duckweeds as a separate family (Engler 1889). In
the nineteenth and twentieth centuries, several
monographs on the duckweeds were published
(Hegelmaier 1868, 1895; Daubs 1962; Landolt
1986; Landolt and Kandeler 1987), making
duckweeds as one of the earliest and most thoroughly investigated model of flowering plants at
that time. Until now, scholars still debate whether
the duckweeds should be recognized at the
familial (Lemnaceae) or subfamilial (Lemnoideae) level (Appenroth et al. 2015; Sree et al.
2016).
The present duckweeds comprise five
well-defined genera (Spirodela, Landoltia,
Lemna, Wolffia and Wolffiella) and altogether 37
species (Sree et al. 2016). Except for Wolffiella
that is restricted to America and Africa, species
of other duckweed genera are cosmopolitan in
distribution from tropical to moderate climates,
excluding polar regions (Landolt 1986). Of all
the continents, the central diversity of duckweeds
seems to be in America. Furthermore, Spirodela,
the genus representing the most primitive characteristics of today’s duckweeds, has its centre of
distribution in South America. Fossil collections
provide evidence that ancestors of the duckweeds
(with the genus Limnobiophyllum Krassilov)
were widely distributed in North America and
Eurasia for approximately 50 million years (from
the Late Cretaceous until the Miocene). To date,
phylogenetic and phylogenomic studies suggest
an early offshoot of duckweeds in the family
Araceae starting about 104 Ma ago in the Early
Cretaceous when the breakup of Pangea was in
its final stage (Nauheimer et al. 2012; Henriquez
et al. 2014). However, it remains unclear why
Limnobiophyllum died out during the Miocene.
The morphology of duckweeds is highly
abbreviated and has evolved by extreme neoteny.
The poorly differentiated leaf-like body (called
frond or thallus) of Spirodela, Landoltia and
Lemna species contains the distal part with the
venation. This structure is homologous to the
veined leaf blade, whereas the pouches (or cavities) at the basal sections of the fronds represent
the sheath of the petiole of Araceae plants. The
duckweed shoot is highly reduced to a vegetative
point (tiny dot) in the middle of the pouches
where new fronds, root(s) and also inflorescence
are produced. The body plans of Wolffiella and
Wolffia species are even more reduced, without
roots and veins in fronds. Generative and vegetative reproductions in these duckweeds are
spatially separated with a floral cavity on the
upper side of the frond and a budding pouch
where the new frond emerges. The shape of
Wolffia fronds is more or less globular to ellipsoid or ovoid boat-shaped, while Wolffiella
fronds are long and flat. Putting together, there is
a trend of reduction in body complexity, leading
from the most ancestors Spirodela, through
Landoltia, Lemna and Wolffiella to Wolffia (cf.
Landolt 1986). It is important to note that most of
the duckweeds propagate mainly or exclusively
via vegetative proliferation (Landolt and Kandeler 1987).
During evolution, the body size of duckweeds
decreased in parallel with a successive reduction
of morphological structures, resulting in the
smallest flowering plants in the genus Wolffia.
Frond dimensions (length/width/thickness) of W.
globose and W. angusta are 0.8/0.4/0.7 mm and
0.8/0.4/1.0 mm, respectively (Landolt 1998b).
Interestingly, the cell size of Wolffia is larger than
the more primitive duckweeds (e.g. Spirodela,
Lemna, Landolt and Kandeler 1987). In another
aspect, the cell number per individual plant or
frond in Wolffia species is much smaller than in
Spirodela (Landolt 1986), and eventually in most
angiosperms plants, providing an excellent
model system for studying genome and karyotype evolution in the relation to the body size,
cell size and nuclear DNA content (Cao et al.
2015, 2016).
