Charophytes as the ancestor of land plant
The emergence of land plants or also known as embryophytes
approximately 480 million years ago was one of the major events in
the history of life on earth. It led to major changes in the earths
environment as for example reducing the atmospheric CO2
concentration, which consequently lowered the earths surface
temperature, initiated the development of the entire terrestrial
ecosystem and set the stage for evolution of other terrestrial
organisms. There are several groups that are closely linked to the
origin and early evolution of land plants which are charophytes,
bryophytes and pteridophytes.
It is widely accepted that embryophytes evolved from green
algae, or more specifically, from a small but diverse group of
green algae known as the streptophyte algae (charophycean algae).
Streptophyte algae and embryophytes together constitute the
division Streptophyta, which likely split from the Chlorophyta (all
other green algae) about 725-1200 MY ago. Streptophyta and
Chlorophyta comprise the Viridiplantae, one of the three
evolutionary lineages derived from the single primary endosymbiosis
of a cyanobacterium and a eukaryotic host cell.
Charophytes are a group of predominantly freshwater green algae
from which land plants are believed to have arisen. They consist of
five orders: Charales, Chlorokybales, Coleochaetales,
Klebsormidiales and Zygnematales (including desmids). The first
evidence linking charophytes and land plants came in the mid-1970s
from ultrastructural studies of reproductive cells and cell
division. Since then, numerous lines of evidence have confirmed
this relationship. Phylogenetic analyses of nuclear small subunit
(nuSSU) rRNA sequences have demonstrated that charophytes and land
plants form a monophyletic group, and that this clade, termed
Streptophyta, is sister to a clade composed of all other green
algae.
However, there is one central question surrounding the
charophytes, that is which specific lineage is the sister of land
plants? Despite many years of phylogenetic studies, this question
has not been resolved and remains controversial.
To discuss this question, I have a review few journals that
propose evidences either supporting or declining the idea that land
plants are closely link with charophytes.The first journal that I
have study is by Richard M. Bateman in EARLY EVOLUTION OF LAND
PLANTS: Phylogeny, Physiology, and Ecology of the Primary,
Terrestrial Radiation
Karol K, McCourt R, Cimino M, Delwiche CF (2001) The closest
living relatives of land plants. Science 294: 23512353.It was
nearly a decade ago that Karol et al. concluded after a four-gene,
three genome analysis that, of the charophytes, the Charales
constitute the closest living relative to land plants. Another data
analysis supported the same notion and, for a time, this appeared
to be a settled matter.M. Turmel et al. in their structural
analyses also revealed that many of the features conserved in land
plant cpDNAs were inherited from their green algal ancestors. The
intron content data predicted that at least 15 of the 21 land plant
group II introns were gained early during the evolution of
streptophytes and that a single intron was acquired during the
transition from charophycean green algae to land plants.
The Closest Living Relatives of Land PlantsKenneth G. Karol,
Richard M. McCourt, Matthew T. Cimino,Charles F. Delwiche1This
analysis supports the hypothesis that the land plants are placed
phylogenetically within the Charophyta, identies the Charales
(stoneworts) as the closest living relatives of plants, and shows
the Coleochaetales as sister to this Charales/land plant
assemblage. The results also support the unicellular agellate
Mesostigma as the earliest branch of the charophyte lineage. These
ndings provide insight into the nature of the ancestor of plants,
and have broad implications for understanding the transition from
aquatic green algae to terrestrial plants.The chloroplast genome
sequence of Chara vulgaris sheds new light into the closest green
algal relatives of land plants.Our phylogenetic analyses of
concatenated nucleotide and amino acid data sets provide
overwhelming support for the hypothesis that the Charales rather
than the Coleochaetales diverged just before the emergence of the
land plants. Eight of the 23 proteins investigated (Atp6, Cox1,
MttB, Nad2, Nad4, Nad5, Nad6, and Rps4) individually support the
monophyly of Charales and land plants but cannot reject alternative
hypotheses. Of these proteins, the one encoded by the single
mitochondrial gene analyzed by Karol et al. (2001), Nad5, provides
the highest bootstrap support (93%) for the sister-group
relationships between the Charales and land plants.
