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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.
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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. A more sophisticated form of asexual reproduction is spore production. Spores, produced by mitosis, are released, often by the hundreds, and develop into new individuals. All progeny resulting from asexual reproduction are genetically identical (clones). Asexual reproduction is a means of increasing an individual’s dominance in its environment.
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About Seaweeds
Figure 8. The three-phased life cycle of the red seaweed Gracilariopsis. Separate haploid gametophytes, male (A) and female (D), produce non-motile sperm (B, C) and attached eggs (E). The sperm encounters a bowling pin–shaped egg and fuses with it (E). The resulting zygote develops into a parasitic diploid phase on the female gametophyte (F, G). This parasitic phase produces spores (H, I), which are released and develop into a free-living diploid phase (J). The free-living phase then produces spores (K, L) by meiosis (reduction cell division). These spores develop into male and female gametophytes—the sexual phase.
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Seaweed Names
Every known species has a scientific name, and some species also have a common name. For example, Nereocystis luetkeana is the scientific name for the species commonly known as “bull kelp.” The common name is the name by which a species becomes known among lay people and can be very useful, especially in cases where the common names are universally applied, such as with breeds of dogs and species of birds. However, in most cases a common name may refer to any of several species and is often haphazardly applied or region specific. For example, ask someone from the Pacific Northwest to show you “bull kelp” and they point to Nereocystis luetkeana; ask the same of someone from Australia, New Zealand or South Africa and they point to Durvillaea antarctica, a different species altogether. On top of that, a single species may have more than one common name: Nereocystis luetkeana is also (though much less frequently) known as “bladder kelp” or the delightfully imaginative “sea otter’s cabbage.”
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