Useful notes on the modes of reproduction in Ectocarpus!
The sexual reproduction ranges from isogamy to Anisogamy.
Majority of the species are monoecious. The most thoroughly worked out species E. siliculosus is dioecious.
The plurilocular gametangia develop from the terminal cell of a lateral branchlet. It divides several times repeatedly producing a vertical row of 6 to 12 cells. This is followed by several other vertical and transverse divisions and ultimately hundreds of small cubical cells are produced arranged in 20 to 40 transverse tiers. As already stated, the gametes are produced on haploid plants which develop from the zoospores produced in the unilocular sporangia. The gametophytic plants resemble the sporophytic plants in shape but quite different in behaviour.
These gametophytes bear plurilocular gametangia. The plurilocular gametangia are quite similar to the plurilocular or neutral sporangia in their shape.
The protoplasts of the cubical cells of the plurilocular gametangia metamorphose into biflagellate gametes which liberate through an apical pore on the gametangium gradually.
In E. siliculosus, the isogamets take part in fusion. The gametes taking part in fusion come from two different plants. In E. Secundus, The anisogametes take part in fusion. Here the female gamete is bigger and inactive and the male is smaller and active. As a rule, the gametes are haploid. Each gamete is ovoid, biflagellate and uninucleate.
Fusion of gametes:
The two isogametes from two different plants take part in fusion in the case of isogamy. In the case of anisogamy the bigger inactive female gamete rests and gets surrounded by several actively motile male gametes. Soon after one of the male gametes fuses with the female gamete and the zygote is formed.
The zygote germinates into a new diploid plant which produces unilocular or neutral sporangia. All cytological investigations in E. siliculosus have revealed that diploid plants bearing unilocular sporangia have a reduction division while those bearing neutral (plurilocular) sporangia have no reduction division.
The diploid zoospores which have been produced from neutral (plurilocular) sporangia germinate into sporophytes bearing unilocular or neutral sporangia. According to Papen fuss (1935), the zoospores produced from unilocular sporangia germinate into gametophytes bearing gametes which may either unite in pairs to produce zygotes, or develop into new gametophytes by means of parthenogenesis. The zygote (2n) on germination produces a sporophyte (2/7).
According to Knight (1929), Schussnig and Kothbauer (1934), the zoospores from unilocular sporangia also unite like gametes, but nothing is known about the formation of zygotes.