some of which gave rise to intermediate-cytotype progenies and some to 56-cytotype progenies, showed aneusomatic variation, possibly due to some inherent mitotic instability. Intermediate cytotypes [special characters omitted] recognised in populations which did not contain B.pendula. A major proportion of this thesis is concerned with the development of a method for the extraction of birch leaf enzymes and overcoming the problems of denaturation of enzymes by tannins. A suitable extraction medium was devised after investigation of various buffers and additives to counteract "tanning". The most useful extraction medium to date, based on a Tris-HCl buffer, includes reagents to complex with tannins, to break down tannin/protein complexes and to inhibit the activity of phenoloxidases which catalyse the polymerisation of phenolic compounds to form tannins. An efficient method for the mechanical breakdown of leaf tissue was sought and starch and polyacrylamide gels, in tube and slab form, were compared for efficiency and clarity in separating isozymes by electrophoresis. Systems were compared by measuring the concentrations of protein in solution and/or the number of general protein or specific isozyme bands which could be detected after electrophoresis. The vertical slab polyacrylamide gel system was found to be the most useful electrophoretic system for routine analysis of birch leaf enzymes. Using this method peroxidase isozymes were shown to be variable and species specific patterns were recognisable. Preliminary results showed that chemotaxonomic relationships between B.pendula, B.pubescens and B.nana were in agreement with classical taxonomic divisions. A naturally occurring hybrid from Glen Prosen was identified by combined cytological and biochemical evidence. Information from suitable polymorphic enzyme systems may now be used to compare artificial and naturally occurring hybrids with intermediate cytotypes of uncertain origin. Isozyme analysis is now seen to have the potential to replace the more limited methods of cytological and morphological analyses in measuring genetic variation within and between populations of birch. The genetic origin of the intermediate-cytotype birches in Glen Gairn remains uncertain but, on balance, cytological evidence and information from the crossing programme indicate that recent hybridity is not involved and that the intermediates are derived from a B.pubescens genome.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.593822  DOI: Not available
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