Mapping morphological genes relative to molecular markers in lettuce (Lactuca sativa L.).

Two F2 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, 'dwarf-2', with the butterhead cultivar, 'Saffier', 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 x 87-109M, nine traits [white seed (w), brown seed (br), salmon flower colour (sa), pale yellow flower 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. 'Calmar' x cv. 'Kordaat'. A tenth trait, golden yellow (gy), remained unlinked. Approximately a third of the RAPD markers analysed segregated in both the 87-25-1M x 87-1090M and 'Calmar' x 'Kordaat' 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.