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Ectocarpus the Brown Algae Evolved Multicellularity Independently Of

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Ectocarpus the Brown Algae Evolved Multicellularity Independently Of Ectocarpus The brown algae evolved multicellularity independently of animals and higher plants, and they include several lineages that rival higher plants in their complexity (de Reviers, 2003). Brown algae develop from cells that are released into the surrounding seawater (de Reviers, 2002), and the early stages of development are therefore easily observable. The growth of the initial cell can be markedly different in different species, involving either unipolar or bipolar germination (emergence of either one or two germ tubes, respectively). Moreover, bipolar germination can result in different growth patterns. When germination produces a symmetric filament, this leads to the development of a prostrate, basal structure before the erect thallus is formed (termed mediate differentiation or heterotrichy, . By contrast, if an asymmetric structure is formed following germination, this leads to the immediate development of an erect thallus without the formation of a prostrate, basal structure (immediate differentiation) (Pedersen, 1984; Fletcher, 1987). The alga has an isomorphic, alternation of generations, the gametophyte and the spoiophvte being essentially alike in appearance. The gametophyte produces plurilo­cular gametangia. The plurilocular gametangium is an elongated structure which develops from the terminal cell of a lateral branchlet . The proto­plast of each cell is metamorphosed into a single pear­shaped swarmer with two later­ally inserted unequal flagella having the longer one directed forward and the shorter in the backward direction. Liberation of the swarmer is through a pore on the cell wall (Fig. 103A). The swarmer’s are the same size and morphologically similar behav­ing as gametes. But some are less active and become passive after a short time. The more active ones cluster around the relatively passive one and fix themselves to it by their forwardly directed flagella. But fusion takes place between the passive one and the one of the more active gametes to form a zygote. This method of fusion and clus­tering of the active gametes around the passive one is known as the clump forma­tion The zygote so formed germinates­ directly producing a diploid plant—the sporophyte which resembles in every respect, the gametophyte that has produced plurilocular gametangia, only difference being the diploid plant bears plurilocular sporangia also known as neutral sporangia (Fig. 104), and more or less oval unilocular sporangia (Fig. 102B to C) that are developed with an enlarge­ment of the terminal cells of short lateral branchlets. The single nucleus of the young unilocular sporangium divides and redivides producing 64 nuclei. The first division of the nucleus is reductional and the rest being equational. With the completion of the nuclear division there is a cleavage into uninucleate protoplasts. Each protoplast then metamorphoses into a pear­shaped zoospore. The zoospores (Fig. 103G) resemble gam­etes, but differ in their behaviour by producing haploid individuals, the gametophytes. Again the plurilocular sporangia borne on the sporophytic plant resemble morpho­logically the plurilocular gametangia, but they produce diploid zoospores and not gametes (Fig. 104). Each mature plurilocular sporangium consists of vertical rows of many small cubical cells or compartments. The protoplast of each compartment is metamorphosed into a single biflagellate zoospore. No reduction division takes place during zoospore formation, hence all the zoospores are diploid. These diploid zoos­pores and the haploid zoospores produced in the unilocular sporangia are morpholo­gically indistinguishable. The diploid zoospores germinate to give rise to diploid individuals which bear plurilocular and unilocular sporangia. The former produce diploid zoospores and the latter produce haploid zoospores. The haploid zoospores produce the haploid individuals—the gametophytes. Whereas, the diploid zoospores serve to multiply the diploid individuals—the sporophytes. This is how the life cycle is completed (Fig. 104). Life Cycle of Ectocarpus sp.
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