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Educational
Communication
Module on
Evolution of Gymnosperms
By
Dr. MANZOOR
AHMAD SHAH
Assistant Professor
Senior Assistant Professor
Department of Botany
University of Kashmir
SRINAGAR-190 006
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Text
Gymnosperms: a brief introduction
Gymnosperms, commonly known as naked-seeded plants, form
vast forests that have dominated the landscape for more than
200 million years. There are four divisions of gymnosperms;
Cycadophyta (commonly known as the cycads), Ginkgophyta
(represented by Ginkgo biloba), Gnetophyta (with some angiosperm
affinitiea), and Coniferophyta (the conifers). The largest division
(Coniferophyta) includes almost all large evergreen trees, such as
pines, firs, spruce, junipers, and redwoods. As against angiosperms
with approximately >250,000 species, there are only about 720
species of conifers, despite this they dominate vast forested
regions, especially in the northern Hemisphere. While Cycadophyta
and Gnetophyta are represented approximately by 289 and 68
species, respectively, and Ginkgophyta harbour a single genus
and only one species. Gymnosperms comprise the longest living
organisms (Pinus longaeva, 5000 yrs), largest single organisms
by volume (1540 m3), tallest organisms (115 m) and even the
toughest organisms. Despite the fact that 4 of the 5 divisions
of seed plants are gymnosperms (Cycads, Ginkgo, Gnetales,
and Conifers), yet compared to angiosperms (Anthophyta), little
is known about the patterns of diversification and evolution,
especially the genome evolution in gymnosperms. The bias for
gymnosperms is evident from the fact that all complete genome
sequencing projects have focused on angiosperms – mainly
cereals (rice, maize, sorghum) and others such as Arabidopsis,
Medicago etc.
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Gymnosperms: a concise sketch of characteristics and
evolutionary trends
Gymnosperms do not constitute an evolutionary line equivalent
to that of angiosperms, but represent a series of evolutionary
lines of seed-bearing plants lacking distinctive characteristics
of flowering plants. Notwithstanding that there are only around
720 species of living gymnosperms, in comparison to >2,50,000
species of angiosperms, the individual gymnosperm are often
dominant over wide areas. One of the fundamental reasons
of dominance of gymnosperms is the seed habit. Since their
first appearance in the Late Devonian, gymnosperms and other
seed plants have come to dominate almost every terrestrial
ecosystem, encompassing a greater range of habit and habitat
than any other group of Tracheophytes. The evolutionary success
of spermatophytes generally stems from their reproductive
system, involving seeds, which facilitated these plants to exploit
habitats not accessible to most of the lower vascular plants. The
seed habit and vegetative traits, such as the production of wood
by a secondary meristem (cambium), contributed decisively to
the evolutionary success of the gymnosperms and angiosperms.
It is important to note that the ancestor of gymnosperms
was heterosporous and the evolution of heterospory was a
significant development because it led to the evolution of seeds.
Moreover, seeds are better than spores because spores not
only have a short lifetime, but also are thinner walled and more
vulnerable to pathogens and damage than seeds. In contrast,
seed coat provides protection and gametophyte tissue provides
nourishment for developing embryo, which due to dormancy can
wait a long time to germinate when conditions are good.
The seed habit differs from free-sporing heterospory fundamentally
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in that it facilitated delivery of the male gametophyte directly to
the female gametophyte through pollination, thereby bypassing
the free release of the female gamete into the environment, and
thus the environmental limitations placed on the free-sporing
system by water-mediated fertilization. In addition to seed habit,
which is common in angiosperms and gymnosperms, in contrast to
angiosperms, gymnosperms have rare polyploidy, more conserved
chromosome number, long life span, low species number, low net
morphological change, wind-based pollination, mainly generalists
and less variation in C-value range.
Progymnosperms: precursors of gymnosperms
So far as the origin is concerned, gymnosperms, the first group of
plants to develop seeds, probably evolved from a heterosporous
group called the progymnosperms in the Devonian Period (408-360
mya). First gymnosperms lacked seeds, but had developed them
by the end of the Devonian. So far as the radiation of gymnosperms
is concerned, They coexisted with the bryophytes, ferns, and other
seedless vascular plants; but their adaptive radiation occurred
during the Carboniferous and early Permian (360-245 mya ago)
when the climate became warmer and drier. Gymnosperms were
the dominant plants during the Age of Dinosaurs (Mesozoic 24565 mya), hence this era is called also as Age of Gymnosperms.
