Spirodela, the most primitive genus with higher
differentiated plant body over Landoltia, Lemna
and Wolffiella to Wolffia, the most derived genus
with the lowest degree of differentiation. The
question about the relationship between nuclear
DNA content (aka genome size) and the level of
body differentiation, that was known a C-value
paradox (Cavalier-Smith 1985) or later referred
to as the C-value enigma (Gregory 2005),
attracted Geber to determine cytophotometrically
the DNA content of five different duckweed
species (Geber 1989). He found a remarkable
small amount of DNA in Spirodela polyrhiza
(0.15 pg/1C) and a surprising increase of DNA
content (over Landoltia punctata (aka. Spirodela
punctata), Lemna minor and Wolffiella oblonga
to Wolffia arrhiza) with successive reduction of
morphological
structures
within
studied
duckweeds.
4.2.1 Genome Size Evolution
An extensive survey of duckweed genome sizes
with 115 accessions (clones) of 23 duckweed
species representing all five genera was performed by using flow cytometry confirmed the
continuous increase of DNA content in parallel
with a morphological reduction and body size
(Wang et al. 2011). Among duckweed species, a
nearly 13-fold range from 150 Mbp in Spirodela
polyrhiza to 1881 Mbp in Wolffia arrhiza was
observed, indicating duckweeds as an interesting
model for studying genome size evolution
(Fig. 3.1). Interestingly, the five genera show
different degrees of interspecific genome size
variation from very small in Spirodela (150–165
Mbp) and Landoltia (372–427 Mbp) to 1.6- or
two-fold variation in Wolffiella (623–973 Mbp)
or Lemna (323–760 Mbp) and up to 5.3-fold in
the genus Wolffia (357–1881 Mbp). In addition,
large intraspecific DNA content variances were
found among Lemna species (e.g. Lemna minor,
L. aequinoctialis, L. trisulca and L. japonica),
while other genera did not show such
intraspecific variation. Furthermore, there was no
significant overall correlation of genome size
values with environmental conditions determined
by latitude, longitude and altitude (Wang et al.
2011).
4.2.2 Chromosome Number
and Karyotype Evolution
There are only few studies of chromosome
numbers in duckweeds. The likely first chromosome count of duckweeds was documented in
1933 with a note “the notable feature of the
chromosomes of the whole group is their extremely small size” (Blackburn 1933). The smallest
chromosomes in Spirodela polyrhiza were measured only 0.1 Â 0.18 µm (cf. Geber 1989),
suggesting the smallest recorded chromosomes in
flowering plants at that time. The largest duckweed chromosomes are in Wolffia species that
have a mean length of about 1.7 µm (UrbanskaWorytkiewicz 1980). In general, duckweed
chromosomes were a very difficult material for
handling in cytogenetic studies and it has been
often the case that different chromosome counts
were reported for the same accessions (clones)
and species, due to different and likely not-yetoptimal preparation techniques.
The most intensive investigation on duckweed
chromosome numbers was accomplished in a
15-year work of Urbanska-Worytkiewicz (1980)
in which the author studied about 1500 accessions (clones) of total 30 duckweed species.
Surprisingly, reported chromosome counts range
from 2n = 20–126, but do not correlate well with
genome size variation (Urbanska-Worytkiewicz
1980; Wang et al. 2011; Cao et al. 2016). Using
the squash technique of mitotic dividing cells and
chromosome staining by lacto-propionic-orcein,
Urbanska-Worytkiewicz proposed the basic
chromosome number of duckweed as x = 10.
