certain. Analysis of plastid data gave modest
support to a monophyletic Wolffia (Les et al.
2002), but the addition of nuclear data caused
W. brasiliensis to associate more strongly with
Wolffiella than with other Wolffia species (Tippery et al. 2015). Landolt (1986) included
W. brasiliensis in section Pigmentatae along with
W. borealis, but these two species do not associate strongly in phylogenetic analyses. Wolffia
australiana and W. microscopica, the latter
belonging to section Pseudorrhizae (Landolt
1986), likewise have uncertain relationships with
other Wolffia species (Les et al. 2002; Tippery
et al. 2015). The phylogenetic distinctness of
these four species is such that each might merit
its own monotypic section; however, we do not
advocate for any taxonomic revisions at this
time. We do retain the sectional classification
according to ICN guidelines, whereby named
sections must contain their respective type species. Thus, W. microscopica belongs to section
Pseudorrhizae. We here designate W. brasiliensis to be the type species of section Pigmentatae,
because Landolt (1986), when establishing the
section, mentioned that Hegelmaier (1868, 1895)
originally considered W. brasiliensis to be the
sole representative of this group. The remaining
Wolffia species, which belong to section Wolffia,
are comfortably monophyletic and distinct from
other lineages.
Some phylogenetic uncertainty inevitably
persists, even after analyzing over 5000 nucleotides of DNA sequence data (Tippery et al.
2015). Next-generation sequencing methods are
poised to drastically amplify the amount of
phylogenetically informative sequence data in
duckweeds (Appenroth et al. 2015). Researchers
have obtained whole-genome nucleotide
sequence data for Spirodela polyrhiza (Wang
et al. 2012, 2014) and two species of Lemna
(L. gibba and L. minor; lemna.org), as well as
plastid genome sequences for Wolffia australiana
and Wolffiella lingulata (Wang and Messing
2011). Such large-scale data may be able to
resolve persistent phylogenetic uncertainty
among duckweed lineages.
2.4 Morphological Evolution
The 37 duckweed species share a large number
of distinctive morphological characters, and they
clearly evolved from a single common ancestor.
Duckweeds are exceptional among flowering
plants for having their vegetative and reproductive organs severely reduced in size, and in some
cases lost entirely (e.g., the roots of Wolffia and
Wolffiella). A unique anatomical synapomorphy
for Lemnaceae among other Arales is the existence of pollen with an ulcerate aperture (Keating
2002; Cusimano et al. 2011). Prior to the advent
of molecular data, many authors considered
water lettuce (Pistia, Araceae) to be the closest
relative of duckweeds, because it shares a
pleustonic habit (i.e., with the entire plant floating on the water surface), reduced flower number, and similar leaf morphology. Pistia indeed is
a close relative of duckweeds, but no closer than
any other “true” Araceae (sensu Mayo et al.
1997; Cusimano et al. 2011), and the morphological similarity between the two kinds of plants
resulted from convergent evolution rather than
descent from a morphologically similar ancestor.
Instead of having a pleustonic growth form, the
ancestor that gave rise to duckweeds would have
looked more like a modern aroid, with an erect
habit supported by terrestrial roots, basal leaves,
and a multi-flowered inflorescence (i.e., features
that are widespread in Araceae, Orontiaceae, and
Tofieldiaceae).
On morphological grounds, duckweed genera
are split broadly between two groups: plants that
produce roots (Landoltia, Lemna, and Spirodela)
belong to subfamily Lemnoideae, and those
lacking roots (Wolffia and Wolffiella) belong to
subfamily Wolffioideae (Landolt 1986). Phylogenetic data support Landolt’s (1986) assertion
that rooted plants represent the ancestral condition, with rootless plants being more derived.
Moreover, the number of roots per frond follows
a reduction series as lineages branch off: Spirodela (8–21 roots) branches first, followed by
Landoltia (2–7 roots), then Lemna (one root),
then the two rootless genera. Wolffia species
26
N. P. Tippery and D. H. Les
support to a monophyletic Wolffia (Les et al.
