Northern Hemisphere disjunctions in Lactuca (Cichorieae, Asteraceae): independent Eurasia to North America migrations and allopolyploidization

Katy E. Jones,Edward E. Schilling,Elisabete F. Dias,Norbert Kilian

doi:10.3372/wi.48.48206

Abstract

The Lactuca lineage is one of nine lineages in the lettuce subtribe (Cichorieae, 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 diploids (x = 8) and allotetraploids (x = 17), the former restricted to Eurasia and the latter to North America and the Azores. In contrast, members of the other 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 these two Eurasian-North American clades and the potential 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 marker (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, and we propose a North American origin for the Azorean endemic. Taxonomic conclusions include L. graminifolia var. mexicana being raised to specific rank with the name L. brachyrrhyncha and the alleged endemic L. jamaicensis in fact represents the SE Asian L. indica, introduced to Jamaica.Citation: Jones K. E., Schilling E. E., Dias E. F. & Kilian N. 2018: Northern Hemisphere disjunctions in Lactuca (Cichorieae, Asteraceae): independent Eurasia to North America migrations and allopolyploidization. – Willdenowia 48: 259–284. doi: https://doi.org/10.3372/wi.48.48206Version of record first published online on 24 August 2018 ahead of inclusion in August 2018 issue.

Highlights

The most striking floristic similarities among Northern Hemisphere plant disjunctions are observed between North America and E Asia (Gray 1859; Wen 1999; Milne & Abbott 2002; Liu & al. 2017)
Plant migration between North America and Eurasia has been facilitated by two major land bridges from the Middle Eocene until the Middle Miocene, and even until much more recently as fluctuating land masses would have persisted until the Last Glacial Maximum (LGM)
Kilian & al. (2017b) revealed that the currently recognized genus Lactuca is not monophyletic, neither in the wider nor narrower circumscription. We focus on this monophyletic “Lactuca lineage”, which, according to Kilian & al. (2017b), contains a number of taxa that had previously not been included in the genus Lactuca, whereas a number of taxa that had always been considered members of the genus were resolved outside of the Lactuca lineage

Summary

Introduction

The most striking floristic similarities among Northern Hemisphere plant disjunctions are observed between North America and E Asia (Gray 1859; Wen 1999; Milne & Abbott 2002; Liu & al. 2017). Disjunct distributions of closely related plant species between continents in the Northern Hemisphere may be explained by factors such as vicariance or long-distance dispersal (LDD; Wen 1999; Manos & Donoghue 2001; Donoghue & Smith 2004). 200 species, has a widespread distribution, occurring in Eurasia, Africa and North America All North American Lactuca species and Northern Hemisphere intercontinental disjunctions are resolved within the so called “Lactuca lineage”, which is one of nine major lineages within the lettuce subtribe and contains c. We focus on this monophyletic “Lactuca lineage”, which, according to Kilian & al. (2017b), contains a number of taxa that had previously not been included in the genus Lactuca, whereas a number of taxa that had always been considered members of the genus were resolved outside of the Lactuca lineage

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