Asexual Reproduction in Diatoms
Imagine a hat box: the lid is the same size and shape as the box itself, except that the lid is just slightly bigger so that it fits snugly over the box. This is how a diatom cell wall, or frustule, is shaped. The "lid" is called the epivalve and the "box" is called the hypovalve. The two valves are held together by a sort of tape, called girdle bands. All three of these components together make up the frustule and all are made of the same material: glass.
In order to divide into two daughter cells, the cell grows and pushes out the two valves to make room. But when it is actually ready to divide, it must do something with the rigid cell wall. Instead of shedding the entire cell wall and each daughter cell having to create a brand new one, diatoms give each daughter cell one valve. This is a great strategy because laying down silicon dioxide from seawater is metabolically costly. However, it has its drawbacks. Unfortunately, the daughter cells can only make a new hypovalve, the smaller half of the hat box and so for each reproductive cycle, a diatoms produces 1 original size cell and 1 slightly smaller cell. Over many generations, diatoms can shrink in size considerably, as shown in the figure below:
Diatoms shrink with successive generations!
© 2001 Caren E. Braby