Tuatara: Volume 2, Issue 3, September 1949
Cytology and Genetics and their Application to New Zealand Plants
Cytology and Genetics and their Application to New Zealand Plants
Before discussing some of the lines of cytological and genetical research which might be carried out in New Zealand, I will describe very briefly some of the types of studies which are now being carried out overseas.
In 1926, twenty-six years after the re-discovery of Mendel's paper, T. H. Morgan summed up knowledge at that time, of heredity and its physical basis in his book “The Theory of the Gene.” He noted that the characters of an individual are referable to genes which occur in a linear order on the chromosomes, and that the genes on each chromosome are held together in a linkage group. In diploids, homologous* chromosomes and their genes are paired, and each gamete contains only one of a pair, thus satisfying Mendel's first law. If genes are on different chromosomes they are assorted independently at game-togenesis, upholding Mendel's second law. Interchange of genes may occur between homologous chromosomes, and as the frequency of this interchange is roughly proportional to the distance between genes, a method was available of making linkage maps.
Research has proceeded in several directions since 1926. Genetic research on diploids has been extended to polyploids*. The statisticians have developed techniques for obtaining the maximum amount of information from a set of figures, as for example, in a linkage experiment, or for estimating the frequency of a gene in a population and the extent of its fluctuation due to selective influences. Refinements of technique continue to appear, but the development of new methods is usually beyond the power of the biologist, though he will naturally keep acquainted with the tools that the mathematician produces for his use.
R. A. Fisher in 1928 suggested that modifying genes were of great importance in the evolution of dominance and this stimulated research into the reaction of the same gene in different genetic environments. A great deal of research in this field has been done on cotton. The recognition that the phenotypic* effect of a main gene can change gradually this way or that under the influence of different modifying complexes, has given valuable support to the Darwinian theory that species evolved gradually by the accumulation of slight changes which have a selective advantage.
Further support for the Darwinian theory has been given by Mather from his recent researches on polygenes in Drosophila melanogaster. The polygenes occur in large numbers on all the chromosomes, even on the so-called “inert” heterochromatic Y chromosome. They page 110 were shown to control bristle number on the ventral abdominal surface of the fly. When the normal polygenic system was gradually disrupted and recombined by selection, flies were produced with a much greater or a much smaller number of bristles than in the wild type. Thus polygenes, because of their large number and the magnitude of the different combinations possible, provide a vast reservoir of variability in the organism and they may be used to change the form gradually.
A full knowledge of the theory of polygenes is of great importance to plant and animal breeders, as most characters of commercial value are under the control of large numbers of genes each with a small effect.
The Ascomycete genus, Neurospora, has come to the fore in genetical research in recent years. It has been used to study the way in which genes control the synthesis of chemical substances. Its merit is that immediately after fertilisation an “ordered” ascus is formed consisting of 8 spores in a row. These may be dissected out in order and on germination tested for the mutant under consideration. The relative position of mutant and normal spores in the ascus show whether or not crossing over has occurred in the meiotic division. Irradiation with ultra-violet light induces biochemical mutants in Neurospora. They will grow only on a complete medium containing all the elaborated substances they could possibly need, and will not grow on a basic medium as they lack the power to synthesise some substance. This substance is identified by adding, in turn, all known vitamins and all known amino-acids to the basic medium until the spores germinate. Thus the mutant “lysineless” will not grow unless lysine is added to the basic medium (showing that the normal allele* of lysineless takes part in lysine synthesis). In the United States it has been shown that at least 7 genes control the synthesis of the amino-acid arginine, the first 4 building up ornithine, the next 2 changing that to citrulline and the last changing citrulline to arginine.
Genetical studies in other simple organisms are appearing in increasing numbers in the journals. Mutants in bacteria are identified by their resistance to various types of bacteriophage, while in America there are several workers who specialise in the genetics of Paramoecium.
Genetical research on micro-organisms is a specialised branch of a specialist subject and would be difficult to start as yet in New Zealand Universities. Such research can be better carried out elsewhere.
Before talking about work in New Zealand just a word about developments in cytology.
In 1926 cytology was not in as good a position as genetics. What happened during prophase of meiosis was still rather a mystery. Such a fundamental point as to whether the chromosomes paired end to end (telosynapsis) or along their whole length (parasynapsis) was not yet settled. However, during the next ten years the position improved rapidly, due mainly to the brilliant work of C. D. Darlington who page 111 synthesised his own observations and those of his predecessors into a coherent system of cytology in which the deductive method could be used as in the physical sciences. Darlington explained chromosome movements in terms of Newtonian mechanics; e.g., chromosomes moving under forces of attraction or repulsion depending on whether they were respectively single or double threads. Cytology has now entered another phase and is being re-written in terms of chemistry. Cell divisions are being investigated, not so much from the point of view of bodies moving under the influence of external and internal forces, but from the point of view of a nucleic acid cycle. This involves a concentration of thymonucleic acid on a protein core to form the visible chromosome during division, and then the diffusion of the acid into the nucleus and its storage during the resting stage. The fascinating inter-relationships which may exist between the chromosomes (euchromatin plus heterochromatin), nucleolar organisers, nucleolus, centromeres and cytoplasm in this cycle, are recorded in the Symposium on Nucleic Acid published in 1948 for the Society of Experimental Biology. Of course, in this new aspect of cytology the organic chemist and the biophysicist play important parts, but it is only the cytologist who can relate their observations to the important cell processes, and it is only the cytologist who can watch the movements of large amounts of nucleic acid on the chromosomes.
