Lemnaceae and Orontiaceae Are Phylogenetically and Morphologically Distinct from Araceae

Abstract

Duckweeds comprise a distinctive clade of pleustophytic monocots that traditionally has been classified as the family Lemnaceae. However, molecular evidence has called into question their phylogenetic independence, with some authors asserting instead that duckweeds should be reclassified as subfamily Lemnoideae of an expanded family Araceae. Although a close phylogenetic relationship of duckweeds with traditional Araceae has been supported by multiple studies, the taxonomic disposition of duckweeds must be evaluated more critically to promote nomenclatural stability and utility. Subsuming duckweeds as a morphologically incongruent lineage of Araceae effectively eliminates the family category of Lemnaceae that has been widely used for many years. Instead, we suggest that Araceae subfamily Orontioideae should be restored to family status as Orontiaceae, which thereby would enable the recognition of three morphologically and phylogenetically distinct lineages: Araceae, Lemnaceae, and Orontiaceae.

Keywords: Araceae; Lemnoideae; aquatic plants; duckweeds; molecular phylogenetics; taxonomy.

Figure 1

Species of Arales, divided among the family-level categories that are described herein: Orontiaceae (A–D), Lemnaceae (E,F), and Araceae (G–K). (A) Vegetative growth and inflorescence (inset) of Orontium aquaticum (photo credit: Wolfgang Pomper), (B) Symplocarpus foetidus (photo credit: Claire O’Neill), (C) Inflorescence of Gymnostachys anceps (photo credit: Leith Woodall), (D) Lysichiton americanus with inflorescence (photo credit: Ryan Kurtz), (E) Fronds of Spirodela polyrhiza surrounded by Wolffia globosa, (F) Light micrograph of Wolffia microscopica bearing the floral organs on the dorsal side (anther lobes seen on the top), (G) Anthurium andraeanum with inflorescence, (H) Monstera deliciosa with inflorescence (photo credit: Wolfgang Pomper), (I) Calla palustris with inflorescence, (J) Amorphophallus konjac with inflorescence (photo credit: Wolfgang Pomper), (K) Inflorescence of Pistia stratiotes (photo credit: Bo-Fu Sun).

Figure 2

Phylogeny of Arales, constructed using combined DNA sequence data from five plastid regions (matK, ndhF, rbcL, rps16, and trnL-F). The tree represents a maximum likelihood tree, constructed in IQ-TREE [18] using default settings. Taxa were trimmed to include only one representative species for most genera. Branch lengths were adjusted to be ultrametric using the chronopl function in the R package ape [19], and the x-axis was scaled to approximate node ages that were reported previously [20]. Ancestral biogeography was reconstructed using the ace function in the ape package, to estimate the likelihood of an ancestor occupying one or more of the biogeographical realms indicated by the colored regions in the inset map. The ancestral likelihood values are shown as pie charts at the nodes of the phylogeny.

Figure 3

Usage of ‘Lemnaceae’ and ‘Lemnoideae’ in publications over time. Important events that may have influenced usage include the initial plastid data suggesting that duckweeds belong to the Araceae clade in 1995 [3] and the APG recommendation to include duckweeds in Araceae s.l., in 1998 [10]. Data were obtained from the Dimensions website (https://app.dimensions.ai/; accessed on 20 August 2021) by searching for each keyword anywhere in an article.

Figure 4

Phylogeny of Arales, constructed using combined DNA sequence data from five plastid regions (matK, ndhF, rbcL, rps16, and trnL-F), showing more complete taxon sampling from Arales genera. The tree represents a maximum likelihood tree, constructed in IQ-TREE [18] using default settings.

Figure 5

Phylogeny of Arales, constructed using combined DNA sequence data from nuclear ribosomal genes and spacers (18S and 5.8S rRNA, and the ITS-1 and ITS-2 spacers). The tree represents a maximum likelihood tree, constructed in IQ-TREE [18] using default settings.

Figure 6

Phylogeny of Arales, constructed using combined DNA sequence data from four mitochondrial regions (atp1, matR, rps3, and nad5). The tree represents a maximum likelihood tree, constructed in IQ-TREE [18] using default settings.