Disentangling Reticulate Evolution of North Temperate Haplostemonous Ludwigia (Onagraceae)

Abstract

While it is known that whole genome duplication (WGD) and reticulate evolution play important roles in plant evolution, the origins and evolutionary histories of most polyploid and reticulate groups are still poorly known. The North Temperate haplostemonous (NTH) Ludwigia L. (sections Isnardia (L.) W. L. Wagner & Hoch, Ludwigia, Microcarpium Munz, and Miquelia P. H. Raven) group, characterized by having 4-merous and haplostemonous flowers, pluriseriate and free seeds, glabrous and convex nectaries, and a north-temperate distribution, is a polyploid complex (2×, 4×, 6×, and 8×) of 24 species with frequent reports of inter- and intrasectional hybridization. Although earlier biosystematics studies postulated some evolutionary scenarios and recent molecular phylogenetic studies have partially tested these propositions, the full history of their reticulate evolution remains puzzling. In this study, we sequenced four chloroplast regions (rpL16, rpoB-trnC, trnL-trnF, and ycf6-psbM) and conducted extensive molecular cloning of the biparentally inherited single-copy nuclear PgiC gene (376 clones in total), sampling 23 of the 24 NTH Ludwigia species whose chromosome numbers and ploidy levels were confirmed. Both the chloroplast and PgiC trees include strongly supported sister clades of section Ludwigia (four diploid species) and the “Microcarpium complex” (composed of sections Isnardia, Microcarpium, and Miquelia), which together are sister to the rest of Ludwigia. In the PgiC tree, eight clades are identified within the Microcarpium complex, with four clades including no extant diploid species. Neither sections Isnardia nor Microcarpium are monophyletic, while the monospecific section Miquelia has a hybrid origin. By integrating our phylogenetic trees with previous cytological hypotheses, the reticulate evolution of NTH Ludwigia is disentangled and four to eight extinct diploid species are inferred. Ancestral area reconstruction supports a North American origin of L. ovalis whose current East Asian distribution reflects a relict of the Arcto-Tertiary Geoflora. Based on our results, we propose to synonymize sections Microcarpium and Miquelia under the expanded section Isnardia.

Blumea chishangensis sp. nov. (Asteraceae: Inuleae) from Taiwan and new insights into the phylogeny of Blumea

“…The conflicts between molecular and morphological data are not rare in plants (e.g., Bremer and Struwe 1992 ; Schuster et al 2018 ; Lu et al 2021 ). These conflicts might infer some convergent evolution and/or hybridization in the evolutionary history (e.g., Ran et al 2018 ; Soreng and Davis 2000 ; Liu et al 2020 ).…”

New insights into polyploid evolution and dynamic nature of Ludwigia section Isnardia (Onagraceae)

“…Isnardia currently contains 22 taxa, including five diploids, 11 tetraploids, four hexaploids, one octoploid, and one taxon with an unknown ploidy level (Table 1 ) (Raven 1963 ; Raven and Tai 1979 ; Peng 1988 , 1989 ; Peng et al 2005 ; Wagner et al 2007 ; Arya et al 2020 ; Liu et al 2020 ). Isnardia are wetland weeds characterized by their creeping or erect stem, 4-merous and haplostemonous flowers, terete or globose capsules, and pluriseriate and free seeds (Wagner et al 2007 ).…”

“…Ludwigia L. (Eyde 1977 , 1978 , 1981 ). Recent molecular phylogenies revealed that this complex is a monophyletic group with strong (Liu et al 2020 ) or weak (Liu et al 2017 ) support, and sect. Ludwigia is sister to the complex (Liu et al 2017 , 2020 ).…”

“…However, none of the three sects. in the complex exhibit monophyly (Hung et al 2009 ; Liu et al 2017 , 2020 ), therefore the three sects. in the complex were combined into the sect.…”

“…in the complex were combined into the sect. Isnardia (Liu et al 2020 ). Examining the combination with different data sets will provide valuable information on this taxonomic treatment.…”