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Educational
Communication
Module on
Origin and Evolution of Sporophyte
in Bryophytes
By
Khursheed Ahmad
Department of Botany
Islamia College of Science and
Commerce Srinagar
Email: [email protected]
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Text
Origin of Sporophyte
There are two conflicting schools of thought concerning the
origin of sporophyte in bryophytes. One is called Antithetic
theory and the other is called as Homologous theory.
Homologous theory
The homologous theory also called as transformation or
modification theory was proposed initially by Pringsheim
and later elaborated by Fritsch. According to this theory, the
sporophyte in bryophytes originated by a direct modification
of the gametophyte and took the specific function of spore
production. The sporophyte thus originated was similar and
homologous to the gametophyte and both were free living and
photosynthetic. The evidence in favour of this theory is the
occurrence of similar but distinct photosynthetic sporophyte
and gametophytic generations in some alage such as Ulva,
Cladophora and Ectocarpus which are believed to be the
ancestors of bryophytes. Thus in the first land plants the
gametophytic and sporophytic generations were independent
and isopmorphic but gradually the sporophyte became
dependent, less complex and attached to the gametophyte.
The other strong evidence of homologous theory or direct
modification is the phenomenon of apogamy occurring in
present day bryophytes where sporophyte develops directly
from gametophyte.
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Antithetic theory
This theory was first proposed by Celkovasky in 1874 and
latter it was strongly supported by Strausberger (1894), Bower
(1908, 1929, 1935), Chamberlin (1935), Campbell(1940) and
Cavers (1970). According to this theory, the gametophytic
generation is the original one and the sporophyte was
added to the life cycle, intercalated between two successive
gametophytic generations. This theory suggested that the
sporophyte originated from the zygote by undergoing mitosis
and elaborate development. The algal ancestors of bryophytes
produce gametes which fuse to form the zygote which soon or
after a period of rest, divides meiotically to produce haploid
cells which give again rise to the gametophytes. Thus, the
sporophyte in them is simply represented by the unicellular
zygote. However, in bryophytes the first sporophyte appeared
when the gametophyte evolved archegonium and retained
the zygote. The retained zygote underwent mitotic divisions
instead of meiosis. This resulted in a mass of diploid cells or a
multicellular structure, the ancestral sporophyte. Successively,
the cells of this cellular body divided meiotically to produce
the haploid spores. This represented the simplest multicellular
sporophyte of bryophytes in which all the cells were fertile
besides being very much diffrent from the gametophyte. Later
the progressive sterilization of this ancestral fully fertile simple
sporophyte lead to the evolution of complex sporophyte in
bryophytes.
Evolution of Sporophyte
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There are two schools of thought which have contrasting theories
to explain the evolution of sporophyte in Bryophytes. The one
school of thought includes bryologists Cavers(1912), Bower and
Campbell(1935,1940) who are the proponents of the Theory of
progressive sterilazation. According to this theory, sporophyte
with maximum fertile tissue and with little amount of sterilization,
such as that of Riccia is the most simple and primitive type.
The complex and the most advanced sporophytes have evolved
from such simple sporophyte by progressive sterilization of the
potential sporogenous or fertile tissue. The second school of
thought believes in the theory of progressive simplification.
According to this theory, the simplest sporophyte of Riccia is
not the primitive type, but represents a reduced and highly
evolved sporophyte which has evolved through the process of
regressive evolution where the complex structure has been
lost through reductive evolution. This theory is supported by
bryologists like Kashyap (1919), Church (1919), Goebal and
Evans (1930, 1939).
Theory of Progressive Evolution
According to this theory, the most primitive and simple
sporophytes are found in the genus Riccia of Hepaticopsida
which evolved from an ancestral sporophyte which consisted
of a mass of sporogenous tissue with all cells being fertile. The
first step in the evolution of sporophyte involved the sterilization
of outer layer of sporogenous tissue which resulted in the
formation of a jacket layer around the sporogenous tissue.
Such a sporophyte consisted of a spherical capsule devoid of
foot and seta. This type of sporophyte is present in the genus
Riccia such as R. glauca. The evolution of sporophyte is seen
a step ahead in R. crystallina where all sporogenous cells are
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not fertile and spore producing. Some sporogenous cells fail to
form spores and thus are sterile. They are called as nurse cells
and serve as nutritive cells.
The sporophyte in Riccia develops from zygote which undergoes
division within the archegonium differentiating an outer
ampithecium and an inner endothecium. The amphitecium
gives rise to one layered outer wall of the capsule while as the
endothecium gives rise to archespoium which divides further
to produce sporogenous cells. All these sporogenous cells are
potentially fertile in R. glauca and give rise to spores. The first
step of sterilization is seen in R. crystalina where some of these
potential sporogenous cells fail to divide and form spores. They
act as nurse cells and perform a nutritive function.
The next stage in the evolution of bryophytic sporophyte
involved the formation of foot and seta. These structures came
at the expense of the enhanced sterilization of the potential
sporogenous tissue of sporophyte.
The occurrence of a foot is first seen in Corisnia where basal part
of the sporophyte is sterilized to form the foot. The sporphyte
thus consists of a small foot and a capsule. The capsule contains
a single layered wall which encloses the sporogenous cells which
produce the spores. There is also sterilization inside the capsule
with many potential sporogenous cells forming nurse cells.
A step further in evolution is the sporophyte of Sphaerocarpos
which is organised into a bulbous foot, a short seta and a capsule.
