1844), Spirodela (Schleiden 1839), Wolffiella
(Hegelmaier 1895), and finally Landoltia (Les
and Crawford 1999). Landolt (1986) contributed
the most comprehensive modern treatment of the
genus, recognizing 34 species in four genera (i.e.,
prior to the establishment of Landoltia). Subsequent to his monograph, Landolt named four
additional species (Lemna yungensis, Wolffia
cylindracea, Wolffia neglecta, Wolffiella caudata;
Landolt 1992a, b, 1994, 1998). Additionally, he
merged Lemna ecuadoriensis with L. obscura
(Landolt 2000), bringing the most recently
accepted number of species to 37 (Sree et al.
2016). Numerous synonyms exist for genera and
species of Lemnaceae, and these were outlined
clearly by Landolt (1986) and then later by Sree
et al. (2016). In this treatment, we provisionally
exclude Lemna landoltii Halder & Venu, a
recently described taxon with affinity to L. perpusilla but known to grow in only one pond in
India (Halder and Venu 2012) and currently
uncorroborated by molecular sequence data.
The majority of duckweed species are clearly
differentiated morphologically and resolve as
monophyletic. However, a series of investigations have used broader taxon sampling, along
with plastid gene sequences and amplified fragment length polymorphism (AFLP) data, to show
that a small number of species are incompletely
differentiated. These methods support the independence of Landoltia punctata, Spirodela
intermedia, and S. polyrhiza (Bog et al. 2015).
However, in the genus Lemna, L. valdiviana and
L. yungensis are intertwined, and L. gibba associates variously with L. japonica and L. turionifera, potentially as a result of interspecific
hybridization (Bog et al. 2010). Wolffia globosa
clusters with W. neglecta and W. borealis,
whereas plastid data place some W. elongata
accessions with W. columbiana (Bog et al. 2013).
Study of the genus Wolffiella has revealed
extensive genetic overlap among two species
groups, W. gladiata + W. lingulata + W.
oblonga and W. hyalina + W. repanda + W.
rotunda (Bog et al. 2018). Tippery et al. (2015)
also sampled multiple accessions for some of
these species and recovered consistent plastid
and nuclear-based topologies, but with varied
levels of internal support for specific
associations.
With the exception of Wolffia, all other
duckweed genera are clearly monophyletic and
well differentiated using molecular data (Les
et al. 2002; Borisjuk et al. 2015; Tippery et al.
2015). In Wolffia, the species W. australiana, W.
borealis, W. brasiliensis, and W. microscopica
resolve within a modestly supported Wolffia
clade on the plastid phylogeny (Les et al. 2002);
however, the nuclear ribosomal phylogeny renders these taxa as unresolved, showing uncertain
affinity for either Wolffia or Wolffiella (Tippery
et al. 2015). Although molecular data have raised
some concerns regarding the monophyly of
Wolffia, in light of the morphological distinctness
of this genus and the lack of a strongly supported
phylogenetic alternative, we continue to recommend the generic recognition of Wolffia without
taxonomic alteration from its current status.
Several taxonomic sections have been established within Lemnaceae genera. Some of these
are congruent with phylogenetic relationships,
whereas others have been reevaluated or discarded because of more recent evidence. For
maximum clarity, we will highlight only those
sections that are reasonably well supported by
phylogenetic evidence (Table 2.1). Within
Wolffiella, Hegelmaier (1868) segregated W.
hyalina and W. repanda into section Stipitatae,
and Landolt (1986) placed W. rotunda into section Rotundae, with the remaining species comprising section Wolffiella. Lemna section
Uninerves (Hegelmaier 1895) contains L. minuta, L. valdiviana, and L. yungensis, while L.
aequinoctialis and L. perpusilla belong to section
Alatae (Hegelmaier 1895). Lemna tenera is the
sole species in section Biformes (Landolt 1986),
and the remaining species occupy section Lemna.
In genus Wolffia, section Wolffia contains seven
species that are consistently monophyletic in
molecular data analyses, whereas the four poorly
resolved species have been assigned variously to
section Pseudorrhizae (W. microscopica; Landolt 1986), the paraphyletic section Pigmentatae
(W. borealis and W. brasiliensis; Landolt 1986),
or are unassigned (W. australiana; Les et al.
2002).