|Danthonia (12)||Clematis (1)|
|Epilobium (17)||Chrysobactron (1)|
|Arthropodium (2)||Dianella (1)|
|Thelymitra (1)||Hebe (21)|
Of these, two would be well worth further study. Chrysobactron Hookerii has 14 large chromosomes ranging from 10 to 18mu long; several have distinctive morphology due to their length, non-staining constrictions and position of the centromere. If plants from widely separated populations were crossed, studies of meiosis in the hybrid might reveal the beginning of structural differentiation between chromosomes from different parts of the species area. Clematis indivisa has 16 quite large chromosomes, and I have recently found that C. colensoi has the same number. A chromosome survey of the rest of the page 112 N.Z. species of Clematis may aid taxonomists, and it would be interesting to search for an XY sex-determining mechanism as the species are dioecious.
Chromosome surveys are of great use to the taxonomist, and the cytologist should keep pressed specimens of the plants which he examines. Chromosome surveys may support groupings that the taxonomist has already made. Thus Cockayne and Allan (1926) created two varieties of Hebe vernicosa, var. canterburiensis, and var. gracilis. This division, made on morphological grounds, was supported by the research of Frankel and Hair (1937) who showed that these varieties had 40 and 42 chromosomes respectively. Frankel and Hair also showed that the two varieties of Hebe buxifolia (var. odora and var. pauciracemosa) created by Cockayne and Allan (1926) differed in chromosome number, var. pauciracemosa having 42 chromosomes, and var. odora having 84.
On the other hand evidence from chromosome numbers may lead the taxonomist to revise his work. A type example comes from work done in N.Z. on the closely related genera Hebe and Veronica (Frankel, 1941). The majority of N.Z. species were placed by Cockayne and Allan in Hebe, while a few were included in Veronica. Frankel and Hair (1937) found that both N.Z. Hebes and Veronicas had basic numbers of 20 or 21, where as the Veronicas of the Northern Hemisphere had basic numbers of 4, 8, 7, 9, or 17. This showed that the separation of Hebe from Veronica by Pennel in 1921 had been justified, and it also suggested that N.Z. Veronicas were closer to Hebe than to the Northern Hemisphere Veronicas. Allan (1939), on re-examining the capsule dehiscence in N.Z. Veronicas found that it was essentially the same as in Hebe and therefore transferred them to the latter genus.
Again Calder's study of chromosome numbers in the N.Z. Danthonias showed that Danthonia semiannularis had 48 chromosomes, but that both D. semiannularis var. setifolia and D. semiannularis var. nigricans had only 24. The difference in chromosome number implies that species and variety are effectively isolated as a hybrid between them would probably be sterile. This fact, together with definite morphological differences, justified him in making two new species, D. setifolia and D. nigricans.
Chromosome numbers also indicate whether an inter-specific cross is likely to be fertile or sterile. Mr. Hair tells me that one of our Agropyron species has 42 chromosomes, and another has 28. A hybrid between them will have 35 chromosomes and probably be sterile.
Enough has been said to show the importance of a knowledge of chromosome numbers, and it is to be hoped that this gap in our knowledge of the N.Z. flora will be filled within the next few years. Of course, many other problems are often unearthed during a chromosome survey. Thus S. Smith-White (1948) in Sydney, while carrying page 113 out a survey of the Epacridaceae, found that Leucopogon juniperinus was a triploid, and that contrary to most of the rules it was stable and reasonably fertile. The reason discovered for this is of great general interest.
The New Zealand flora should provide ideal material for studies in the evolution of species. The pattern of distribution in most of the genera has not been appreciably modified by man.
The beginnings of species differentiation might be sought for among the numerous cases of discontinuous distribution known within the country (see Wall, 1927). Individuals from two widely separated populations could be brought together in an experimental garden and studied for signs of cytological, taxonomical and, if possible, genetic divergence.
Ecotypes*, considered by many to be the first marked stage in species differentiation, have not been studied much in this country, possibly because of lack of experimental gardens. That such studies can produce interesting results has been shown in some work by Mr. H. Conner of the Botany Division, D.S.I.R., who has collected a number of distinct forms of Agropyron scabrum from both Islands. These have retained their distinctive characters when grown together under uniform conditions in Wellington.
