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Genetic structure and diversity of natural populations of prickly lettuce (Lactuca serriola) were studied using AFLP markers and then compared with the diversity of the RGC2 disease resistance gene cluster. Screening of 696 accessions from 41 populations using 319 AFLP markers showed that eastern Turkish and Armenian populations were the most diverse populations and might be located in the origin and center of diversity of L. serriola. Screening 709 accessions using the microsatellite MSATE6 that is located in the coding region of most RGC2 homologs detected B66 different haplotypes. Again, the eastern Turkish and Armenian populations had the highest diversities at the RGC2 cluster. The diversities at the RGC2 cluster in different populations were significantly correlated with their genomewide diversities. There was significant variation of copy number of RGC2 homologs in different populations, ranging from 12 to 22 copies per genome. The nucleotide diversities of two conserved lineages (type II) of RGC2 genes (Kand L) were not correlated with diversifies calculated using the MSATE6 or AFLP data. We hypothesize that the high genomewide diversity and diversity of the RGC2 cluster in eastern Turkish and Armenian populations resulted from high abiotic and biotic stresses in the regions of origin of L. serriola.

We have sequenced two complete chloroplast genomes in the Asteraceae, Helianthus annnus (sunflower), and Luctuca sativa (lettuce), which belong to the distantly related subfamilies, Asteroideae and Cichorioideae, respectively. The Helianthus chloroplast genome is 151 104 bp and the Lactuca genome is 152 772 bp long, which is within the usual size range for chloroplast genomes in flowering plants. When compared to tobacco, both genomes have two inversions: a large 22.8-kb inversion and a smaller 3.3-kb inversion nested within it. Pairwise sequence divergence across all genes, introns, and spacers in Helianthus and Lactuca has resulted in the discovery of new, fast-evolving DNA sequences for use in species-level phylogenetics, such as the trnY-rpoB, trnL-rpl32, and ndhC-trnV spacers. Analysis and categorization of shared repeats resulted in seven classes useful for future repeat studies: double tandem repeats, three or more tandem repeats, direct repeats dispersed in the genome, repeats found in reverse complement orientation, hairpin loops, runs of A's or T's in excess of 12 bp, and gene or tRNA similarity. Results from BLAST searches of our genomic sequence against expressed sequence tag (EST) databases for both genomes produced eight likely RNA edited sites (C [right arrow] U changes). These detailed analyses in Asteraceae contribute to a broader understanding of plastid evolution across flowering plants. Key words: Asteraceae; chloroplast DNA; comparative genomics; divergent sequence; genomic repeats; Helianthus annnus; Lactuca sativa; RNA editing.

The inheritance, epistatic effects, and linkages of 6 chlorophyll deficient mutants of lettuce is studied. Chlorophyll-deficient-3, -4, -5, -6, -7 and dappled influence the development of the genetic map for lettuce. Chlorophyll-deficient-5 and -6 are epistatic to chlorophyll-deficient-7. Chlorophyll-deficient-3 is epistatic to light green and is recessive. Dappled is a recessive allele. Chlorophyll-deficient-4 is linked to an anthocyanin gene.

A diallele study, using hypocotyl elongation assay, of nine dwarf mutants from an M2 population of Lactuca sativa L. to evaluate stem elongation control in lettuce reveals the presence of four dwarfing loci, dwf-1 to 4. The mutants exhibit a reduction in stem length, smaller size and altered flower morphology. Allele dwf-2 induces the expression of dwf-4. The height of dwarf phenotypes ranges between 3 and 30 cm.

Analysis of lettuce seedlings for Dm mutations reveal that there are four different downy mildew resistance (Dm) gene activities which undergo 16-independent mutations. The genetic and molecular marker data indicate that several closely linked genes are involved in the resistant phenotype and that the deletions are intergenic. The majority of the mutants are induced by the ionizing radiation of neutrons.

Experiments on lettuce help determine the effects of genes and growth conditions on resistance costs of lettuce. Unlike agricultural fields which offer perfect growth conditions for plants, stressful conditions increase resistance costs of lettuce cultivation. Genetic linking or pairing also induce variations in resistance cost, and this is attributed to the gene background which exhibits resistance.

An AFLP data set comprising 95 accessions from 20 species of Lactuca s.1. (sensu lato) and related genera was generated using the primer combinations E35/M48 and E35/M49. In phenetic analyses of a data subset, clustering with UPGMA based on Jaccard's similarity coefficient resulted in the highest cophenetic correlation, and the results were comparable to those of a principal coordinates analysis. In analyses of the total data set, phenetic and cladistic analyses showed similar tree topologies for the well-supported parts of the trees. The validity of cladistic analysis of AFLP data is discussed. The results do not support a distinction among the serriola-like species L. sativa, L. serriola, L. dregeana, and L. altaica, which is in line with previous results. Therefore, we postulate that these species are conspecific. The serriola-like species L. aculeata occupies a clearly separate position, making it an ideal outgroup for studies of the closest relatives of L. sativa. The subsect. Lactuca as a group is well supported by our data, but the positions of L. saligna and L. virosa relative to the serriola-like species remain unclear. The close relationship between the sect. Mulgedium species L. tatarica and L. sibirica is corroborated by the present AFLP results and by additional crossability data. Key words: AFLPs; Asteraceae; Compositae; Lactuca; Lactuceae; lettuce; molecular phylogeny; phenetic relationships.

Internal transcribed spacer (ITS-1) sequences from 97 accessions representing 23 species of Lactuca and related genera were determined and used to evaluate species relationships of Lactuca sensu lato (s.l.). The ITS-1 phylogenies, calculated using PAUP and PHYLIP, correspond better to the classification of Ferakova than to other classifications evaluated, although the inclusion of sect. Lactuca subsect. Cyanicae is not supported. Therefore, exclusion of subsect. Cyanicae from Lactuca sensu FeraSkova is proposed. The amended genus contains the entire gene pool (sensu Harlan and De Wet) of cultivated lettuce (Lactuca sativa). The position of the species in the amended classification corresponds to their position in the lettuce gene pool. In the ITS-1 phylogenies, a clade with L. sativa, L. serriola, L. dregeana, L. altaica, and L. aculeata represents the primary gene pool. L. virosa and L. saligna, branching off closest to this clade, encompass the secondary gene pool. L. virosa is possibly of hybrid origin. The primary and secondary gene pool species are classified in sect. Lactuca subsect. Lactuca. The species L. quercina, L. viminea, L. sibirica, and L. tatatica, branching off next, represent the tertiary gene pool. They are classified in Lactuca sect. Lactucopsis, sect. Phaenixopus, and sect. Mulgedium, respectively. L, perennis and L. tenerrima, classified in sect. Lactuca subsect. Cyanicae, form clades with species from related genera and are not part of the lettuce gene pool. Key words: Asteraceae; Cichorium; gene pool; Lactuca; Lactuceae; phylogenetic relationships; ribosomal DNA internal transcribed spacer (ITS); Taraxacum.