of an auxospore. This is usually, if not always, coupled with sexual reproduction.
Drebes (1977) provides a thorough discussion of sexual processes in diatoms. In
centric forms (an example of a life-history sequence is shown in Fig. 2.11) many
species produce four flagellated sperm by meiosis. A sperm fertilizes a single oocyte
produced inside the shell of the reduced vegetative cell. The resulting zygote drops
the frustule, grows to a large size, and forms a heavy cell membrane of organic
material containing siliceous scales (Edlund & Stoermer 1991), and finally develops a
shell of the vegetative type internally. This new frustule, surrounding the so-called
“initial cell”, is produced in association with several mitoses, indicating a relationship
between the control of frustule formation and the control of mitosis per se. All of the
daughter nuclei, save one, simply degenerate. Pennates exhibit similar sexual
processes, although motile sperm are not involved. Rather the gametes are
“isogametes”, cells of the same size (sometimes amoeboid) produced by meiosis that
fuse to form a zygote when exchanged between touching parent frustules. The zygote
produces a new frustule by an elaborate process (see Mann 1984; Pickett-Heaps et al.
1990).
Fig. 2.11 Events in the life-cycle of Stephanodiscus. Cell reduced to small size (a)
forms motile gametes. Gametes fuse with haploid oogonial protoplasts, which form
enlarged auxospores (b). Auxospores expand (c), then break open revealing
sculptured shell of the initial cell (d). (e) The initial cell divides, producing vegetative
cells (f then g). Size reduction then proceeds again through many mitotic division
cycles (h and i). Formation of motile gametes is only supposed for this genus, not
observed directly.
(^) (After Round et al. 1990.)