Phylogeny of angiosperm: Origin, evolution and inter-relationships in dicots and monocots

Phylogeny of angiosperm: Origin, evolution and inter-relationships in dicots and monocots:-

Origin of Angiosperms:-

> A fundamental problem that must be dealt with is whether the angiosperms are monophyletic (i.e. a group consisting of all descendants derived from a single ancestor) or polyphyletic (i.e. a group that does not have a common ancestor).

> Due to inadequate fossil records, the question of phyla of the angiosperms still remains unsolved. However, angiosperms are a natural group and contain characters, which make them unique from all other vascular plants.

Monophyletic Origin:-

i. As a group, the angiosperms have typically been viewed as being monophyletic. However, no definite fossil evidences are available in favour of the monophyletic origin.

This view is based on the fact that present-day angiosperms show remarkable consistency in their characters, i.e. presence of sieve tubes in all, uniform staminal structure, characteristic endothedial layer of the anther wall, double fertilization, and formation of triploid endosperm, which are considered defining features of angiosperms and support the monophyletic grouping.

ii. The monophyletic origin of angiosperms is also supported by Hickey & Doyle on the basis of their studies of mono-sulcate pollen.

iii. Dahlgren believes that the ancestor of the present-day angiosperms was a gymnospermous member.

Polyphyletic Origin:-

i. Several phylogenists including Cronquist, Hughes, Games, Krassilov and Meeuse have argued that the angiosperms are polyphyletic i.e. dicots and monocots originated from different primitive stocks at different times, and attained their present status through parallel or convergent evolution.

ii. The theory of polyphylesis is also supported by the fossil records of variety in perianth and the nature of carpel in both dicots and monocots.

iii. The polyphyletic origin of angiosperms is further supported by the fact that primitive orders of both the monocots and dicots do not show any close relationship in their characters.

> Thus fossil records suggest that angiosperms, as a group, are monophyletic, and their families or groups of families are polyphyletic. However, recently, phylogenetic analyses using nuclear, mitochondrial, and plastid gene sequences have aided in clarifying relationships between the various angiosperm families.

> The monocots and eudicots are each supported as being monophyletic. The angiosperms as a whole were found to be monophyletic to the exclusion of the gymnosperms.

Evolution of Angiosperms:-

> Angiosperms (flowering plants) appeared about 130 million years ago and today dominate the plant world, with approximately 235,000 species.

> In early Devonian-age rocks, approximately 363- 409 million years old, fossils of simple vascular and nonvascular plants can be seen. Ferns, lycopods, horsetails, and early gymnosperms became prominent during the Carboniferous period (approximately 290-363 million years ago).

> The gymnosperms were the dominant flora during the Age of Dinosaurs, the Mesozoic era (65-245million years ago). More than 130 million years ago, from the Jurassic period to early in the Cretaceous period, the first flowering plants, or angiosperms (phylum Anthophyta), arose. Over the following 40 million years, angiosperms became the world’s dominant plants.

> The angiosperms show high species diversity, and they occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. Angiosperms range in size from eucalyptus trees well over 100 meters (328 feet) tall with trunks nearly 20 meters (66 feet) in circumference to duckweed, simple floating plants barely 1 millimeter (0.003 inch) long.

Special Characteristics:-

- Some of the defining characteristics of angiosperms involve their physical appearance or morphology and internal anatomy: the presence of flowers and fruits containing seeds, stamens with two pairs of pollen sacs, a microgametophyte (the male, haploid stage of the life cycle contained in the pollen) with three nuclei, a megagametophyte (the female, haploid stage of the life cycle enclosed in the ovary) with eight nuclei, companion cells, and sieve tubes in the phloem (vascular tissue important in the transport of organic molecules).

- Some of these characteristics involve life-cycle features, such as double fertilization, that are distinct from almost all other members of the plant kingdom. (Double fertilization is also known in the genera Ephedra and Gnetum, members of the gymnosperms.)

- Because angiosperms possess so many unique features, plant taxonomists have long believed that angiosperms originated from a single common ancestor. Because the first flowers and pollen grains appear in fossils from the early Cretaceous period, up to about 130 million years ago, it is probable that angiosperms actually arose more than 130 million years ago.

- As the findings of paleobotanists (botanists who study plants in the fossil record) have been combined with more recent knowledge from evolutionary genetics and biochemistry, a clearer picture of angiosperm evolution has emerged.

Proposed Ancestors:-

- Because gymnosperms (the other large group of seed plants) have long been considered ancestral to the angiosperms, researchers have attempted to develop models for the evolution of the ovule-bearing structures of flowering plants from the similar, naked ovule-bearing structures of gymnosperms.

- Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the gnetophytes, a modern division of the gymnosperms which show up in the fossil record about 225 million years ago, are the seed plants most closely related to angiosperms.

- All three groups, the Bennettitales, the gnetophytes, and the angiosperms, share, or shared, superficially similar flower-like reproductive structures. The strobili, or cones, of some gnetophytes closely resemble flowers, and the xylem (vascular tissue specialized for transporting water) of some gnetophytes is similar to the xylem found in angiosperms.

Seed Ferns:- Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. The seed ferns, which predate the angiosperms by many millions of years, had seed-bearing cupules and specialized organs that produced pollen. Many plant taxonomists believe that the seed-bearing cupules in some groups of seed ferns could have evolved into the carpels of flowers.

Earliest Flowers:-

- Most paleobotanists assume that the first flowers were small, simple, and green in color and by modern standards were rather unattractive. Their petals and sepals were probably not clearly differentiated.

- In November of 1998, Ge Sun and David Dilcher and their colleagues published their discovery of the oldest angiosperm fossil to date, estimated to be at least 122 million years old and possibly as old as 145 million years. Either age qualifies it as the oldest.

- The fossils were discovered in China, and the fruits show the characteristic enclosed ovule (a carpel) that is distinctive to angiosperms. It was given the scientific name Archaefructus liaoningensis. Given its great age, this find implies that angiosperms may have arisen as early as the Jurassic period, more than 145 million years ago.

- Other early flowers produced pollen with a single aperture, or opening, a trait that the monocot branch of the angiosperms shares with cycads and ginkgos. Plant taxonomists believe that pollen with a single opening is an ancestral feature that some plants have kept as they evolved. The pollen of eudicots,with its three apertures, is thought to be a derived feature (that is, a later evolutionary development).

- Recent studies of angiosperm evolution, using data from deoxyribonucleic acid (DNA) sequences, have led to the proposal that an obscure shrub from the South Pacific island of New Caledonia, called Amborella trichopoda, represents what is left of the ancestral sister group (a related organism that branched off before the evolution of another group of organisms) to all the angiosperms.

- As a sister group to all the angiosperms, it is considered to be the most primitive (in an evolutionary sense) of the angiosperms and therefore should resemble what the ancestor to the angiosperms was like. It does possess some of the expected primitive traits for a primitive angiosperm, such as small, greenish-yellow flowers and a lack of vessels for conducting water from the ground to the leaves.

Inter-relationships in dicots and monocots:-

Flowering plants are divided into monocots (or monocotyledons) and dicots (or dicotyledons). This comparison examines the morphological differences in the leaves, stems, flowers and fruits of monocots and dicots.