Mapping unexplored genomes: A genetic linkage map of the woody Sonchus alliance (Asteraceae: Sonchinae) in the Macaronesian Islands

As an initial step to mapping quantitative trait loci for species differences and adaptive radiation of insular endemics in Macaronesia, a genetic linkage map was constructed from an intergeneric backcross between Lactucosonchus webbii and Sonchus radicatus, core members of the tree lettuces in the Macaronesian Islands. A total of 152 amplified fragment length polymorphism markers were mapped into 10 major and 3 minor linkage groups for a total map length of 644 cM with an average distance of 4.53 cM for the 10 major groups. The genetic linkage map length is considerably less than the estimated, and this may reflect incomplete genomic coverage in the current study or reduced recombination, which is a common feature of maps for hybrids of divergent taxa. Segregation distortion occurred in 34% of the mapped markers, and they were located primarily in 4 linkage groups. Segregation distortion in the current BC₁ intergeneric population is slightly lower than average (40%) for BC1 interspecific populations. This level of segregation distortion implies that unlike what we normally assume no to few reproductive barriers, oceanic island plant taxa do exhibit some degree of postmating reproductive isolation.