Evolution and Phylogenetic Relationships in Chlorophyceae!
From the view point of their evolution and phylogenetic relationships three tendencies volvocine, tetrasporine and chlorococcine have been taken into consideration.
They are as follows:
Volvocine line:
The Volvocales possess a unique type of vegetative organization of the individual cells. The group is also specialized for the biflagellate mechanism of motility possessed by the species of the order.
There are great differences in the specialization of the cells within the colony and the complexity of the cell within the colony and the complexity of the colony formation.
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There are also great variations in the reproduction which ranges from isogamy to oogamy through Anisogamy. The similarities have been placed in a separate line of evolution which has been known as volvocine line.
In the order Conjugales, the protoplasts taking part in conjugation are quite similar, and this method of reproduction has been treated as unique among the different groups. The Charales also constitute a unique group on the basis of the complexity of antheridium (globule) and oogonium (nucule), which makes a definite classification of the group difficult.
In the order Charales, the vegetative organization is also unique, and because of the presence of all these characteristics sometimes the order is included in a separate class, the Charophyceae. But the cellulose walls, pigmentation and food reserves are clearly of chlorophycean type and therefore, they have been placed in the class Chlorophyceae.
Tetrasporine line:
In this line of evolution, the organisms probably began with a unicellular form much like Chlamydomonas, but motility was soon restricted to reproductive cells and vegetative body became an unbranched filament like Ulothrix. In the tetrasporine line of evolution, the members of Ulotrichales play an important role.
According to this tendency, the species of Tetraspora are supposed to be unicellular members of the Volvocales in which the cells are in an immobile cytoplasmic pseudocilia on the anterior surface. The cell also contains a single nucleus, a cup-shaped chloropolast containing single pyrenoid. The cellular contents of Tetraspora indicate a relationship to the Volvocales, and the lack of motility indicates that they have given rise to an unbranched filamentous genus such as Ulothrix.
The evolution of unbranched filamentous species was followed by the branching filaments. The branching of the filaments becomes more and more complex in the genera such as Draparnaldia and Chaetophora. In this line of evolution, the simple isogamy also terminates into oogamy.
The tetrasporine line also gives rise to another tendency where the filamentous condition changes into leaf-like thallus in the development of Ulva and Enteromorpha. The gametes of these genera are of primitive type but they show a type of alternation of generations. It is also thought that in this line of evolution a form similar to Coleochaete could have given rise to the liverworts.
Chlorococcine line:
This line of evolution has been named after Chlorococcum and Chlorococcales. In this line, the simple chlamydomonad flagellate type of organism has been thought to be ancestor. Here the line of evolution has proceeded in more than one direction. The motility of the vegetative parts was soon lost and Chlorella is an example of this type.
The species of Pediastrum and Hydrodictyon demonstrated the characteristic of being siphonaceous as they possess coenocytic cells and the formation of daughter colonies within the individual cells and the mother colonies. The second divergence of this tendency is shown in Codium where the coenocytic cells are found in branched forms.
If such a form could have developed a vascular cylinder in its central axis and the sporangia at the apex of its branches, it could have given rise to a plant quite similar to simplest known Tracheophyta. It is believed that the chlorococcine line of evolution is the most logical solution of the relationship of Chlorococcales with the Tracheophyta. They have been treated to be Chlorococcales the ancestors of simplest known Tracheophyta.