Page 31 - Algae

14 Algae: Anatomy, Biochemistry, and Biotechnology

Diplontic or Gametic Life Cycle

This cycle has a single predominant vegetative diploid phase, and the meiosis gives rise to
haploid gametes. Diatoms (Figure 1.20) and Fucus (Heterokontophyta) (Figure 1.21) have a diplon-
tic cycle.

Diplohaplontic or Sporic Life Cycles
These cycles present an alternation of generation between two different phases consisting in a
haploid gametophyte and a diploid sporophyte. The gametophyte produces gametes by mitosis;
the sporophyte produces spores through meiosis. Alternation of generation in the algae can be iso-
morphic, in which the two phases are morphologically identical as in Ulva (Chlorophyta)
(Figure 1.22) or heteromorphic, with the predominance of the sporophyte as in Laminaria
(Heterokontophyta) (Figure 1.23) or with the predominance of the gametophyte as in Porphyra
(Rhodophyta) (Figure 1.24).

FIGURE 1.20 Life cycle of a diatom: 1, vegetative cell; 2, 3, vegetative cell division; 4, minimum cell size;
5, gametogenesis; 6, 7, fertilization; 8, auxospores; 9, initial cells. R!, meiosis.