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Biotin-labelled DNA probes for tandemly repeated sequences have been used in in situ hybridization experiments as chromosome-specific markers in pea. Six of the seven chromosome pairs can be marked at single sites in this way. Translocations from a standard karyotype are revealed as chromosomes that have two hybridization sites, rather than one. By probing a tester set of reciprocal translocation (or interchange) lines, some of the markers can be assigned to chromosomes. The method is rapid and simple and, in the absence of well-resolved chromosome bands, provides a means for clarifying some of the problems in pea cytology.Key words: in situ hybridization, translocations, chromosome markers.

The organization of three classes of legumin gene has been studied by exploiting restriction fragment length polymorphisms. Hybridization of cDNA probes, representing different sub-families of legumin gene, to DNA from F2 and F6 plants from selected crosses showed that there was a tight linkage of individual genes within each of two sub-families. One of these sub-families has been mapped to a locus on chromosome 7 close to r, while the other maps to a locus near a on chromosome 1. The results from one of the crosses analyzed indicated that a third class of legumin gene is also linked to a.

In order to detect low copy number sequences in pea using biotin-labelled probes we optimised some aspects of the in situ hybridization technique. We found protoplast preparations to be superior to standard squashes in terms of their signal: noise ratio. Heat and alkali denaturation of chromosomal DNA were both more effective than acid denaturation. A comparison of antibody-fluorochrome and streptavidin-enzyme conjugates showed the streptavidin-alkaline phosphatase conjugate to be the most sensitive detection system. Using the optimised method, we were able to detect a single site for a 13.5 kb legumin gene clone.

The extent of variation in cell mass, and in the size of the cell population of cotyledons has been examined to determine their relative roles in the evolution of variation in seed weight within the genus Vicia. In the 12 species examined there were highly significant correlations between seed weight and cotyledon cell number, and between seed weight and mean cell mass. The extremes differed by a factor of 625 fold in dry seed weight; of this variation, most (140 fold) was attributable to variation in the size of the cell population and only a 4.47-fold change was due to differences in cell mass. The cotyledon cells of all the species, irrespective of their seed weight showed very high levels of replication of their DNA. The implications of these data in terms of the evolution of seed mass in Vicia is considered. There was a constant relationship between cell mass and DNA amount per cell for all species, irrespective of the basic genome size, extent of DNA replication or of the information content of the genome. The basis of this relationship, and its evolutionary implication, is considered.

Variation in cell population size and cell weight in the cotyledons are important factors in determining seed weight in Pisum sativum. The regulation of these parameters as well as that of growth rate has been examined. The use of reciprocal crosses between varieties of contrasting seed size has allowed the recognition of two systems of control-an intrinsic one dependent on the seed's own genotype, and the extrinsic control of the maternal environment. It is shown that the use of particular kinds of crosses can aid in discriminating the separate roles of sinks and sources as determinants of seed size.

A comparison has been made of the amount of DNA coding for ribosomal RNA present in a polyploid series of plants of Datura innoxia. The plants which were produced by in vitro culture of pollen grains were haploid, diploid, triploid, tetraploid and hexaploid. In all instances there was a similar proportion of DNA coding for ribosomal RNA. The implications of these results are discussed, and the data compared with that available from other series of plants of different DNA content or ploidy.

Summary Culturing young hybrid embryos on a synthetic medium proved, in four out of six crosses attempted, to be a successful method of overcoming inter-specific incompatibility within the genus Hordeum. Hybrids of diploid H. vulgare and tetraploid H. bidbosum were studied morphologically and cytologically.

Different varieties of peas have different proportions of rDNA in their genomes; there is no obvious correlation between the proportions and seed or leaf size. The rDNA proportions in root tips, seedlings, leaves and in the cotyledon cells of high DNA content, were compared in four varieties. In three, there was no difference between tissues; the fourth showed an amplification of rRNA genes in the cells of high DNA content of the seed cotyledon, and also in the cells of young but not of older leaves. The fourth variety was the one that had the lowest proportion of rDNA of all those examined. Studies of the tissues of hybrids between genotypes with "low" and "high" proportions of rDNA showed that heterozygotes had the "high" value, showing again the occurrence of an amplification phenomenon.

MANY difficulties and disadvantages are involved in undertaking a programme of improvement of crop plants by the artificial induction of mutations. Among those which have been recognized are (a) the necessity for screening very large populations of plants ; (b) the considerable degree of sterility found in mutant material; (c) the inefficiency of existing mutagens, and (d) in inbreeding forms, the desirability of isolating the frequently male-sterile R 1 generation, to avoid contamination with foreign pollen. The first of these difficulties is less troublesome in certain instances, as for example, when screening seedlings for disease resistance, for large numbers are easily handled. No doubt, for this reason, many reports1 of the successful induction and selection of disease resistant mutants do in fact exist; resistance to Erysiphe graminis f. hordeii in barley, to Puccinia glumarum and Puccinia graminis f. tritici in wheat, and to Puccinia graminis f. avenae in oats, are among the most important.

Examination of data from a set of diallel crosses between a number of inbred lines was used for analysing the genetic control of radiosensitivity in six tomato varieties. The six varities were crossed in all possible combinations, including reciprocals, and the parental and F1 seed were used in the experiments. Seeds were dried and exposed to 28,116 rad of Co60 gamma rays at a rate of 2000 rad/hr. The characters scored were the distance from the apex of one cotyledon to the apex of the other at the 12th day, and the lengths of the 1st and 2nd leaves at the 19th day after sowing. The results showed the response of tomato seeds to radiation to be genetically controlled. The nature of this control was polygeric, with evidence of additivity and dominance. The different varieties had various proportions of dominant and recessive alleles, and dominance and ambidirectional. Pronounced consistent and inconsistent maternal effects influenced the radiation response; these affected dominance values to produce a complex genetic situation. The genetic control of radiosensitivity was different from that of the simple growth characters, though there was some similarity in that both showed maternal effects. There was also evidence that the control ofmore » these characters varied according to whether they were measured in the cotyledon, first or second leaf. Relationships between spontaneous mutation and chromosome breakage, and induced mutation and breakage is discussed. It is concluded that radiosensitivity per se may not be a true character, and that what was measured may be some quite different character, which could control such factors as oxygen tension, metabolic rate, or production of radioprotective substances; and that different levels of expression of this character could result in different levels of radiosensitivity. (H.H.D.)« less

Summary(1) The radiosensitivity of soaked barley seeds was directly dependent on their stage of development in relation to the cell cycle.(2) At any given stage of development there was a pronounced oxygen effect, but no effect of irradiation temperature in the range 2° to 20°c. There was no evidence of any temperature-dependent recovery process.(3) Similar responses were obtained when radiobiological damage was measured in terms of induced mutation, sterility, or growth inhibition.

The nuclear DNA content of prophase nuclei in root tips of two cultivars and two primitive lines of Pisum sativum and of Pisum fulvum have been determined, using a scanning microdensitometer. The nuclear DNA contents differed significantly between the genotypes investigated but there was no correlation with their supposed phylogenetic positions.A loss of 73% of the DNA from cells of aseptically cultured excised pea roots has been recently reported (Abbott, 1971). In marked contrast to this claim, our measurements of the nuclear 4C DNA content of root tip meristematic cells have shown that there is no significant loss in excised roots compared with attached roots.