An integrated map for lettuce comprising of 2,744 markers was developed from seven intra- and inter-specific mapping populations. A total of 560 markers that segregated in two or more populations were used to align the individual maps. 2,073 AFLP, 152 RFLP, 130 SSR, and 360 RAPD as well as 29 other markers were assigned to nine chromosomal linkage groups that spanned a total of 1,505 cM and ranged from 136 to 238 cM. The maximum interval between markers in the integrated map is 43 cM and the mean interval is 0.7 cM. The majority of markers segregated close to Mendelian expectations in the intra-specific crosses. In the two L. saligna x L. sativa inter-specific crosses, a total of 155 and 116 markers in 13 regions exhibited significant segregation distortion. Data visualization tools were developed to curate, display and query the data. The integrated map provides a framework for mapping ESTs in one core mapping population relative to phenotypes that segregate in other populations. It also provides large numbers of markers for marker assisted selection, candidate gene identification, and studies of genome evolution in the Compositae.

Polymerase chain reaction (PCR) based random amplified polymorphic DNA (RAPD) markers were used to study the extent of redundancy (duplication of genetic materials) within a genetic resources collection. Nine nearly phenotypically and identical accessions of butterhead lettuce (Lactuca sativa L.) were assayed for their genetic identities. A nonuniform, heterogeneous butterhead line and a crisphead cultivar were added for population comparison. PCR amplification using 13 oligonucleotide primers generated 93 polymorphic bands. The percentage of segregating bands was used to determine within-line variation; values ranged from 0.0 to 12.0%, except for the nonuniform line at 22.6%. Between-line similarity was measured using similarity coefficients and ranged from 0.919 to 0.985. The relationship between the crisphead accession and a composite of all butterhead accessions was 0.84. Selfed progeny of each line were measured for morphological uniformity. The variation obtained from these biological data was compared with variation detected at the DNA level and each was positively correlated. Results demonstrate that RAPD analyses may serve as a major source of information for separation of closely related accessions, especially when integrated with phenotypic measures.

Two F 2 populations were generated by crossing morphologically diverse genetic stocks in order to map 10 morphological traits relative to polymerase chain reaction-based molecular markers (RAPDs). Using one segregating population generated from crossing the experimental line, Ôdwarf-2Õ, with the butterhead cultivar, ÔSa erÕ, the dwarf phenotype conditioned by the dwf2 locus was mapped using bulked segregation analysis to within 38 cM of the Adh3 locus. Using the second segregating population generated by crossing two experimental lines, 87-25-1M´87-109M, nine traits [white seed (w), brown seed (br), salmon¯ower colour (sa), pale yellow¯ower colour (pa), virescent juvenile leaf colour (vi), plump involucre (pl), yellow seed (y), one of two complementary genes for anthocyanin expression (C or G) and anthocyanin spotting (Rs)] were linked to RAPD loci, but only six of them could be placed on an existing genetic map of lettuce generated by analysis of cv. ÔCalmarÕ´cv. ÔKordaatÕ. A tenth trait, golden yellow (gy), remained unlinked. Approximately a third of the RAPD markers analysed segregated in both the 87-25-1M´87-1090M and ÔCalmarÕ´ÔKordaatÕ populations. In the genomic regions with multiple segregating loci in common, their relative orders and distances were mostly conserved. In one instance, linkage detected in the present study consolidated two separate groups on the earlier genetic map.Keywords: bulked segregant analysis, gene mapping, lettuce, morphological traits, RAPD. IntroductionLettuce (Lactuca sativa L.) is a rosette plant which is harvested for its leaves. Genetic studies in this species have identi®ed many morphological genes controlling leaf, ower and seed characteristics, as well as numerous genes for disease resistance (Robinson et al., 1983, Waycott & Taiz, 1991, Michelmore et al., 1994, Ryder, 1996. Several of these characters have been shown to be linked (Ryder, 1975(Ryder, , 1983(Ryder, , 1989(Ryder, , 1992 Kesseli et al., 1994). However, few morphological genes have been placed on the genetic maps developed using molecular markers.The majority of recent mapping studies on lettuce have focused on populations segregating for disease resistance (Landry et al., 1987, Paran & Michelmore, 1993, Kesseli et al., 1994Michelmore et al., 1994). These studies have produced genetic maps containing isozyme, RFLP and RAPD markers as well as four major clusters of disease resistance genes. The core map was generated from an intraspeci®c cross between cultivars ÔCalmarÕ and ÔKordaatÕ that is currently comprised of over 500 markers spanning more than 1200 cM, distributed in 13 major and four minor linkage groups (Kesseli et al., 1994 and unpubl. data). Additional populations have been analysed using bulked segregant analysis (BSA) (Michelmore et al., 1991) to map individual genes and develop linked markers (Kesseli et al., 1993).In this paper, we report the mapping of 11 morphological genes of lettuce using two di erent mapping strategies and compare two intraspeci®c mapping populations for the similarity of...

Nine selected strawberry genotypes (Fragaria ×ananassa Duch.) from the University of California, Davis, strawberry improvement program were intercrossed and their seedling offspring evaluated for five production traits. Plants were evaluated in annual hill culture, with and without preplant soil fumigation using a mixture of 67 methyl bromide: 33 chloropicrin (wt/wt, 392 kg/ha). Plant mortality was <1% for seedlings grown in either soil environment, indicating that the main effects of fumigation treatment in this experiment were due to the consequences of sublethal soil organisms. Plants grown in nonfumigated soils measured from 74% to 77% of the diameter of those grown in fumigated soils and yielded 59% as much fruit. Significant cross × fumigation interactions were not detected for fruit yield, fruit size, and weighted fruit appearance. Moreover, genetic correlations for these three traits calculated by comparing seedling performance in fumigated and nonfumigated soil environments were at or near unity, suggesting that the same genes condition genetic variability for these traits in both soil environments. Together, these findings demonstrate that strawberry fruit yield and vigor are increased substantially by fumigation, even in the absence of an identifiable major pathogen problem. Further, there may be little promise for developing cultivars with genetic adaptation specific to the sublethal effects of nonfumigated soils, as selection in either soil fumigation environment is likely to affect the same sets of genes.