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.
Four monogenic, recessive dwarf mutants of lettuce (Lactuca sativa L.), previously isolated from a population induced by ethyl methanesulfonate, were compared with the normal genotype (E-1) for plant height, weight, leaf area, as well as hypocotyl length and root length. These nonallelic dwarfs (dwf1, dwf2, and dwf3) exhibited reduced hypocotyl length, smaller, dark green leaves, and reduced stem length. Another mutant, dwf2', allelic with dwf2, exhibited an intermediate phenotype. Epidermal cells on hypocotyls and mature leaves were counted for both normal E-1 and dwf2 plants. The total number of epidermal cells per unit area for hypocotyls and for leaves from these plants was very similar, implying the dwarf's smaller size was due to an inhibition of cell expansion and not due to decreased cell divisions. Both dwarf and normal hypocotyls elongated normally in response to exogenous gibberellin A3 (GA3). In the rosette stage, only E-1 and dwf2' responded similarly to lower concentrations of GA3, while the other dwarfs required higher concentrations to respond. Hypocotyls of dwf2 and E-1 elongated equally with applied entkaurenol, ent-kaurenoic acid, GA53-aldehyde, GA53, GA19, GA20, and GA1 indicating that the biochemical block in dwf2 occurs at a very early step in the GA-biosynthetic pathway.
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...
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