obviously represents an extreme case of heterochrony, a change in the relative timing of
developmental events. Because the timing of
reproductive relative to vegetative development
is changed, so that a juvenile stage becomes
reproductively mature, this represents a clear
case of paedomorphosis. This kind of heterochrony comes in two different flavours, neoteny and progression (Arthur 2011). Duckweeds
have been considered as cases of neoteny, the
retention of juvenile traits into the adult form as a
result of retardation of somatic development
(Bogner 2009; Wang et al. 2014). However,
since duckweeds develop flowers on juvenile
tissue, they might be better considered as cases of
progenesis, the acceleration of developmental
processes such that the juvenile form becomes a
sexually mature adult.
The evolution of duckweeds involved dramatic deviations from regular Araceae in terms
of the developmental trajectory, growth habit,
and body plan. Did this dramatic developmental
change leave footprints in the duckweed genome? More specifically, were the evolutionary
changes of the duckweed body plan accompanied, and thus potentially causally linked, to a
loss of function of genes that control the development of structures that were subjected to
miniaturization, simplification or even loss? To
address these questions, we focus here on a gene
family that brought about numerous developmental control genes during plant evolution, the
MADS-box genes.
9.2 MADS About Development:
MTF Phylogeny
and the Ontogeny
of Angiosperms
Many aspects of angiosperm development are
controlled by MADS-box genes, encoding
MADS-domain transcription factors (MTFs) (for
a review, see Gramzow and Theißen 2010;
Smaczniak et al. 2012; Theißen et al. 2000,
2016). MTFs are characterized by a highly conserved DNA-binding domain, the MADS
domain, named after the four founding members
of this family: MINICHROMOSOME MAINTENANCE FACTOR1 (MCM1) from baker’s
yeast (Saccharomyces cerevisiae), AGAMOUS
(AG) from thale cress (Arabidopsis thaliana),
DEFICIENS (DEF) from snapdragon (Antirrhinum
majus)
and
SERUM
RESPONSE FACTOR (SRF) from human
(Homo sapiens). MTFs appear to be absent from
prokaryotes, but two types existed probably
already in the most recent common ancestor
(MRCA) of extant eukaryotes, termed Type I and
Type II MTFs (Gramzow et al. 2010). Plant
Type II MTFs acquired a characteristic domain
structure very likely in the stem group of extant
streptophytes (charophyte green algae and land
plants). These streptophyte Type II proteins
exhibit a domain structure in which the MADS
domain is followed by an Intervening, a Keratin-like and a C-terminal domain, and have
hence been coined MIKC-type proteins (Theißen
et al. 2000).
Type I MTFs in flowering plants are further
subdivided in three groups, Ma, Mb, and Mc
(Gramzow and Theißen 2010). The genes
encoding angiosperm Type I MTFs have higher
birth and death rates than Type II genes. In line
with this, many of them have only quite subtle
functions in female gametophyte, embryo and
seed development, and some might even be
pseudogenes (reviewed by Gramzow and
Theißen 2010). The Type II (MIKC-type) genes
of land plants, including angiosperms, are subdivided into MIKC
C -group and MIKC*-group
genes based on phylogeny reconstructions and
structural features (reviewed by Gramzow and
Theißen 2010). In euphyllophytes (ferns and
their allies, and seed plants), two clades of
MIKC*-type genes exist, termed S and P clade
(Gramzow et al. 2012). In angiosperms, MIKC*group genes appear to have a conserved role in
pollen development (Liu et al. 2013).
In contrast to the relatively limited interest
that Type I and MIKC*-group MTFs have found
so far, MIKC
C -group genes took plant biology
by storm, not least due to the spectacular
homeotic and heterochronic phenotypes of some
mutants. The most well-known MIKC
C -group
genes include those genes that confer floral organ
9 Stranger than Fiction: Loss of MADS-Box Genes …
93
developmental events. Because the timing of
reproductive relative to vegetative development
is changed, so that a juvenile stage becomes
reproductively mature, this represents a clear
case of paedomorphosis. This kind of heterochrony comes in two different flavours, neoteny and progression (Arthur 2011). Duckweeds
have been considered as cases of neoteny, the
retention of juvenile traits into the adult form as a
result of retardation of somatic development
(Bogner 2009; Wang et al. 2014). However,
since duckweeds develop flowers on juvenile
tissue, they might be better considered as cases of
progenesis, the acceleration of developmental
processes such that the juvenile form becomes a
sexually mature adult.
The evolution of duckweeds involved dramatic deviations from regular Araceae in terms
of the developmental trajectory, growth habit,
and body plan. Did this dramatic developmental
change leave footprints in the duckweed genome? More specifically, were the evolutionary
changes of the duckweed body plan accompanied, and thus potentially causally linked, to a
loss of function of genes that control the development of structures that were subjected to
miniaturization, simplification or even loss? To
address these questions, we focus here on a gene
family that brought about numerous developmental control genes during plant evolution, the
MADS-box genes.
9.2 MADS About Development:
MTF Phylogeny
and the Ontogeny
of Angiosperms
Many aspects of angiosperm development are
controlled by MADS-box genes, encoding
MADS-domain transcription factors (MTFs) (for
a review, see Gramzow and Theißen 2010;
Smaczniak et al. 2012; Theißen et al. 2000,
2016). MTFs are characterized by a highly conserved DNA-binding domain, the MADS
domain, named after the four founding members
of this family: MINICHROMOSOME MAINTENANCE FACTOR1 (MCM1) from baker’s
yeast (Saccharomyces cerevisiae), AGAMOUS
(AG) from thale cress (Arabidopsis thaliana),
DEFICIENS (DEF) from snapdragon (Antirrhinum
majus)
and
SERUM
RESPONSE FACTOR (SRF) from human
(Homo sapiens). MTFs appear to be absent from
prokaryotes, but two types existed probably
already in the most recent common ancestor
(MRCA) of extant eukaryotes, termed Type I and
Type II MTFs (Gramzow et al. 2010). Plant
Type II MTFs acquired a characteristic domain
structure very likely in the stem group of extant
streptophytes (charophyte green algae and land
plants). These streptophyte Type II proteins
exhibit a domain structure in which the MADS
domain is followed by an Intervening, a Keratin-like and a C-terminal domain, and have
hence been coined MIKC-type proteins (Theißen
et al. 2000).
Type I MTFs in flowering plants are further
subdivided in three groups, Ma, Mb, and Mc
(Gramzow and Theißen 2010). The genes
encoding angiosperm Type I MTFs have higher
birth and death rates than Type II genes. In line
with this, many of them have only quite subtle
functions in female gametophyte, embryo and
seed development, and some might even be
pseudogenes (reviewed by Gramzow and
Theißen 2010). The Type II (MIKC-type) genes
of land plants, including angiosperms, are subdivided into MIKC
C -group and MIKC*-group
genes based on phylogeny reconstructions and
structural features (reviewed by Gramzow and
Theißen 2010). In euphyllophytes (ferns and
their allies, and seed plants), two clades of
MIKC*-type genes exist, termed S and P clade
(Gramzow et al. 2012). In angiosperms, MIKC*group genes appear to have a conserved role in
pollen development (Liu et al. 2013).
In contrast to the relatively limited interest
that Type I and MIKC*-group MTFs have found
so far, MIKC
C -group genes took plant biology
by storm, not least due to the spectacular
homeotic and heterochronic phenotypes of some
mutants. The most well-known MIKC
C -group
genes include those genes that confer floral organ
9 Stranger than Fiction: Loss of MADS-Box Genes …
93