plants that currently exist. However, they are not
simply miniature versions of regular angiosperms, but rather represent a highly modified
structural organization, involving the simplification, and loss of many anatomical features. The
peculiar morphology of duckweeds obviously
reflects an extreme adaptation to aquatic life as
free-floating plants (Bogner 2009).
Several lines of evidence, including molecular
data, suggest that duckweeds are an early offshoot of the family Araceae (arum family,
aroids), subfamily Lemnoideae. Traditionally,
however, duckweeds have been treated as a
separate family (Lemnaceae), a view that is still
quite fashionable (Bogner 2009; Michael et al.
2017; Hoang et al. 2018). In any case, duckweeds are relatively basal representatives of the
Alismatales, the most basal order of monocotyledonous plants except Acorales (Wang et al.
2011, 2014; Olsen et al. 2016).
There are currently 37 duckweed species in
five genera recognized, Spirodela, Landoltia,
Lemna, Wolfiella and Wolffia (Hoang et al.
2018). The first three genera constitute the tribe
Lemneae, the latter two the Wolfieae (Bogner
2009). Among duckweeds, Spirodela represents
the most ancestral and Wollfia the most derived
lineage (Wang et al. 2011). There is a successive
reduction of morphological structures and size in
parallel with evolutionary advancement within
Lemnaceae, ranging from the 1.5 cm long Spirodela polyrhiza to the less than 1 mm long
Wolffia globosa (Wang et al. 2011). Interestingly,
the DNA content continuously increases in
duckweeds parallel to the reduction in body size,
from a genome size of 150 million base pairs
(Mbp) in case of the ancestral Spirodela polyrhiza to 1881 Mbp in case of the highly derived
and miniaturized Wolffia arrhiza (Wang et al.
2011).
The duckweed plant body is organized as a
‘thalloid’ or ‘frond’ lacking a stem. Lemneae
have only simple roots; the Wolffieae are even
rootless; the usually very short stipe of the new
fronds of the Lemneae has been interpreted as a
stolon. The tribes Lemneae and Wolffieae show,
in their evolution, a clear line to the reduction of
their organs. Historically, three morphological
interpretations have been proposed for the frond
of duckweeds, (a) that it corresponds to a leaf,
(b) that it corresponds to a shoot of leaf-like
shape, (c) that the basal part of the frond represents a shoot, and the distal part a foliar organ
(phyllome) (reviewed by Bogner 2009). The
flower-like structures have previously been
interpreted as a single flower, or alternatively, as
an inflorescence in which all flowers are reduced
to either a single stamen or a single gynoecium
(reviewed by Bogner 2009). Recently, the
detailed analysis of both extant and fossil plants
revealed in some cases clear trends of reduction
and allowed the identification of homologies.
According to Bogner (2009), the pouch of the
Spirodela, Landoltia, and Lemna frond is
homologous to the petiole sheath of typical
Araceae. The frond´s distal part is homologous to
the veined leaf blade. The shoot is reduced to a
vegetation point that generates both new fronds
and inflorescences. The membranous envelope
around the inflorescence in the Lemneae was
interpreted as the spathe (lacking in Wolffieae);
the spadix is reduced to a single bisexual flower
in the Wolffieae and to one bisexual flower and
one male flower (with one stamen only) in the
Lemneae (Bogner 2009).
Nevertheless, duckweed evolution involved
not only miniaturization, simplification, and
reduction. Some duckweeds have even evolved
novel structures, such as a protrusion on the
ventral surface termed a ‘pseudoroot’ in case of
Wolffia microscopica (Sree et al. 2015).
Duckweeds flower only occasionally, with
remarkable exceptions such as the frequently
flowering but very rare Wolffia microscopica
(Sree et al. 2015). Duckweeds usually reproduce
by vegetative daughter fronds initiated from the
mother frond. They do so in a very efficient way,
with doubling time of the fastest growing species
under optimal growth conditions of less than
30 h, nearly twice as fast as other ‘fast-growing’
flowering plants and more than double that of
conventional crops (Wang et al. 2014).