2002), but the addition of nuclear data caused
W. brasiliensis to associate more strongly with
Wolffiella than with other Wolffia species (Tippery et al. 2015). Landolt (1986) included
W. brasiliensis in section Pigmentatae along with
W. borealis, but these two species do not associate strongly in phylogenetic analyses. Wolffia
australiana and W. microscopica, the latter
belonging to section Pseudorrhizae (Landolt
1986), likewise have uncertain relationships with
other Wolffia species (Les et al. 2002; Tippery
et al. 2015). The phylogenetic distinctness of
these four species is such that each might merit
its own monotypic section; however, we do not
advocate for any taxonomic revisions at this
time. We do retain the sectional classification
according to ICN guidelines, whereby named
sections must contain their respective type species. Thus, W. microscopica belongs to section
Pseudorrhizae. We here designate W. brasiliensis to be the type species of section Pigmentatae,
because Landolt (1986), when establishing the
section, mentioned that Hegelmaier (1868, 1895)
originally considered W. brasiliensis to be the
sole representative of this group. The remaining
Wolffia species, which belong to section Wolffia,
are comfortably monophyletic and distinct from
other lineages.
Some phylogenetic uncertainty inevitably
persists, even after analyzing over 5000 nucleotides of DNA sequence data (Tippery et al.
2015). Next-generation sequencing methods are
poised to drastically amplify the amount of
phylogenetically informative sequence data in
duckweeds (Appenroth et al. 2015). Researchers
have obtained whole-genome nucleotide
sequence data for Spirodela polyrhiza (Wang
et al. 2012, 2014) and two species of Lemna
(L. gibba and L. minor; lemna.org), as well as
plastid genome sequences for Wolffia australiana
and Wolffiella lingulata (Wang and Messing
2011). Such large-scale data may be able to
resolve persistent phylogenetic uncertainty
among duckweed lineages.
2.4 Morphological Evolution
The 37 duckweed species share a large number
of distinctive morphological characters, and they
clearly evolved from a single common ancestor.
Duckweeds are exceptional among flowering
plants for having their vegetative and reproductive organs severely reduced in size, and in some
cases lost entirely (e.g., the roots of Wolffia and
Wolffiella). A unique anatomical synapomorphy
for Lemnaceae among other Arales is the existence of pollen with an ulcerate aperture (Keating
2002; Cusimano et al. 2011). Prior to the advent
of molecular data, many authors considered
water lettuce (Pistia, Araceae) to be the closest
relative of duckweeds, because it shares a
pleustonic habit (i.e., with the entire plant floating on the water surface), reduced flower number, and similar leaf morphology. Pistia indeed is
a close relative of duckweeds, but no closer than
any other “true” Araceae (sensu Mayo et al.
1997; Cusimano et al. 2011), and the morphological similarity between the two kinds of plants
resulted from convergent evolution rather than
descent from a morphologically similar ancestor.
Instead of having a pleustonic growth form, the
ancestor that gave rise to duckweeds would have
looked more like a modern aroid, with an erect
habit supported by terrestrial roots, basal leaves,
and a multi-flowered inflorescence (i.e., features
that are widespread in Araceae, Orontiaceae, and
Tofieldiaceae).
On morphological grounds, duckweed genera
are split broadly between two groups: plants that
produce roots (Landoltia, Lemna, and Spirodela)
belong to subfamily Lemnoideae, and those
lacking roots (Wolffia and Wolffiella) belong to
subfamily Wolffioideae (Landolt 1986). Phylogenetic data support Landolt’s (1986) assertion
that rooted plants represent the ancestral condition, with rootless plants being more derived.
Moreover, the number of roots per frond follows
a reduction series as lineages branch off: Spirodela (8–21 roots) branches first, followed by
Landoltia (2–7 roots), then Lemna (one root),
then the two rootless genera. Wolffia species
26
N. P. Tippery and D. H. Les