Progymnosperms (phylum Progymnospermophyta), comprised
a group of plants that existed in the Late Paleozoic era with
intermediate characteristics between those of seedless vascular
trimerophytes and those of the seed plants. Although the
progymnosperms reproduced through freely dispersed spores,
they resembled living conifers in the production of wood (secondary
xylem). Amongst the woody Devonian plants, progymnosperms
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were quite unique in that they also produced secondary phloem.
It is believed that both the progymnosperms and Paleozoic
ferns probably evolved from the more ancient Trimerophytes,
from which they, however, differed primarily in possessing more
elaborate and more highly differentiated branch systems, and
correspondingly more complex vascular system.
The most significant evolutionary advance of progymnosperms
over both Trimerophytes and the ferns is the presence of bifacial
vascular cambium, which produced both secondary xylem and
secondary phloem. This type of vascular cambium, characteristic
of seed plants, evolved first in the progymnosperms. Notable
progymnosperms include Aneurophyton type and Archaeopteris
type.
Aneurophyton type cccurred in the Devonian period, approximately
362 to 380 million years ago, and are characterized by three
dimensional branching pattern. Moreover, this type possessed
a solid cylinder of vascular tissue, or protostele. In view of
the resemblance of branching system of Aneurophyton type
progymnosperms to early seed ferns, some paleobotanists consider
the later as their precursors. The best example of Aneurophyton
type progymnosperms is Triloboxylon ashlandicum.
Archaeopteris type appeared in the Devonian period, some 370
m years ago, and extended upto the Mississipian period, about
340 million yrs ago. Lateral branch system was flattened in one
plane and bore laminar structures considered to be leaves. An
evolutionarily advanced type of stele, Eustele (characterized by
arrangement of vascular tissues in discrete strands around pith)
apparently evolved in this group of progymnosperms. This strong
similarity links this group with the living seed plants. Unlike
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most progymnosperms which were homosporous, some species
of Archaeopteris were heterosporous, a further evolutionary
adavaced step. The best example of this type is Archaeopteris
macilenta.
Notwithstanding that enough evidence has accumulated over the
years for seed plants having been evolved from the progymnosperms
after the appearance of seed, still many problems remain to be
solved in fully understanding the early evolution of seed plants.
Extinct gymnosperms
Pteridospermophyta (seed ferns or pteridosperms) is a highly
diverse relatively unnatural group that range in age from
the Devonian to the Jurassic. In form they range from Late
Devonian slender branched plants with ovules, such as Elkinsia
and Archaeosperma, to tree like Carboniferous plants, such as
Medullosa. Cordaitales represent primitive conifer-like plants.
How exactly different groups of seed ferns are linked to living
gymnosperms is still not clearly known.
Cycadeoids (or Bennettitales) – consisted of plants with palm-like
leaves, somewhat resembling the living cycads. Bennettitales are
an enigmatic group of Mesozoic gymnosperms that disappeared
from the fossil record during the Cretaceous. Though the exact
phylogenetic position of Bennettitales is still uncertain, some
paleobotanists believe that they might have been the members of
the same evolutionary line as angiosperms. Though Bennetitales
lived at the same time as extinct cycads and both produced almost
similar leaves, they were, however, reproductively distinct from
cycads in several respects such as the presence of flower like
reproductive structures that were bisexual in some species.
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Living gymnosperms
Living (or modern) gymnosperms are gymnosperms with living
representatives. These gymnosperms have “naked” seeds that
are not enclosed in an ovary (as angiosperm seeds are) but their
ovules and seeds are exposed on the surface of sporophylls and
analogous structures. There are four phyla of gymnosperms with
living representatives:
Cycadophyta (Cycads)
Ginkophyta (Ginkgo)
Coniferophyta (Conifers)
Gnetophyta (Gnetophytes)
Cycads, with about 140 species in 11 genera and 2 families, are
palmlike plants with trunks and sluggish growth and characterized
by large palm-like leaves and large cones. Cycads appeared at
least 250 mys ago during Permian, and became so numerous in
Mesozoic era that this period is often called as the age of cycads
and dinosaurs. Cycads are distributed mainly in the Old World
tropics, extending away from the equator to warm tropics.
