Diatoms like all of protists are able to undergo sexual and asexual reproduction; also, like all protists they predominately reproduction asexually. The mechanism they use to reproduce asexually is simple mitosis. But they have a slight problem. They are incased in a class shell. They way its put together though is very useful for reproduction though (see more in facts). The frustules fits together like a Petrfound at dish, a larger valve called the epitheca and the smaller valve called the hypotheca. When they divide each cell ends up taking one of the valves for its epitheca. They then proceed to secreting the hypotheca.
    There are several problems their form of asexual reproduction. Such as one of the cells is going to be continuously be getting smaller and smaller. That isn’t constructive for the cells. They find the solution to that though their method of sexual reproduction.
    Like many protest for sexual reproduction there has to be particular triggers that cause diatoms to take the sexual reproduction path way such as unfavorable conditions, predations, time of year, ect. In the case of diatoms there are several possible triggers. They could be the cell has gotten too small, one of the valves was damaged (often the larger older one), the conditions are unfavorable and they have to form a vegetation cyst, and sometimes it’s just random. Usd with permision from Hasle and Syvertsen; Found at
    Well to process by which diatoms go through sexual reproduction is rather complex. First of all one of the cells forms the male gametes, this if often a smaller cell, first the cell divides into several smaller diploid cells that are called spermatogonia and they remain within the frustule until it opens and then they undergo meiosis to form haploid sperm. Then for the female gametes there is three pathways in which it can go depending on species. The first is they form oogonia (protective mucus around diatom eggs ) containing two eggs. This way it starts out undergoing and equal cytokinesis in the first division of meiosis and the during the second meiosis there is no cytokinesis. The second is an oogonia containing a single egg and a polar body. This way starts by undergoing and unequal cell division after the first cell division and then the second division it does the same as the first form. Then the third form is an oogonia containing a single egg. It is formed by reductions divisions where there is no cytokinesis present and ends up forming one large active nucleus formed by two smaller pycnotic nuclei around it. These are also evolutionary steps and the third form is the most derived form of how the eggs are developed and the first form is the most primitive form of egg development.
    Then fertilization takes place by the sperm cell approaching the egg and interning the oogonial  thecae (outer coat of the oogonia) through a sperm channel. It then goes and gets to the egg cell and binds to it and from a zygote.
The zygote of a diploid cell is called an auxospore and it is virtually a diatom without the silica shell. During this time the cell grows rapidly but doesn’t divide. It undergoes and incomplete mitosis lacking cytokinesis forming the epitheca on the inside of the auxospore. Then one of the nuclei is removed or broken down by the cell. The cell repeats the process again to produce the hypotheca. This process ending with a complete internalized diatom cell with everything including frustule. Then the auxospore lyses open releasing the newly formed full size diatoms cell.
    There are variations of this between the different phyla, classes, and orders.

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