Cai et al. 2015). Hence, all other analysed species
are missing less clades of MIKC
C -group genes
than duckweeds.
9.5.4 Link Between the Simplified
Body Plan of Duckweeds
and Loss of Clades
of MIKC
C -Group Genes
Our analyses confirm some previous reports
about the absence of clades of MIKC
C -group
MADS-box genes in duckweed genomes. They
thus provide a solid starting point for future
investigations on the functional importance of
MADS-box gene loss in flowering plants in
general, and especially in duckweeds. Some of
our findings provide already quite some food for
thought. For example, the AGL12-like gene of
Arabidopsis thaliana (also known as XAANTAL1, XAL1) has been shown to be involved in
root development as a promoter of cell proliferation in the root apical meristem (García-Cruz
et al. 2016). Considering that duckweeds only
have simple or even no roots makes it tempting
to speculate that the loss of this gene clade is
linked to the loss of complex roots. Since XAL1
plays also a role in the transition to flowering
(Tapia-López et al. 2008), this involvement must
be dispensable in duckweeds, possibly due to
functional redundancy to other genes.
OsMADS32 has been studied in rice where it
has functions in flower development. In
OsMADS32 mutants, the lodicules (organs
homologous to petals) are transformed into
hull-like organs and the number of stamens is
reduced (Wang et al. 2015), revealing a function
similar to that of class B floral homeotic genes, to
which OsMADS32 is closely related (Gramzow
et al. 2014). OsMADS32-like genes may have
become dispensable during duckweed evolution
as duckweeds have strongly reduced flowers
without petals. Note, however, that also eudicots
have lost their OsMADS32-like genes (Fig. 9.1;
Gramzow et al. 2014), possibly because of
redundancy to class B floral homeotic genes
proper (specifying petal and stamen identity).
When AGL9-like (SEP3-like) genes are
mutated in rice, the corresponding plants flower
late and show homeotic changes of lodicules,
stamens, and carpels into palea/lemma-like
organs, as well as a loss of floral determinacy
(Cui et al. 2010). Hence, the loss of AGL9-like
genes may be correlated to the rare flowering of
duckweeds. The fact that duckweeds still are able
to produce fertile stamens and carpels may be
explained by functional redundancy. AGL9-like
genes form a superclade with AGL2- (SEP1-),
and AGL6-like genes which are still present in
duckweeds. For AGL2- and AGL6-like genes in
petunia, it has been shown that they function
redundantly in floral organ formation (Rijpkema
et al. 2009), and the SEP genes of Arabidopsis
thaliana are also largely redundant (Pelaz et al.
2000). SEP proteins may be able to substitute
each others in the transcription factor complexes
(‘floral quartets’) that specify organ identity in
the flower (Theißen et al. 2016).
Taking together, our study suggests that the
loss of specific MADS-box genes clades is correlated to the simplification of the duckweed
body plan. Demonstrating a causal link between
both observations in the one or other direction
will require detailed studies in the future. In
addition to investigations on gene loss it will also
be interesting, however, to figure out as to why
some clades, such as AGL17-like genes with a
major function in root development, have been
retained in duckweeds. Since some duckweed
species (Landoltia punctata, Lemna gibba, and
L. minor) can be transformed already, genome
editing tools such as CRISPR-Cas9 could possibly be used to determine the function of the
retained genes.
Acknowledgements We thank Hieu Cao for his kind
invitation to write this article, and his patience.
References
Albalat R, Cañestro C (2016) Evolution by gene loss. Nat
Rev Genet 17:379–391
Arthur W (2011) Evolution—a developmental approach.
Wiley-Blackwell
9 Stranger than Fiction: Loss of MADS-Box Genes …
99
are missing less clades of MIKC
C -group genes
than duckweeds.
9.5.4 Link Between the Simplified
Body Plan of Duckweeds
and Loss of Clades
of MIKC
C -Group Genes
Our analyses confirm some previous reports
about the absence of clades of MIKC
C -group
MADS-box genes in duckweed genomes. They
thus provide a solid starting point for future
investigations on the functional importance of
MADS-box gene loss in flowering plants in
general, and especially in duckweeds. Some of
our findings provide already quite some food for
thought. For example, the AGL12-like gene of
Arabidopsis thaliana (also known as XAANTAL1, XAL1) has been shown to be involved in
root development as a promoter of cell proliferation in the root apical meristem (García-Cruz
et al. 2016). Considering that duckweeds only
have simple or even no roots makes it tempting
to speculate that the loss of this gene clade is
linked to the loss of complex roots. Since XAL1
plays also a role in the transition to flowering
(Tapia-López et al. 2008), this involvement must
be dispensable in duckweeds, possibly due to
functional redundancy to other genes.
OsMADS32 has been studied in rice where it
has functions in flower development. In
OsMADS32 mutants, the lodicules (organs
homologous to petals) are transformed into
hull-like organs and the number of stamens is
reduced (Wang et al. 2015), revealing a function
similar to that of class B floral homeotic genes, to
which OsMADS32 is closely related (Gramzow
et al. 2014). OsMADS32-like genes may have
become dispensable during duckweed evolution
as duckweeds have strongly reduced flowers
without petals. Note, however, that also eudicots
have lost their OsMADS32-like genes (Fig. 9.1;
Gramzow et al. 2014), possibly because of
redundancy to class B floral homeotic genes
proper (specifying petal and stamen identity).
When AGL9-like (SEP3-like) genes are
mutated in rice, the corresponding plants flower
late and show homeotic changes of lodicules,
stamens, and carpels into palea/lemma-like
organs, as well as a loss of floral determinacy
(Cui et al. 2010). Hence, the loss of AGL9-like
genes may be correlated to the rare flowering of
duckweeds. The fact that duckweeds still are able
to produce fertile stamens and carpels may be
explained by functional redundancy. AGL9-like
genes form a superclade with AGL2- (SEP1-),
and AGL6-like genes which are still present in
duckweeds. For AGL2- and AGL6-like genes in
petunia, it has been shown that they function
redundantly in floral organ formation (Rijpkema
et al. 2009), and the SEP genes of Arabidopsis
thaliana are also largely redundant (Pelaz et al.
2000). SEP proteins may be able to substitute
each others in the transcription factor complexes
(‘floral quartets’) that specify organ identity in
the flower (Theißen et al. 2016).
Taking together, our study suggests that the
loss of specific MADS-box genes clades is correlated to the simplification of the duckweed
body plan. Demonstrating a causal link between
both observations in the one or other direction
will require detailed studies in the future. In
addition to investigations on gene loss it will also
be interesting, however, to figure out as to why
some clades, such as AGL17-like genes with a
major function in root development, have been
retained in duckweeds. Since some duckweed
species (Landoltia punctata, Lemna gibba, and
L. minor) can be transformed already, genome
editing tools such as CRISPR-Cas9 could possibly be used to determine the function of the
retained genes.
Acknowledgements We thank Hieu Cao for his kind
invitation to write this article, and his patience.
References
Albalat R, Cañestro C (2016) Evolution by gene loss. Nat
Rev Genet 17:379–391
Arthur W (2011) Evolution—a developmental approach.
Wiley-Blackwell
9 Stranger than Fiction: Loss of MADS-Box Genes …
99