Duckweeds develop flowers after rapid vegetative development on juvenile tissues. The
considerable modification of an Araceae body
plan during the origin of duckweeds thus
92
L. Gramzow and G. Theißen
simply miniature versions of regular angiosperms, but rather represent a highly modified
structural organization, involving the simplification, and loss of many anatomical features. The
peculiar morphology of duckweeds obviously
reflects an extreme adaptation to aquatic life as
free-floating plants (Bogner 2009).
Several lines of evidence, including molecular
data, suggest that duckweeds are an early offshoot of the family Araceae (arum family,
aroids), subfamily Lemnoideae. Traditionally,
however, duckweeds have been treated as a
separate family (Lemnaceae), a view that is still
quite fashionable (Bogner 2009; Michael et al.
2017; Hoang et al. 2018). In any case, duckweeds are relatively basal representatives of the
Alismatales, the most basal order of monocotyledonous plants except Acorales (Wang et al.
2011, 2014; Olsen et al. 2016).
There are currently 37 duckweed species in
five genera recognized, Spirodela, Landoltia,
Lemna, Wolfiella and Wolffia (Hoang et al.
2018). The first three genera constitute the tribe
Lemneae, the latter two the Wolfieae (Bogner
2009). Among duckweeds, Spirodela represents
the most ancestral and Wollfia the most derived
lineage (Wang et al. 2011). There is a successive
reduction of morphological structures and size in
parallel with evolutionary advancement within
Lemnaceae, ranging from the 1.5 cm long Spirodela polyrhiza to the less than 1 mm long
Wolffia globosa (Wang et al. 2011). Interestingly,
the DNA content continuously increases in
duckweeds parallel to the reduction in body size,
from a genome size of 150 million base pairs
(Mbp) in case of the ancestral Spirodela polyrhiza to 1881 Mbp in case of the highly derived
and miniaturized Wolffia arrhiza (Wang et al.
2011).
The duckweed plant body is organized as a
‘thalloid’ or ‘frond’ lacking a stem. Lemneae
have only simple roots; the Wolffieae are even
rootless; the usually very short stipe of the new
fronds of the Lemneae has been interpreted as a
stolon. The tribes Lemneae and Wolffieae show,
in their evolution, a clear line to the reduction of
their organs. Historically, three morphological
interpretations have been proposed for the frond
of duckweeds, (a) that it corresponds to a leaf,
(b) that it corresponds to a shoot of leaf-like
shape, (c) that the basal part of the frond represents a shoot, and the distal part a foliar organ
(phyllome) (reviewed by Bogner 2009). The
flower-like structures have previously been
interpreted as a single flower, or alternatively, as
an inflorescence in which all flowers are reduced
to either a single stamen or a single gynoecium
(reviewed by Bogner 2009). Recently, the
detailed analysis of both extant and fossil plants
revealed in some cases clear trends of reduction
and allowed the identification of homologies.
According to Bogner (2009), the pouch of the
Spirodela, Landoltia, and Lemna frond is
homologous to the petiole sheath of typical
Araceae. The frond´s distal part is homologous to
the veined leaf blade. The shoot is reduced to a
vegetation point that generates both new fronds
and inflorescences. The membranous envelope
around the inflorescence in the Lemneae was
interpreted as the spathe (lacking in Wolffieae);
the spadix is reduced to a single bisexual flower
in the Wolffieae and to one bisexual flower and
one male flower (with one stamen only) in the
Lemneae (Bogner 2009).
Nevertheless, duckweed evolution involved
not only miniaturization, simplification, and
reduction. Some duckweeds have even evolved
novel structures, such as a protrusion on the
ventral surface termed a ‘pseudoroot’ in case of
Wolffia microscopica (Sree et al. 2015).
Duckweeds flower only occasionally, with
remarkable exceptions such as the frequently
flowering but very rare Wolffia microscopica
(Sree et al. 2015). Duckweeds usually reproduce
by vegetative daughter fronds initiated from the
mother frond. They do so in a very efficient way,
with doubling time of the fastest growing species
under optimal growth conditions of less than
30 h, nearly twice as fast as other ‘fast-growing’
flowering plants and more than double that of
conventional crops (Wang et al. 2014).
Duckweeds develop flowers after rapid vegetative development on juvenile tissues. The
considerable modification of an Araceae body
plan during the origin of duckweeds thus
92
L. Gramzow and G. Theißen