Northern Hemisphere disjunctions in Lactuca (Cichorieae, Asteraceae): independent Eurasia to North America migrations and origins of a North American allopolyploid lineage. Lactuca Northern Hemisphere Disjunctions

The Lactuca lineage is one of nine lineages in the lettuce subtribe (Lactucinae, Cichoreae, Asteraceae) distributed in Europe, Africa, Asia and North America. Within the Lactuca lineage two clades show disjunct Eurasian-North American distributions. One disjunct clade consists of both diploids (x=8) and allotetraploids (x=17), the former restricted to Eurasia and the latter to North America and the Azores archipelago. In contrast, members of the second Eurasian-North American disjunct clade are all diploid (x=9), like the remainder of the Lactuca lineage (diploid, x=8 or 9). The aims of the present study were to investigate the migration pathways that led to the disjunct distributions of the two Eurasian-North American clades and to decipher the progenitors of the allopolyploid taxa. We conducted deep taxon sampling and multi-locus phylogenetic analyses using nuclear ribosomal DNA (ETS and ITS), a low-copy nuclear (A44) and five non-coding plastid markers. Divergence time estimations with BEAST and ancestral biogeographic estimations with BioGeoBEARS suggested that both lineages reached North America by the late Miocene. Cloning of the A44 region revealed two sequence copies within allopolyploid individuals that were resolved in divergent clades and this helped to identify potential progenitors. We provide competing hypotheses for the progenitor species and biogeographic pathways that gave rise to the allotetraploid lineage. Taxonomic conclusions of the present study include L. graminifolia var. mexicana being raised to species rank for which the name L. brachyrrhyncha is available and the alleged endemic L. jamaicensis actually representing L. indica introduced to Jamaica in historical times.