22
N. P. Tippery and D. H. Les
(Hegelmaier 1895), and finally Landoltia (Les
and Crawford 1999). Landolt (1986) contributed
the most comprehensive modern treatment of the
genus, recognizing 34 species in four genera (i.e.,
prior to the establishment of Landoltia). Subsequent to his monograph, Landolt named four
additional species (Lemna yungensis, Wolffia
cylindracea, Wolffia neglecta, Wolffiella caudata;
Landolt 1992a, b, 1994, 1998). Additionally, he
merged Lemna ecuadoriensis with L. obscura
(Landolt 2000), bringing the most recently
accepted number of species to 37 (Sree et al.
2016). Numerous synonyms exist for genera and
species of Lemnaceae, and these were outlined
clearly by Landolt (1986) and then later by Sree
et al. (2016). In this treatment, we provisionally
exclude Lemna landoltii Halder & Venu, a
recently described taxon with affinity to L. perpusilla but known to grow in only one pond in
India (Halder and Venu 2012) and currently
uncorroborated by molecular sequence data.
The majority of duckweed species are clearly
differentiated morphologically and resolve as
monophyletic. However, a series of investigations have used broader taxon sampling, along
with plastid gene sequences and amplified fragment length polymorphism (AFLP) data, to show
that a small number of species are incompletely
differentiated. These methods support the independence of Landoltia punctata, Spirodela
intermedia, and S. polyrhiza (Bog et al. 2015).
However, in the genus Lemna, L. valdiviana and
L. yungensis are intertwined, and L. gibba associates variously with L. japonica and L. turionifera, potentially as a result of interspecific
hybridization (Bog et al. 2010). Wolffia globosa
clusters with W. neglecta and W. borealis,
whereas plastid data place some W. elongata
accessions with W. columbiana (Bog et al. 2013).
Study of the genus Wolffiella has revealed
extensive genetic overlap among two species
groups, W. gladiata + W. lingulata + W.
oblonga and W. hyalina + W. repanda + W.
rotunda (Bog et al. 2018). Tippery et al. (2015)
also sampled multiple accessions for some of
these species and recovered consistent plastid
and nuclear-based topologies, but with varied
levels of internal support for specific
associations.
With the exception of Wolffia, all other
duckweed genera are clearly monophyletic and
well differentiated using molecular data (Les
et al. 2002; Borisjuk et al. 2015; Tippery et al.
2015). In Wolffia, the species W. australiana, W.
borealis, W. brasiliensis, and W. microscopica
resolve within a modestly supported Wolffia
clade on the plastid phylogeny (Les et al. 2002);
however, the nuclear ribosomal phylogeny renders these taxa as unresolved, showing uncertain
affinity for either Wolffia or Wolffiella (Tippery
et al. 2015). Although molecular data have raised
some concerns regarding the monophyly of
Wolffia, in light of the morphological distinctness
of this genus and the lack of a strongly supported
phylogenetic alternative, we continue to recommend the generic recognition of Wolffia without
taxonomic alteration from its current status.
Several taxonomic sections have been established within Lemnaceae genera. Some of these
are congruent with phylogenetic relationships,
whereas others have been reevaluated or discarded because of more recent evidence. For
maximum clarity, we will highlight only those
sections that are reasonably well supported by
phylogenetic evidence (Table 2.1). Within
Wolffiella, Hegelmaier (1868) segregated W.
hyalina and W. repanda into section Stipitatae,
and Landolt (1986) placed W. rotunda into section Rotundae, with the remaining species comprising section Wolffiella. Lemna section
Uninerves (Hegelmaier 1895) contains L. minuta, L. valdiviana, and L. yungensis, while L.
aequinoctialis and L. perpusilla belong to section
Alatae (Hegelmaier 1895). Lemna tenera is the
sole species in section Biformes (Landolt 1986),
and the remaining species occupy section Lemna.
In genus Wolffia, section Wolffia contains seven
species that are consistently monophyletic in
molecular data analyses, whereas the four poorly
resolved species have been assigned variously to
section Pseudorrhizae (W. microscopica; Landolt 1986), the paraphyletic section Pigmentatae
(W. borealis and W. brasiliensis; Landolt 1986),
or are unassigned (W. australiana; Les et al.
2002).
22
N. P. Tippery and D. H. Les