Technological advances in molecular biology have contributed substantially to our understanding of plant genetic diversity. Early studies of allozyme variation employing protein electrophoresis revealed that plant populations have high levels of genetic diversity, most of the variation at polymorphic loci is found within populations, and geographic range and breeding system explain the largest proportion of variation in genetic diversity. With the discovery of restriction endonucleases, the first DNA-based markers allowed the detection of variation in DNA sequences in plant population studies. More recently, techniques that utilize the polymerase chain reaction have allowed a more representative assessment of genetic variation in plants by screening multiple loci distributed throughout the genome. The analyses reveal sufficient polymorphism for the examination of fine-scale genetic differences among individuals. Information on plant genetic diversity is also emerging from studies of plant genome structure. Comparative genetic mapping studies of members of the Brassicaceae, Poaceae, and Solanaceae show that gene content is highly conserved between closely related species, although gene order on a chromosomal segment may differ between species. Comparative sequencing studies reveal higher degrees of diversity at the microstructural (less than 1 million base pairs) level than predicted at the genetic map level and suggest that genes are densely packed in gene-rich regions, rather than randomly distributed along chromosomes in species with large genomes. Sequencing of the entire genomes of rice and Arabidopsis thaliana will help identify genes controlling agronomically important traits, improve our understanding of genetic variation for fitness-related traits in wild plant populations including weed species, resolve evolutionary relationships among plant taxa, and potentially revolutionize current ideas on plant diversity and evolution.

Aegilops tauschii, the D genome donor of wheat, is an invaluable source of genetic variability, which can be utilized for broadening the wheat gene pool. Linked leaf rust resistance and non-glaucousness genes transferred from Ae. tauschii to cultivated wheat variety WH542 were mapped in the present study. Genetic analysis in an F2 population from a BC3 plant derived from the cross Triticum durum cv. PBW114/Ae. tauschii acc. pau14195//4*T. aestivum cv. WH542 revealed monogenic dominant inheritance for both the traits. The leaf rust resistance and the non-glaucousness gene were tentatively named LrT and IwT, respectively. Leaf rust resistance gene exhibited all stage resistance. SSR markers Xbarc124, Xgdm5, Xgdm35, Xcfd51 and EST-derived markers Xcau96 and Xte6 on chromosome 2DS were linked with both genes. Chromosomal assignments of the genes were confirmed by testing linked SSR markers on Chinese Spring nulli-tetrasomics lines. SSR markers Xcau96 (1.6 cM) and Xbarc124 (0.6 cM) flanked LrT and Xgdm35 (4.1 cM) and Xte6 (2.5 cM) flanked non-glaucousness gene. LrT and IwT showed a recombination distance of 3.4 cM. Hence, IwT can be used as an easy to score morphological marker of LrT during its transfer to other glaucous backgrounds.

The pattern of inheritance of powdery mildew resistance expressed by two bread wheat (cultivar ‘Alcedo’)/Aegilops markgrafii introgression lines was explored using F2 populations bred from crosses made with the powdery mildew-susceptible cultivar ‘Kanzler’. Disease reaction was tested at both seedling and adult plant stages. Two resistance loci, designated QPm.ipk-1A and QPm.ipk-7A, were identified as mapping to the distal ends of chromosome arms 1AS and 7AL, respectively. Whereas QPm.ipk-1A was expressed throughout the plant's life, QPm.ipk-7A was only effective at the seedling stage. The map location of both resistance loci indicated that resistances originated from A. markgrafii. The possible genetic relationship of these disease-resistant genes to known Pm genes is discussed in the context of synteny.

Segregation distortion, identified as a strong evolutionary force, is common in genetic mapping research. In plants, the percentage, degree, origin and genetic effects of segregation distortion vary significantly with species, population types, crosses and marker types. The exhibition, common features, causes, methods of mapping segregation distortion loci, effects of segregation distortion on map construction and corresponding mapping strategies in plants, mainly in crops, are reviewed in this paper.

The male sterile mutant, ms35, of Arabidopsis thaliana was produced by X-irradiation of seeds. The mutant produces fertile pollen, but is male sterile because the anthers do not dehisce. Anther development in ms35 plants occurs as in wild-type Arabidopsis until shortly after microspores are released from meiotic tetrads. Thereafter, in the wild type, bands of lignified, cellulosic secondary wall thickenings are laid down around the cells of the anther endothecium. In contrast, wall thickenings are not formed in the endothecium of the ms35 mutant. Development of other lignified tissues, for example the vascular tissue of the stamen, occurs normally in ms35 plants. In mutant anthers, as pollen maturation is completed, the stomium is cleaved but the anther wall does not retract to release pollen. The block in anther dehiscence in ms35 plants is specifically correlated with the absence of endothecial wall thickenings. The ms35 mutation represents the first genetic evidence in support of the proposed role of the endothecium in anther dehiscence. The ms35 gene was mapped to the top arm of chromosome 3 (hy2-(4.17±2.31 cM)-ms35-(32.14±5.45 cM)-gl1).

A linear DNA molecule may be labelled at a fixed locus by a minute complementary radioactive molecule. A collection of identical molecules is to be so labelled and each one independently cut at a random number, N, of random places, X (N Poisson, X uniform). Fragments containing the label are to be collected and assayed by length. It is shown that the recovery pattern (fragment length distribution) contains a jump discontinuity at the fixed locus and may be used to determine the distance between the attachment site and the nearest end of the molecule.

The recovery pattern under the hypothesis that the collection of molecules are circularly permuted, i.e. the labelled locus is uniformly distributed over the length of the molecule, does not contain such discontinuities. The case where the labelling molecule has a non-negligible extent is also treated.