Evolutionary relationships of liverworts with a special focus ... - Doria

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support to Schuster’s view (Garbary et al.
1993, Forrest and Crandall-Stotler 2004,
Crandall-Stotler et al. 2005, Heinrichs et al.
2005). According to Crandall-Stotler et al.
(2005), on the other hand, prostrate bilateral
leafy growth form may be ancestral. Thus,
only after establishing the evolutionary order
of the major bryophyte groups will we be
able to state with certainty the growth form
of the first liverwort.
The erect or thallous growth form is
probably related to the form and division of
the apical cell (Renzaglia et al. 2000).
Charales, the algae regarded as the sister
group of land plants (Karol et al. 2001), are
filamentous and branched. Their apical cell
produces daughter cells only at its lower
surface (Graham et al. 2000,
http://www.greenbac.org/tree.html). The
mosses, which in some analyses have
appeared as a sister group of liverworts
(Garbary et al. 1993, Mishler et al. 1994,
Malek et al. 1996, Nishiyama and Kato
1999, Renzaglia et al. 2000), have mostly a
tetrahedral apical cell which divides from
three cutting faces and produces an erect
gametophyte. Thalloid hornworts have a
four-sided apical cell, resembling in this
respect many thalloid liverworts. Changes in
apical cell and body plan may, in turn, be
regulated by auxin, possibly via MADS box
genes (Cooke et al. 2003). MADS box is a
domain of conserved sequences, which are
important in developmental processes.
The sporophytes of bryophytes are less
variable in their form than their
gametophytes and are always composed of a
foot penetrating the gametophyte tissue, a
capsule containing the spores, and often an
unbranched stalk between them. Although
all bryophyte groups have developed their
own special features, e.g. the liverwort
sporophyte being more ephemeral and
simpler in structure than the moss
sporophyte, sporophytes do not seem to
serve as a basis for deducting large-scale
evolutionary trends, unlike the
gametophytes. In addition, the features of
the sporophyte apical and other meristematic
cells are less well known than those of
gametophyte apical cells. Renzaglia et al.
(2000) postulated that the original bryophyte
sporophyte was a small mass of cells with a
fertile internal region, with all bryophyte
groups independently developing their own
special sporophyte features. However, no
extant liverwort group has a sporophyte
resembling this allegedly original
sporophyte form. Actually, in liverworts,
sporophytes with a massive foot, seta (the
stalk), and many-stratose capsule wall have
traditionally been considered primitive
(Schuster 1966, 1981).
Liverwort synapomorphies
Possibly the best-known liverwort
synapomorphy, lacking in both “green
algae” and the other land plants, is the
presence of oil-bodies. Oil-bodies are
peculiar membrane-bound cell organelles
that consist of ethereal terpenoid oils
suspended in a carbohydrate- or protein-rich
matrix (Crandall-Stotler and Stotler 2000).
They are derived from endoplasmic
reticulum in meristematic cells (Duckett and
Ligrone 1995). The function of oil-bodies is
not known, but they may protect the plant
from herbivores or ultraviolet radiation.
However, only about 90% of the liverworts
have oil-bodies. The oil-bodies may thus
have disappeared many times during the
course of evolution, and they are not present