The dominant view has been that Charales, are the sister
lineage, but an alternative hypothesis supports the Zygnematales
(often referred to as pond scum) as the sister lineage. From a
morphological standpoint, the relationship between charophytes and
land plants tells a good story: as the charophyte lineages diverge,
their body plans grow increasingly complex from unicellular
(Mesostigmatales) to sarcinoid packets (Chlorokybales) to
un-branched filaments (Klebsormidiales) to branched filaments
(Zygnematales), to parenchematous tissue (Coleochaetales) and
finally to the macrophytes (Charales). From there, the body plans
evolve into early land colonizers equipped with complex tissues
allowing life out of water. Similarly, sexual reproduction evolves
from isogamy in the ancestral lineages to oogamy into the more
derived charophyte lineages. But in spite of morphological support
for Charales as sister to land plants, other data conflict with
this interpretation. Plastid gene phylogenies provide support for
Zygnematales as sister to land plants. In addition, new data based
on nuclear genes support this alternative topology. Sabina Wodniok
et al in Origin of land plants: Do conjugating green algae hold the
key? The researchers use a large data set of comprise of
nuclear-encoded genes (129 proteins) from 40 green plant taxa
(Viridiplantae) including 21 embryophytes and six streptophyte
algae, representing all major streptophyte algal
lineages.Phylogenomic analyses of nuclear and chloroplast data
indicate that the Charales are most likely not the closest living
extant relatives of the embryophytes despite their morphological
complexity. Instead, the analyses favor either the Zygnematales or,
less likely, a clade consisting of the Zygnematales and
Coleochaetales as the sister group of embryophytes. It seems
plausible that the simpler morphology of extant Zygnematales
represents a secondary simplification, similar to the loss of
flagellate cells in this group, which may actually represent an
adaptation to ensure sexual reproduction in the absence of free
water. Alternatively, the morphological complexity of the Charales
and Coleochaetales might have evolved independently after the three
evolutionary lines (Coleochaetales,Charales, and Zygnematales)
diverged.An extended taxon sampling and/ or analyses of larger data
sets such as complete genomes/ transcriptomes will likely be
necessary to shed further light on the intangible sister group of
the embryophyte plants.
In line with this finding, another group of researchers come out
with a more solid evidence supporting Zygnematales as the closest
living relatives to land plants. Ruth E. Timmel and some other
researchers were conducting a research to address the uncertainty
of the sister lineages of land plants. They sought a comprehensive
genome scale analysis using a deep sampling of many genes drawn
from seven species distributed across all major charophyte
lineages: Charales, Coleochaetales, Zygnematales, Klebsormidiales,
and Chlorokybales.The taxon sampling used included a total of 14
taxa: eight charophytes, four land plants and two chlorophytes.
This study, which includes all charophyte lineages provides a
robust, well-supported result that land plant and Zygnematales are
sister lineages. Previous hypotheses of increasing morphological
complexity[20,21] are not congruent with the results of our study.
However,multiple gains and losses of multicellularity across all
green algaehave been well documented, as has the reduction of
characters inthe Zygnematales.Evidence1) Broad Phylogenomic
Sampling and the Sister Lineage of Land PlantsRuth E. Timme1,
Tsvetan R. Bachvaroff, Charles F. Delwiche (a research article that
support the previous research)The research paper provide a well
supported, 160-nuclear-gene phylogenomic analysis supporting the
Zygnematales as the closest living relative to land plants.
Bryophytes as the first land plantThere is little doubt that
land plants are monophyletic in origin, and that bryophytes
represent the first land plants. However, their exact phylogeny
remains unresolved, especially with regard to which group of
bryophytes (liverworts, mosses or hornworts) represents the
earliest form of land plants. Recent studies have also questioned
the monophyly of liverworts themselves.
ConclusionVirtually all data support paraphyly of bryophytes.
Genomic structural information and at least some sequence data
identify liverworts as the earliest land plants. Relationships
among mosses, hornworts and vascular plant are not resolved at
present. Monophyly of mosses and hornworts seem to be well
established, but not that of liverworts.Transition from algae to
bryophytes to pteridophytesHowever, additional phylogenetic
dilemmas are the evolution of bryophytes from algae and the
transition from these first land plants to the pteridophytes.
Land plants (embryophytes) are most closely related to the
Charophyceae, a small group of predominantly freshwater green
algae, within which either Coleochaetales (,15 living species;
Fig.1a) or Charales (,400 living species;Fig. 1b), or a group
containing both, is sister group to land plants.
Land-plant monophyly is supported by comparative morphology and
gene sequences (18S rRNA, mitochondrial DNA: cox III).
Relationships among the major basal living groups are uncertain but
the best supported hypothesis resolves liverworts (Fig. 1c) as
basal and either mosses (Fig. 1e) or hornworts (Fig. 1d) as the
living sister group to vascular plants (tracheophytes). Less
parsimonious hypotheses recognize bryophyte monophyly and either a
sister-group relationship with vascular plants or an origin from
within basal vascular plants