On the species level, the hybrid swarms so characteristic of our flora show that many species, though morphologically distinct, are not yet differentiated enough to be inter-sterile. Cytological studies of F1 hybrids would show the nature of the chromosome differences between the species. Genetical studies would be difficult in some of the most interesting hybrids. The theory is that an estimation of the number of genes controlling, for example, the leaf shape difference between two species can be obtained by selfing the F1 hybrid and noting the proportion of original parent leaf types obtained in the progeny. Leaf shape and most other characters would doubtless be under the control of several genes, and to re-obtain the parent types many hundreds of F2 plants would need to be cultivated. In the case of shrubs, this would require much garden space for several years, until the plants matured. Other difficulties arise. In attempting to self-pollinate Corokia buddleoides X cotoneaster by bagging last season, no seed was set, though on open pollination it is usually prolific. It may be self sterile, or else bagging does not agree with it. The only chance of getting an F1 intercross would be to find two hybrids growing close together in an isolated garden, and let them cross-pollinate naturally. However, there is much scope for work on the synthesis of hybrids. Many of our puzzling plants have been called natural hybrids because of very strong circumstantial evidence. The forms are intermediate between two good species, and only occur when these species grow in proximity. The final experiment to clinch the argument is to synthesise the hybrid from the supposed parents.page 114
A genus with its main representation in New Zealand could be the subject of an evolutionary study in the style of that of Babcock and others on the genus Crepis (Babcock, 1947). This genus has been under study for about thirty years. Some 113 species have been brought into cultivation at Berkley, California, and have been studied by taxonomists and cytologists, and the pattern of its evolutionary history is now fairly complete. Crepis has evolved from primitive perennial rhizomatous types with large flowers, leaves and achenes to more advanced tap-rooted annuals with small leaves and flowers and with small beaked achenes. The centre of origin was probably the northern part of central Asia, whence it has migrated over the Northern Hemisphere. Chromosome complements have evolved by decrease in basic numbers from 6 to 5, 4 and 3; by polyploidy; by increase in asymmetry*, and by decrease in size.
I had thought that the genus Celmisia might be studied in this manner. There is a large number of species, all but one of them confined to New Zealand; there is a good range of form, and the distribution of species is interesting, for example, a number of endemic species confined to the Nelson mountains. However it may prove very difficult to determine chromosome numbers accurately in this genus. In three species that I have studied, two had chromosome numbers between 80-90, and one about 120. This is very different from, say, Crepis capillaris with 6 chromosomes.
The breeding systems of plants is another important topic. Are some of our polymorphic species variable because they are obligately cross-fertile, thus continually reshuffling genetic material, or do they consist of a number of apomictic* strains each of which will inevitably breed true?
It should also be remembered that N.Z. Universities have quite a good tradition of research in plant anatomy. This could be used in collaboration with a geneticist in studying the control of plant form by genes. This branch of genetics is developing, and reference should be made to the second Symposium of the Society for Experimental Biology which deals with Growth and Differentiation (1948).
Cytological and genetical work on natural populations of Drosophila could be carried out in New Zealand, but first the taxonomy of the species would need to be clarified. Dr. Frankel has pointed out to me the good field for cytological research in insects here, and I have recently found that a male weta (species not yet determined) has 19 pairs of quite large chromosomes which are easy to study.
* Some Definitions
- Allele.—One of two or more forms of a gene which can exist at a given genelocus.
- Apomixis.—A system of reproduction in which a diploid egg cell forms an embryo without fertilisation.page 115
- Asymmetry.—As applied to chromosomes it means that the chromosome arms on each side of the centromere are unequal in length.
- Ecotype.—A genetically determined form within a species, specialised for growth in a particular habitat; e.g., a sand dune ecotype.
- Homologous Chromosomes.—All chromosomes or parts of chromosomes which carry similar genes or alleles of those genes are said to be homologous. In diploids there are pairs of homologous chromosomes, one homologue derived from the male parent, the other from the female parent.
- Phenotype.—The external appearance of an organism caused by a given set of genes, together with any modifications in appearance caused by the environment.
- Polyploids.—Organisms with more than two sets of chromosomes. A triploid has three sets a tetraploid has four sets, etc.
Allan, H. H., 1939.—Notes on New Zealand Floristic Botany. Trans.N.Z.Inst., Vol. 69.
Babcock, E. B., 1947.—The Genus Crepis. Univ. of California Press.
Calder, J. W., 1937.—A cytological study of some New Zealand species and varieties of Danthonia. Journ. Linn. Soc. (Botany). Vol. 51.
Frankel, O. H., and HAIR, J. B., 1937.—Studies on the cytology, genetics and taxonomy of New Zealand Hebe and Veronica. N.Z. Journ. Sci. and Tech. (Pt. 1), Vol. 18.
Frankel, O. H., 1941.—Cytology and taxonomy of Hebe, Veronica and Pygmaea. Nature, Vol. 147.
Hair, J. B., 1942.—The chromosome complements of some N.Z. plants. Trans. Roy. Soc. N.Z., Vol. 71.
Wall, A., 1927.—Some problems of Distribution of Indigenous plants in N.Z. Trans.N.Z.Inst. Vol., 57.
Smith-White, S., 1948.—Polarised Segregation in the Pollen Mother Cells of a stable triploid. Heredity. Vol. 2, Pt. 1.