Ginkophyta is represented by just one species (Ginkgo bilobacommonly known as maidenhair tree), the only living member
of phylum Ginkophyta.This plant species is unknown in wild,
but known only in cultivation. Though G. biloba was previously
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widespread, the species is currently very rare. Moreover, G.
biloba has changed very little for more than 150 m years, hence
is commonly called as Living Fossil. Its living members share
features with other gymnosperm genera of Permian era (some
270 mys ago). Its seed coat is fleshy and t comprises a widely
planted street tree.
Conifers, with about 600 species in 7 families, comprise the
largest and most widespread phylum. They harbour important
gymnosperms including pine, spruce, fir, cedar, etc. and dominate
many plant communities throughout the world, especially wide
stretches of the North. The coniferophytes harbour complex seed
cones, needled leaves reduce water loss.
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Gnetophytes have about 90 species in 3 living genera (Gnetum,
Ephedra & Welwitschia) having angiosperm-like features. From
an evolutionary standpoint, they comprise an important link
and transition to angiosperms. Double fertilization, unique
characteristic of angiosperms, also occurs in Ephedra, suggesting
thereby common ancestory of gymnosperms and angiosperms.
The four living gymnosperm orders (Coniferales, Cycadales,
Ginkgoales, and Gnetales) are morphologically so highly divergent
from one another, that their evolutionary relationships have been
uncertain. There are at least three different views.
a. First, from morphological data, it is proposed that the
Coniferales and Ginkgoales are sister groups. Accordingly,
Page (1990) classified the extant gymnosperms into two
subdivisions: Coniferophytina, which include Coniferales and
Ginkgoales, and Cycadophytina, which include Cycadales and
Gnetales.
b. Second, some researchers are of different view point and have
drawn a different conclusion from the morphological data. They
regard the Coniferales and Ginkgoales as two separate groups
and consider Cycadales as the most archaic seed plants.
c. Third, on the basis of chloroplast and nuclear DNA sequences,
it is suggested that the conifers and cycads form a clade,
representing the earliest lineage among seed plants. In
addition to uncertainty about the interordinal relationships,
the interfamilial relationships within each gymnosperm order
are also controversial.
Are gymnosperms paraphyletic or monophyletic?
On the basis of morphological and fossil data, many authors
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have suggested that the gymnosperms are paraphyletic, arguing
that the ancestor of angiosperms was derived from a certain
gymnosperm lineage, presumably including Mesozoic Bennettitales,
Glossopteridales, or the Gnetales. The last view was affirmed by
cladistic analyses of morphological data, and parsimony analyses
of partial nuclear 18s and 26s rRNAs, &CL gene sequences, and
combined morphological and molecular analyses. However, in
contrast to the above viewpoint, phylogenetic analyses of 5s rRNA
sequences, partial fragments of the small subunits of the nuclear
and the plastid rRNAs, rbcL gene sequences, and chloroplast
intergenic spacer (cpZTS) sequences suggested that the extant
gymnosperms are monophyletic, implying that none of the extant
gymnosperm lineages is a direct ancestor of angiosperms. On this
basis, it has been suggested that the ancestral form of angiosperms
should be searched for among progymnosperms.
Key points learned
The key points learned in the present module include:
• Notwithstanding that there are only about 720 species of
living gymnosperms, in comparison to >250,000 species of
angiosperms, yet individual gymnosperm are often dominant
over wide areas.
• Gymnosperms were the dominant plants during the Age
of Dinosaurs and one of the fundamental reasons for their
dominance was the seed habit.
• Progymnosperms comprise the precursors of gymnosperms
• Pteridospermophyta (seed ferns or pteridosperms) Cordaitales
(primitive conifer-like plants) Cycadeoids (or Bennettitales)
represent the extinct gymnosperms.
• There are four phyla of gymnosperms with living representatives,
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viz. Cycadophyta, Ginkophyta, Coniferophyta, and
Gnetophyta. Of these four phy;a-conifers are the most
widespread, Ginkgo is the living fossil and Gnetophytes
have angiosperm affinities. The four living gymnosperm
orders (Coniferales, Cycadales, Ginkgoales, and Gnetales)
are morphologically highly divergent from one another, so
that their evolutionary relationships have been uncertain.
• On the basis of morphological and fossil data, many authors
have suggested that gymnosperms are paraphyletic, whilst
the molecular phylogenetic analyses suggested that the
extant gymnosperms are monophyletic.