Stramenopila

Diatom vegetative cells are diploid, and repeated mitotic divisions lead to a reduction in cell volume as daughter cells synthesize new frustules that fit within the inherited parent frustule. Once a 30% reduction in volume has been reached diatoms either produce a resting spore (or auxospore) to regain cell size or they reproduce sexually. Sexual reproduction begins by the formation of gametes after meiosis. In round, centric diatoms a single or multiple macrogamete forms within an oogonium. Motile microgametes are formed within the other mating-type diatom. These microgametes are released and fertilization of the oogonium leads to the formation of a zygote within an auxospore. The auxospore enlarges and secretes a new pair of full-size frustules. The diploid, vegetative life cycle  then continues.

      Figure:  The life cycle of Plasmodium spp.  Indicating vertebrate and invertebrate hosts.

The long, thin diatoms, termed pennate, do not form motile gametes, but after a meiotic division fuse somatically to form a zygote.

Chrysophyte life cycles are characterized by the formation of the statospore. Details of life cycles vary in this phylum, but in Dinobryon isogamous cells seem to function as gametes. Spore formation is termed intrinsic and it is independent of external conditions. Around 10% of the population will encyst as a zygotic spore in any one generation, allowing populations in optimal growth conditions to maintain genetic diversity and reduce intraspecific competition.

Oomycetes are filamentous, coenocytic organisms. Asexual reproduction is characterized by the production of flagellate, motile, diploid zoospores from terminal sporangia. Sexual reproduction occurs after meiosis and gamete production in terminal or sub- terminal antheridia and oogonia. A few species are terrestrial and produce nonmotile gametes from sporangia and oogonia. The life cycle of the plant pathogen Phytophthora infestans is typical of this terrestrial group.

Hyphochytrids reproduce by the   formation of multinucleate, unwalled protoplasts within an enlarged host cell. Cyst formation within the  host  cell occurs, and  these are released into  the  soil on  breakdown of the  plant root  where they  can  persist for years. The presence of a suitable host  plant root  breaks dormancy, and  the  cyst germinates by producing anteriorly biflagellate zoospores that swim  to the  host  root  and  actively  penetrate it.

                                       Figure :  Life cycle of the  diatoms.