In this article we will discuss about the life cycle of ectocarpus, explained with the help of suitable diagrams.
Family Ectocarpaceae of Phaeophyta:
Plants are filiform and more or less branched, or sub-simple from a creeping, penetrating, or disciform base; generally uniseriate but occasionally some segments in the lower part with one or two longitudinal septa; reproductive organs lateral, replacing branchlets, or intercalary from transformed vegetative cells.
In general, zoospores are produced in unilocular sporangia, followed by a generation producing gametes in plurilocular gametangia. Both sporangia and gametangia are occasionally found on the same plant.
Genus Ectocarpus of Phaeophyta:
This is one of the best-known brown algae containing many species. It is a marine alga of world-wide distribution being abundant particularly along the Atlantic Coast but rather scarce along the Pacific Coast. The plant body is composed of an irregularly prostrate portion and small sparsely or profusely branched filaments which grow upright in tufts (Fig. 102A).
The ultimate branch-lets of the erect portion are generally attenuated to an acute point (Fig. 102D).
They may also taper into hairs. Plants are usually attached to other submerged plants, sometimes to stones or similar other objects. The individual cell uninucleate and contains plate-like or band-shaped chloroplast with or without pyrenoids. Growth of the prostrate system is apical, but that of erect threads shows considerable diversity. Apical growth is rather rare in the erect threads.
In most species the erect threads exhibit diffuse growth. In a limited number of species growth is trichothallic, during which intercalary meristems are found at the base of the hairs. The cells of the meristem cut off segments both above and below, the former add to the length of the terminal hair, while the latter give rise to branches.
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 (Fig. 102 D to F).
The protoplast of each cell is metamorphosed into a single pear-shaped 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 (Fig. 103 B to F).
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 pear-shaped 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).
This typical life cycle of Ectocarpus exhibits morpholigically identical filaments representing sporophyte and gametophyte—isomorphic alternation of generations.
The filaments can only be differentiated from each other through cytological studies and the nature of reproductive structure a particular filament bears. The filament produced by the germination of haploid zoospore bears pluriloculcar reproductive structure which produces gametes, is the gametophyte, and its cells are haploid.
While the one formed by the direct germination of the zygote bearing unilocular sporangia and plurilocular sporangia is the sporophyte having diploid cells.
Reduction division takes place during the production of zoospores in the unilocular sporaxlgia which behave as zoosporangia. This type of life cycle has a great resemblance with the life cycle of the green alga Ulva. Besides this, the nature and development of swarmer’s in Ectocarpus possibly throw light on the origin of sex in algae.
The gametes and zoospores are similar in structure, only difference being in their behaviour. It has been thoroughly investigated in Ectocarpus siliculosus that environmental conditions, probably temperature commonly inhibit the development of one of the two generations. During warmer part of the year haploid filaments producing gametangia and gametes are developed.
While during colder part diploid filaments bearing unilocular and plurilocular sporangia are produced.
Knight, Kylin and others working on Ectocarpus siliculosus got some interesting results. They found diploid plants bearing both unilocular and plurilocular reproductive structures having a reduction division in the former and none in the latter. They named the plurilocular reproductive structures as neutral sporangia. The neutral sporangia produce diploid swarmers which germinate to form sporophytes.
Whereas, the unilocular sporangia produce haploid zoospores. Zoospores from unilocular sporangia develop into gametophytes (Fig. 105).
