A multi-locus time-calibrated phylogeny of the brown algae ...

672 T. Silberfeld et al. / Molecular Phylogenetics and Evolution 56 (2010) 659–674
4.4.2. Macromorphological features
Regarding macroscopic thallus growth (Fig. 4C), it appears that
growth by an intercalary meristem may have appeared in the
ancestor of the BACR, and that it is a symplesiomorphy of the Desmarestiales,
Sporochnales, Laminariales and Ectocarpales. The
ancestral states are reconstructed with higher uncertainty at nodes
connecting Ascoseirales, Ralfsiales, Tilopteridales, Nemodermatales
and Fucales.
As pointed out by Phillips et al. (2008a), the Desmarestiales and
the Sporochnales have long been considered to be closely related,
mainly because they share trichothallic growth by a bidirectional
intercalary meristem (Fritsch, 1945; Clayton, 1984). This traditional
opinion even led some taxonomists to merge these taxa in
a single order Desmarestiales (Russell and Fletcher, 1975) and
the close relationship of these two orders was even confirmed in
the early molecular work of Tan and Druehl (1996). However, in
our analyses the Desmarestiales and the Sporochnales do not form
a monophyletic lineage. As commented in Phillips et al. (2008a),
the intercalary meristem observed in the Desmarestiales and the
Sporochnales may also be homologous to the multifacial intercalary
meristems (stipo-frondal zone) characteristic of the Laminariales
and also observed in the Ascoseirales. Thus, with this
hypothesis, the terminal growth characterizing the BACR orders
Fucales, Nemodermatales, Ralfsiales and Scytothamnales probably
results from homoplasic evolution. The observation that young
seedlings of Fucus exhibit a hair resulting from an early trichothallic
meristem (Fritsch, 1945) is consistent with this hypothesis.
The haplostichous thallus architecture (i.e. fundamentally made
of filaments only growing by transversal cleavages) is likely to be
the ancestral condition of the BACR (Fig. 4D) and is also observed
in related classes such as Schizocladiophyceae (Kawai et al.,
2003) and Phaeothamniophyceae (Bailey et al., 1998). A polystichous
architecture (i.e. fundamentally made of filaments in which
longitudinal cleavages also occur), can be anticipated to be a synapomorphy
of the SSDO clade because early diverging brown algal
orders outside our taxon sampling exhibit a haplostichous construction:
for example the Discosporangiales (Kawai et al., 2007)
and Ishigeales (Cho et al., 2004), as well as the more distant
Schizocladiophyceae (Kawai et al., 2003) and Phaeothamniophyceae
(Bailey et al., 1998). Within the BACR, a parenchymatous organization
(i.e. produced by an initial cell or group of cells that
undergo cleavages on all faces) is likely to have appeared in a common
ancestor to Fucales, Nemodermatales, Tilopteridales, Ralfsiales,
Ascoseirales, Laminariales and Ectocarpales from a
haplostichous condition as displayed in Desmarestiales, Sporochnales
and Scytothamnales. Subsequently the parenchymatous
organization has probably reversed multiple times to a haplostichous
(Ectocarpales, Ralfsiales) or polystichous (Tilopteridales Cutleriaceae
and Tilopteridaceae) architecture. Describing the
pattern in more detail is precarious due to uncertainties in the
ancestral state reconstructions; e.g. it is impossible to state
whether the parenchymatous organization of Fucales was secondarily
gained, or acquired in an earlier intra-BACR ancestor.
4.4.3. Cytological features
Finally, plastid-related characters correspond much better with
the phylogenetic framework, clearly showing less homoplasy
(Fig. 4E and F). From an ancestor displaying the ancestral condition
with several discoid, pyrenoid-free plastids per cell, reductions to a
single plastid occurred in three lineages: the Ralfsiales, Scytothamnales
and the Scytosiphonaceae within the Ectocarpales. Only the
Ectocarpalean plastids have developed a single protruding pyrenoid
(Rousseau and Reviers, 1999). Pyrenoids are also reported in
members of the Scytothamnales–Bachelotia lineage. Moreover pyrenoids
have recently been reported in the plastids of Nemoderma
tingitanum (Nemodermatales) (Parente et al., 2000), although this
observation has not been confirmed with ultrastructural observation.