4.2 Genome Size, Chromosome
Number and Karyotype
Evolution
A passive adaptation, which is already originated
at an early stage of monocotyledons evolution
more than 100 My ago, to the highly specialized
way of life in the water contributed to the low
degree of differentiation in duckweeds. It likely
comprises one or more reduction series from
48
X. H. Cao and G. T. H. Vu
the nineteenth and twentieth centuries, several
monographs on the duckweeds were published
(Hegelmaier 1868, 1895; Daubs 1962; Landolt
1986; Landolt and Kandeler 1987), making
duckweeds as one of the earliest and most thoroughly investigated model of flowering plants at
that time. Until now, scholars still debate whether
the duckweeds should be recognized at the
familial (Lemnaceae) or subfamilial (Lemnoideae) level (Appenroth et al. 2015; Sree et al.
2016).
The present duckweeds comprise five
well-defined genera (Spirodela, Landoltia,
Lemna, Wolffia and Wolffiella) and altogether 37
species (Sree et al. 2016). Except for Wolffiella
that is restricted to America and Africa, species
of other duckweed genera are cosmopolitan in
distribution from tropical to moderate climates,
excluding polar regions (Landolt 1986). Of all
the continents, the central diversity of duckweeds
seems to be in America. Furthermore, Spirodela,
the genus representing the most primitive characteristics of today’s duckweeds, has its centre of
distribution in South America. Fossil collections
provide evidence that ancestors of the duckweeds
(with the genus Limnobiophyllum Krassilov)
were widely distributed in North America and
Eurasia for approximately 50 million years (from
the Late Cretaceous until the Miocene). To date,
phylogenetic and phylogenomic studies suggest
an early offshoot of duckweeds in the family
Araceae starting about 104 Ma ago in the Early
Cretaceous when the breakup of Pangea was in
its final stage (Nauheimer et al. 2012; Henriquez
et al. 2014). However, it remains unclear why
Limnobiophyllum died out during the Miocene.
The morphology of duckweeds is highly
abbreviated and has evolved by extreme neoteny.
The poorly differentiated leaf-like body (called
frond or thallus) of Spirodela, Landoltia and
Lemna species contains the distal part with the
venation. This structure is homologous to the
veined leaf blade, whereas the pouches (or cavities) at the basal sections of the fronds represent
the sheath of the petiole of Araceae plants. The
duckweed shoot is highly reduced to a vegetative
point (tiny dot) in the middle of the pouches
where new fronds, root(s) and also inflorescence
are produced. The body plans of Wolffiella and
Wolffia species are even more reduced, without
roots and veins in fronds. Generative and vegetative reproductions in these duckweeds are
spatially separated with a floral cavity on the
upper side of the frond and a budding pouch
where the new frond emerges. The shape of
Wolffia fronds is more or less globular to ellipsoid or ovoid boat-shaped, while Wolffiella
fronds are long and flat. Putting together, there is
a trend of reduction in body complexity, leading
from the most ancestors Spirodela, through
Landoltia, Lemna and Wolffiella to Wolffia (cf.
Landolt 1986). It is important to note that most of
the duckweeds propagate mainly or exclusively
via vegetative proliferation (Landolt and Kandeler 1987).
During evolution, the body size of duckweeds
decreased in parallel with a successive reduction
of morphological structures, resulting in the
smallest flowering plants in the genus Wolffia.
Frond dimensions (length/width/thickness) of W.
globose and W. angusta are 0.8/0.4/0.7 mm and
0.8/0.4/1.0 mm, respectively (Landolt 1998b).
Interestingly, the cell size of Wolffia is larger than
the more primitive duckweeds (e.g. Spirodela,
Lemna, Landolt and Kandeler 1987). In another
aspect, the cell number per individual plant or
frond in Wolffia species is much smaller than in
Spirodela (Landolt 1986), and eventually in most
angiosperms plants, providing an excellent
model system for studying genome and karyotype evolution in the relation to the body size,
cell size and nuclear DNA content (Cao et al.
2015, 2016).
4.2 Genome Size, Chromosome
Number and Karyotype
Evolution
A passive adaptation, which is already originated
at an early stage of monocotyledons evolution
more than 100 My ago, to the highly specialized
way of life in the water contributed to the low
degree of differentiation in duckweeds. It likely
comprises one or more reduction series from
48
X. H. Cao and G. T. H. Vu