The most common karyotype within duckweed
species is a diploid number of 2n = 40. In
addition, the euploid numbers (2n = 20, 30, 50,
60, 70, 80) and the aneuploid numbers (2n = 36,
42) have been recorded. Furthermore, three
levels of cytological variations including intraindividual (e.g. aneusomaty, mixoploidy),
4 Cytogenetics, Epigenetics and Karyotype Evolution of Duckweeds
49
differentiated plant body over Landoltia, Lemna
and Wolffiella to Wolffia, the most derived genus
with the lowest degree of differentiation. The
question about the relationship between nuclear
DNA content (aka genome size) and the level of
body differentiation, that was known a C-value
paradox (Cavalier-Smith 1985) or later referred
to as the C-value enigma (Gregory 2005),
attracted Geber to determine cytophotometrically
the DNA content of five different duckweed
species (Geber 1989). He found a remarkable
small amount of DNA in Spirodela polyrhiza
(0.15 pg/1C) and a surprising increase of DNA
content (over Landoltia punctata (aka. Spirodela
punctata), Lemna minor and Wolffiella oblonga
to Wolffia arrhiza) with successive reduction of
morphological
structures
within
studied
duckweeds.
4.2.1 Genome Size Evolution
An extensive survey of duckweed genome sizes
with 115 accessions (clones) of 23 duckweed
species representing all five genera was performed by using flow cytometry confirmed the
continuous increase of DNA content in parallel
with a morphological reduction and body size
(Wang et al. 2011). Among duckweed species, a
nearly 13-fold range from 150 Mbp in Spirodela
polyrhiza to 1881 Mbp in Wolffia arrhiza was
observed, indicating duckweeds as an interesting
model for studying genome size evolution
(Fig. 3.1). Interestingly, the five genera show
different degrees of interspecific genome size
variation from very small in Spirodela (150–165
Mbp) and Landoltia (372–427 Mbp) to 1.6- or
two-fold variation in Wolffiella (623–973 Mbp)
or Lemna (323–760 Mbp) and up to 5.3-fold in
the genus Wolffia (357–1881 Mbp). In addition,
large intraspecific DNA content variances were
found among Lemna species (e.g. Lemna minor,
L. aequinoctialis, L. trisulca and L. japonica),
while other genera did not show such
intraspecific variation. Furthermore, there was no
significant overall correlation of genome size
values with environmental conditions determined
by latitude, longitude and altitude (Wang et al.
2011).
4.2.2 Chromosome Number
and Karyotype Evolution
There are only few studies of chromosome
numbers in duckweeds. The likely first chromosome count of duckweeds was documented in
1933 with a note “the notable feature of the
chromosomes of the whole group is their extremely small size” (Blackburn 1933). The smallest
chromosomes in Spirodela polyrhiza were measured only 0.1 Â 0.18 µm (cf. Geber 1989),
suggesting the smallest recorded chromosomes in
flowering plants at that time. The largest duckweed chromosomes are in Wolffia species that
have a mean length of about 1.7 µm (UrbanskaWorytkiewicz 1980). In general, duckweed
chromosomes were a very difficult material for
handling in cytogenetic studies and it has been
often the case that different chromosome counts
were reported for the same accessions (clones)
and species, due to different and likely not-yetoptimal preparation techniques.
The most intensive investigation on duckweed
chromosome numbers was accomplished in a
15-year work of Urbanska-Worytkiewicz (1980)
in which the author studied about 1500 accessions (clones) of total 30 duckweed species.
Surprisingly, reported chromosome counts range
from 2n = 20–126, but do not correlate well with
genome size variation (Urbanska-Worytkiewicz
1980; Wang et al. 2011; Cao et al. 2016). Using
the squash technique of mitotic dividing cells and
chromosome staining by lacto-propionic-orcein,
Urbanska-Worytkiewicz proposed the basic
chromosome number of duckweed as x = 10.
The most common karyotype within duckweed
species is a diploid number of 2n = 40. In
addition, the euploid numbers (2n = 20, 30, 50,
60, 70, 80) and the aneuploid numbers (2n = 36,
42) have been recorded. Furthermore, three
levels of cytological variations including intraindividual (e.g. aneusomaty, mixoploidy),
4 Cytogenetics, Epigenetics and Karyotype Evolution of Duckweeds
49