By all means they all have been acquired independently in
these lineages.
It is worth focusing on the interesting case of the Scytothamnales–Bachelotia
lineage. Bachelotia as well as the members of the
Scytothamnales exhibit a stellate configuration of their plastidomes.
The three genera of the Scytothamnales sensu stricto
(Scytothamnus Hooker et Harvey, Splachnidium Greville and
Stereocladon Hooker et Harvey) are characterized by a single lobate,
star-shaped chloroplast with a large central pyrenoid embedded in
the stroma (Peters and Clayton, 1998; Tanaka et al., 2007). Bachelotia
antillarum exhibits several elongated chloroplasts with apical
embedded pyrenoid, joined at their pyrenoids to form a stellate
arrangement (Magne, 1976; Asensi et al., 1977; Uwai et al.,
2005). This is of particular interest since the present study fully resolves
the position of Bachelotia antillarum, as sister to the Scytothamnales.
To date, the phylogenetic position of the species
Bachelotia antillarum has always remained unresolved and/or
poorly supported in all analyses previously performed (Rousseau
et al., 2001; Uwai et al., 2005; Phillips et al., 2008a). This resulted
in its uncertain ordinal assignment. It is beyond the scope of this
paper to examine further ultrastructural characters likely to help
building a consistent scenario of the evolution of the stellate plastidomes,
all the more the remaining brown algal taxa featuring a
stellate plastidome could not be included in our sampling: that
is, the genus Asterocladon Müller, Parodi et Peters, closely related
to Ectocarpales, and the genera Asteronema Delépine et Asensi
and Stereocladon Hooker et Harvey (Müller and Parodi, 1994; Müller
et al., 1998; Peters and Clayton, 1998; Uwai et al., 2005; Tanaka
et al., 2007). Nevertheless, we suggest that Bachelotia should be
merged within the Scytothamnales.
4.5. Concluding remarks
Our phylogenetic analyses of a 72 taxa by >10000 nt alignment
of brown algae has allowed us to characterize the evolutionary nature
of the ‘‘Brown Algal Crown Radiation” in more detail. First,
regarding the possible effect of data quality and quantity on the
resolution of the BACR, we showed that a substantial increase of
the alignment size resolved a good amount of the relationships
within the BACR. Nonetheless, several branches still did not obtain
satisfactory support. Second, our application of a relaxed molecular
clock showed that the BACR corresponds to an episode of accelerated
cladogenesis during the Lower Cretaceous, resulting in several
short, though not zero-length, internal branches. The ca 30 Ma period
of evolutionary time over which the BACR lineages diverged
from each other suggests that the BACR is a soft polytomy that
could not previously be resolved due to alignment size limitations.
This observation along with the fact that, through the history of
brown algal phylogenetic studies, increasing amounts of data have
contributed to provide more support in the BACR, suggests that a
complete resolution of the BACR is not inconceivable.
Acknowledgments
We acknowledge Tristan Le Goff, Alan Millar, Kathy Miller, Richard
Moe, Claude Payri, Akira Peters, Nathalie Simon and Nick Yee
for their kind supply of silica-gel dried brown algal samples. We
thank the Computational biology service unit of the Cornell University
(http://cbsu.tc.cornell.edu/) for making available online
their MrBayes and BEAST cluster facilities. Thomas Silberfeld is
most grateful to Michaël Manuel (UPMC) for enlightening scientific
discussions, Marie-Lilith Patou and Julien Lorion for their assistance
with the molecular dating methods, Anna Plassart and Lucie
Bittner for their undivided support. Heroen Verbruggen thanks
Stefano Draisma and Steve LoDuca for sharing their insights and