H. Rees

Nuclear DNA variation in Allium

Now that DNA is firmly established to be the carrier of genetic information much can be learned about the nature and origin of heritable variation by directly investigating qualitative and quantitative changes in the DNA itself. Qualitative differences in the base sequences and the base ratios of nucleic acids are reported for a number of species (e.g. Chargafl 1955; Reddi, 1959). A growing body of evidence also testifies to widespread variation in the quantity of nuclear DNA between species (e.g. McLeish and Sunderland, 1961; Keyl, 1964, 1965; Rees et al., 1966; Rothfels et al., 1966; Martin and Shanks, 1966; Martin, 1966; John and Hewitt, 1966). Such variation is especially common among Angiosperms and may be quite independent of change in chromosome number. The DNA differences are often large— even between species closely related. For example, Vicia faba has seven times more DNA than V. sativa (Rees et al., bc. cit.). Both are diploids, yet the variation in DNA content is equivalent to that between 2x and 1 4x. The following is an investigation of nuclear DNA variation in Allium. It reveals that widespread changes in nuclear DNA content accompanied the divergence and evolution of species within the genus. In addition it provides information about the nature and the distribution of the chromosome structural changes which account for the variation in nuclear DNA.

Genotypic control of chromosome behaviour in rye XI. The influence of B chromosomes on meiosis

Genotypic control of chromosome behaviour in rye XI. The influence of B chromosomes on meiosis

Nuclear DNA variation in Lathyrus

In the genus Lathyrus the divergence and evolution of species was accompanied by large scale changes in nuclear DNA amount. All the species are diploids with 14 chromosomes so that the DNA changes were the result of amplification or deletion of segments within chromosomes. Our evidence indicates that the quantitative changes involve mainly the repetitive, as distinct from the non-repetitive fraction of the chromosomal DNA and, on a cytological basis, mainly heterochromatin in contrast to euchromatin. There is an element of discontinuity in the distribution of DNA amounts among species which suggests that the DNA variation results from a series of separate, spasmodic events. The discontinuities may be viewed, also, as “steady states” from the standpoint of genetic balance and biological fitness.

Chiasmata and variability in Lolium and Festuca populations

There are significant differences in mean pollen mother cell chiasma frequencies between populations within Lolium perenne, L. multiflorum and Festuca pratensis. The differences are genotypically controlled. With low chiasma frequencies the chiasmata are distally located. With increasing chiasma frequency the frequency of chiasmata in interstitial segments increases. Shorter lived populations have higher chiasma frequencies than the more perennial. — The higher the chiasma frequency of a population the lower the phenotypic and genetic variance for characters under polygenic control, such as flowering time, and the less effective also is the response to selection for such characters. These observations are interpreted on the premise that high chiasma frequencies are instrumental in the breaking up of supergene sequences in interstitial chromosome segments.

Induced and developmental variation in chromosomes of meristematic cells

Change in chromosome size in root tip meristems of rye and Allium cepa are induced by growing the plants in solutions differing in phosphorus content. The chromosomes are 50% larger by volume in a “high” phosphate as compared with a “no” phosphate solution. Alteration of other elements supplied in culture also induces change in the size of chromosomes. — The size variation is a reflection of change in the chromosome dry mass. In part at least this change in mass is attributable to alteration in the amount of protein. The DNA component of the chromosomes remains unchanged. — A consistent pattern of change in chromosome size, quite independent of that induced by varying the culture solution, is related to age and development. For example, the root tip chromosomes double in size during the first three weeks of growth in rye seedlings. Thereafter the size decreases. As with the induced chromosome changes the protein content alters, the DNA amount remains constant. — Variation in the “non-permanent” component of the chromosomes in meristems appears to be closely correlated with the rate of cell metabolism.

Constraints upon the composition of supplementary DNA

Speciation in eukaryotes is often accompanied by massive changes in nuclear DNA amount resulting from the accumulation or deletion of DNA base sequences within chromosomes. Our evidence shows that, among related species, the DNA lost or gained during divergence is of a remarkably consistent and restricted composition in respect of the ratio of repetitive to non-repetitive base sequences. It is argued that the restriction may in part at least be imposed by natural selection, in the sense that the only changes tolerable are confined to DNA fractions of particular composition and organisation.

Fertility and selection in tetraploid Lolium

Selection over five generations increased the fertility (seed set) in a population of tetraploid Lolium perenne from 67.57 per cent in the second (C2) to 73.95 per cent in the seventh (C7) generation. The increased fertility is associated with a high quadrivalent frequency and a low frequency of bivalents, trivalents and univalents. The high quadrivalent frequency in the selected population was achieved partly by a slight overall increase in chiasma frequency but mainly by a redistribution of chiasmata.

The influence of B-chromosomes upon the nuclear phenotype in rye

Quantitative studies on the ffects of B-chromosomes in rye upon variation of the nuclear phenotype showed that Bs: (i) influence the phenotype of the A-chromosomes at metaphase of mitosis, (ii) increase the amount of nuclear DNA in proportion to their number present, (iii) have a structural organisation different to that of the A-chromosomes, (iv) influence the structure of the interphase nucleus in respect of non-permanent chromosome material.

Nuclear DNA variation in eu-sorghums.

There are significant differences in nuclear DNA amount between Eu-Sorghum species. The DNA variation is considerable. For example Sorghum durra has 63 per cent more DNA than Sorghum roxburghii. Increase in nuclear DNA is accompanied by increase in the mass of the nucleolus and of non-nucleolar material including the chromosomes. The ratio of DNA to non-DNA material is constant between species. The DNA concentration per unit volume at interphase in somatic nuclei and per unit of chromosome length is significantly lower in the wild species S. sudanense and S. virgatum than in the cultivated species S. caudatum, S. conspicuum, S. durra and S. roxburghii. It is suggested that the lower DNA concentration may reflect a lower proportion of heterochromatin relative to euchromatin in the wild as compared with the cultivated forms.

An anomalous variation due to B Chromosomes in rye

B CHROMOSOMES in rye are heterochromatic in that they replicate their DNA later in interphase than do the A chromosomes (Darlington and Haque, 1966). In common with other heterochromatic chromosomes the Bs carry no genes of major effect. They are not, however, inert. They cause a delay in germination (Moss, 1966) and in flowering (Kishikawa, 1965), a general reduction in plant vigour and in fertility (Muntzing, 1963) and, at cell level, influence the distribution of chiasmata at meiosis (Jones and Rees, 1967) and the composition and organisation of interphase nuclei in somatic tissue (Jones and Rees, 1968). While, in general, the influence of B chromosomes upon the rye phenotype is proportional to the number of Bs carried, the following evidence indicates that their genetic activity is not explainable in terms of straightforward additive effects. The indication is that the information vested in Bs is expressed differentially according to whether they are present in odd or even numbers in the nucleus.