Macrocystis Evolution

Algae are often referred to as "plants" even though they are not closely related to vascularized land plants. Phylogenetic studies of ribosomal DNA have concluded that the three major algal groups arose independently over evolutionary time. The green (chlorophyte), brown (chromophyte), and red (rhodophyte) algae are therefore called polyphyletic. This is shown in the phylogenetic tree of eukaryotes. It is thought that these groups appeared as eukaryotic host cells formed symbiotic relationships with photosynthetic bacterial cells. This has been termed the "serial endosymbiosis hypothesis".

Taxonomy

Division: Chromophyta

Macrocystis pyrifera is a member of the group loosely called the brown algae, or Division Chromophyta (this division was formerly named phaeophyta) The chromophyta are actually believed to be the least related to land plants and most closely related to oomycete fungi (McFadden, Gilson, and Hill 1994). The chromophytes have chlorophyll a, c1, c2, and fucoxanthin as their major photosynthetic pigments. They include the golden brown algae, diatoms, and the multicellular macro algae.

Class: Phaeophyceae

The next classification is the Phaeophyceae. This includes the algae that are multicellular and have cellulose walls containing alginic acid (commercially used to produce algin). The phaeophyceae include members such as the Ectocarpales, Dictyotales, Desmerestiales, Laminariales, and Fucales.

Order: Laminariales

The Laminariales as a group are generally called kelps (although the Fucales are also sometimes called kelps). These are usually large, conspicuous brown algae that live in low intertidal and subtidal habitats. The kelps grow by cell division at a meristoderm. This order contains the families: Alariaceae, Chordaceae, Laminariaceae, and Lessonaceae.

Family: Lessoniaceae

Macrocystis pyrifera is ultimately placed in the family Lessoniaceae. The characteristics of the family include branched stipes, each branch terminating in a single blade, and blades that split at an intercalary meristem. Some of the genera in this family have gas filled floats called pneumatocysts that hold the plants erect in the water. Other Genera include: Postelsia, Pelagophycus, and Nereocystis.

The most recently constructed phylogenetic tree has shown that Macrocystis is part of a group of closely related taxa (see tree at right). Using ribosomal DNA data, Druehl et al. (1997), concluded that the molecular tree does not agree with the current morphological classification of kelps. This has led to a realization that kelps are a very recently evolved group and proper classification needs further investigation.

Kelps are thought to have appeared in the Miocene epoch, 23 to 5 million years ago. Brown algal fossils were found in the Monterey Formation of California and documented by Parker and Dawson in 1965. Click the image at the right to see the fossils found in the Monterey formation (and check out the rest of the UC Museum of Paleontology site). The morphologies of these fossils very closely resemble those of present day kelps.

A case of convergent evolution: Every algal cell potentially has the ability to photosynthesize and there is generally no complete specialization of different parts of the algae. This is in contrast to terrestrial plants where there are roots, stems, and leaves containing xylem and phloem that have specific, exclusive functions. Kelps however, are the only group of algae that seem to have developed specialized transport tissue. Kelps have trumpet cells that are thought to be similar to the conductive tissue, or phloem, in vascular plants. If you think about the similarities between Macrocystis in a kelp forest and a tree in a terrestrial forest, there is a need in both systems to be able to transport photosynthetic products from a sun-lit source far above to a shaded sink down below. Algae and terrestrial plants are entirely unrelated yet have a similar way of solving the problem. This is an example of convergent evolution.

Evolution of the name, Macrocystis pyrifera

The alga that was to eventually be named Macrocystis pyrifera was apparently known by navigators for a long time before it was formally recognized. Johann Gerhard Koenig (1728-1785), a missionary and plant collector, was the first to officially collect the plant. In 1771 Linnaeus named Koenig's specimen, Fucus pyriferus. Linnaeus was the first scientist to formalize the system of naming organisms by genus and species but at the time, had only three categories for all of the algae.

As more specimens were described and greater diversity recognized, a man named Lamouroux created more genera. Lamouroux is responsible for discerning the divisions of green, red, and brown algae that we still recognize today. In 1813 Lamouroux created the genus Laminaria and placed renamed the plant, Laminaria pyriferus. In his publication he references Turner's 1809 picture showing the blades and pneumatocysts. The first picture of a blade by Esper in 1802 was made from a specimen actually collected by Turner.

In 1820 Carl Agardh of Sweden published his book on species of algae with descriptions including new the genera: Sargassum, Cystoseira, Macrocystis. In this book he renames L. pyriferus to Macrocystis pyrifera, meaning large (macro) bulbs (cysts). His collections came from the Ethiopian Sea, Cape of Good Hope, and the Pacific.

 

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