Sphaerocarpos is thus having additional sterilized tissue in the
form of seta which is two-celled thick.
Targonia sporophyte displays a further step of progressive
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sterilization of potentially sporogenous tissue. The sterile regions
of the sporophyte comprise of a broad foot, a well-developed
seta and a large number of elaters in the capsule. Half of the
potentially sporogenous tissue in the capsule is transformed into
sterile elaters with 2-3 spiral thickenings.
The next stage in the progressive sterilization of sporogenous
tissue and evolution of sporophyte is seen in the sporophyte
of Marchantia. Here the hypobasal region of the embryo forms
the foot and lower part of the seta, while as the epibasal part
forms the upper part of seta and the capsule. The capsule
is having a single layered wall derieved from ampithecium
which becomes multilayered at the apex of the capsule. This
multilayered apex splits halfway down into many segements
at maturity thus helping in dehiscence.The endothecium gives
rise to archesporium which forms the sporogenous tissue. Only
half of this sporogenous tissue produces spores while as the
rest remain sterile and transform into elongate elaters. Some
sporogenous cells at the top differentiate into sterile apical cap.
Thus the sporophyte of Marchantia is having a foot, seta, elaters,
several layered apex and apical cap as the sterile tissue. Here
the additional sterile tissue is exploited for capsule dehiscence
and spore dispersal.
The next stage in sporophyte evolution and structure complexity
is met in the members of order Jungermanniales. In Pellia and
Riccardia, the sporophyte differentiated into foot, seta and
capsule develops from the epibasal half of the zygote only. The
hypobasal half of the zygote does not play any role in sporophyte
development. The wall of the capsule is 2-8 layered thick, with
thickeneing bands and is derived from ampithecium. Here also the
entire endothecium produces archesporium which by repeated
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divisions gives rise to sporogenous tissue and elaters, further
sterilization of sporogenous tissue gives rise to the formation of
an elateriophore which occurs at base in Pellia and at apex in
Riccardia. Thus further sterilization or reduction of sporogenous
tissue results in new structure with more efficient mechanism
of spore dispersal.
Further sterilization of sporogenous tissue is found in the highly
specialized sporophyte of Anthoceros where more specilaized
structures develop at the cost of reduction in sporogenous tissue.
The sporophyte of Anthoceros consists of a long cylindrical
capsule and a foot with a meristematic zone at its base. The
capsule wall in Anthoceros is multilayered with stomata and
airchambers. The cells contain chloroplast and thus the capsule
wall is photosynthetic. The capsule wall develops from the outer
layers of ampithecium while as the inner layer of ampithecium
gives rise to the spore sac. The sporogenous sac contains spores
and pseudoelaters.The whole endothecium is tansformed into a
column of sterile tissue of elongated cells called columella. The
sporogenous tissue is restricted to a small portion of the capsule
which overarches and surrounds the central columella.
The ultimate stage of sporophyte evolution in bryophytes is
seen in the mosses such as polytrichum and Funaria, where
progressive sterilization of sporogenous tissue has reached its
maximum. In these sporophytes, the sterile tissue is specialized
into varied and complex structures which perform diverse
functions. Thus the highest degree of structural complexity in
these sporophytes is directly the outcome of maximum amount
of sterilization of sporogenous tissue. The sporophyte of Funaria
is differentiated into a sterile bulbous foot, sterile long seta and a
complex capsule. The sterile capsule wall, which is multilayered
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and photosynthetic, is derived from the ampithecium. The
endothecium gives rise to archesporium from its outer layer
only, while the rest of it develops into a massive central sterile
columella. The extremely reduced archesporium consists of a
single layer of barrel-shaped cells and is held between operculum
and apophysis. Thus in Funaria major portion of sporophyte is
sterile and the sterile structures include, foot, seta, capsule wall,
apophysis, peristome, operculum and colummela. This structural
complexity has evolved for an efficient spore dispersal mechanism.
The fertile zone in this type of sporophyte is restricted to a much
smaller portion.
Reduction theory or Theory of Progressive Simplification
This theory is supported by Kashyap, Church, Goebel, and Evans
who believe that the evolution of sporophyte in bryophyta has
occurred through reduction or progressive elimination of sterile
tissue. According to this theory the sporophytes of Funaria and
Polytrichum are primitive owing to huge amount of sterile tissue
and structural complexity. This theory holds that the sporophyte
of Riccia is the most advanced sporophyte which has evolved from
complex and elaborate sporophytes of mosses such as Funaria
by progressive elimination of sterile tissue, retrogression or
progressive simplication. According to this theory the progressive
simplification and evolution of sporophyte has occurred through
following stages
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1. The sporophytes of Bryopsida such as Funaria and Polytricum
are the most primitive
2. The peristome and peristome-facilitated spore dispersal has
been eliminated e.g. Sphagnum.
3. The photosynthetic tissue in the capsule wall has reduced
gradually and ultimately eliminated from the wall of liverworts,
such as in Marchantia, Riccia, etc.
4. The multilayered wall gradually changed into single layered
wall together with elimination of stomata and airspaces in wall
layers e.g. in Riccia and Marchantia.
5. Gradual disappearance of foot and seta resulted in a sporophyte
which remained completely embedded in the gametophyte
thallus, as in Riccia. This type of reduced sporophyte with little
sterile tissue and maximum fertile tissue is regarded as the
most advanced according to this theory of sporophyte evolution.