Speaking of kelp sex, legend has it that Professor Dieter Müller, University of Konstanz, Germany, discovered a gin-like smell in his lab that he attributed to his graduate students. The students denied any wrong-doing. Müller determined the aroma was a brown algal pheromone. In kelp, the pheromone lamoxirene, produced by the female gametophyte, has two
19
About Seaweeds
Figure 5. The isomorphic life cycle of Ulva (sea lettuce). The haploid sexual plants, gametophytes (A, B and D, E), produce morphologically similar gametes (C, F), which sexually fuse (G, H). The resulting zygote (fertilized gamete) (I) develops into the diploid spore-producing phase, the sporophyte, (J–M), which produces spores by meiosis (reduction cell division) (N, R). These spores develop into the new gametophyte generation (O–Q, S–U).
A
B
C
G
syngamy
meiosis
Q
D
E
F
H
I
J
K
L
M
N
R
O
P
S
T
U
Pacific Seaweeds
20
Figure 6. The animal-like life cycle of Fucus (rockweed). The macroscopic plant (A) develops special cells that produce sperm (D) and eggs (H) in pits (conceptacles) (C) located on its branch tips (recep-tacles) (B). Meiosis (reduction cell division) occurs in these special cells, producing eggs (I–K) and sperm (E–G). The sperm is attracted to the released egg, with which it fuses (K). The resultant zygote (L) develops into the macroscopic sexual plant (M, N).
A
B
C
G
D
E
F
H
I
J
K
L
M
N
syngamy
meiosis
21
About Seaweeds
Figure 7. The heteromorphic life cycle of Nereocystis (bull kelp). The macroscopic spore-producing phase, the sporophyte (A), produces spores in well-defined patches (sori) by meiosis (re-duction cell division) (B, C). These spores are released from the sori after the sori have been dropped (C, D), and they develop into microscopic male (E–G) and female (I–K) gametophytes. The sperm (H) is attracted to the retained egg (L) and fertilization occurs (M). The resultant zygote (fertilized egg cell) (N) develops into the new sporophyte generation while attached to the female gametophyte.
A
B
C
G
Q
D
E
F
H
I
J
K
L
M
N
O
P
syngamy
meiosis
Pacific Seaweeds
22
functions: to cause the explosive release of sperm from the male and to attract the sperm to the egg. Interesting, but of no evolutionary significance, is the observation that similar-smelling substances (gin, lamoxirene) elicit similar responses in humans and kelp.
Subsequent studies have shown that the perfume, lamoxirene, produced by any species of kelp will attract sperm of any other kelp species. This raises the question, if lamoxirene is a universal sexual attractant, what prevents universal hybridization? A study of intergeneric hybridization in kelp indicated hybridization is common. Furthermore, this study suggested that the putative hybrids bore the female parent’s morphology. This unusual genetic expression, where the male’s contribution seems insignificant, is foreign to plants and animals, and may be a unique feature to algae.
The life cycles of most red seaweeds have an added twist. The gametophytes release their sperm but retain their eggs. The eggs are fertilized while attached to the gametophyte and the new sporophyte generation develops parasitically on the gametophyte. This sporophyte produces spores by mitosis (no reduction in chromosome sets). These spores are released and establish a free-living sporophyte generation. The sporophyte undergoes meiosis (reduces the chromosome sets to one) in the production of spores that will give rise to a new gametophyte generation. As with other seaweeds, the red seaweeds may have isomorphic or heteromorphic generations (Figure 8).
The various types of life cycle may convey survival advantages. For example, some red and brown seaweeds have two morphologies, a persistent crust and a short-lived erect bladed (leaf-like) structure. The crust may be less susceptible to grazing or summer drying than the more delicate exposed blade.
Many seaweeds are capable of reproducing outside of their life cycle. This type of reproduction is referred to as asexual reproduction. The simplest form of asexual reproduction is fragmentation, where broken-off bits of plant develop into new individuals.