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Taxonomy
and Sytematics of Algae
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Depending
on methodology and philosophy, there are between 50,000 and 10 million
different algal species
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Algal
taxonomy and systematics is under constant discussion and change, even
more after introduction of molecular techniques
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The history
of algal classification went from morphology to ultrastructure to genomic
analyes; sometimes genetic and morphological classifications contradict
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Sometimes
morphology can mislead when one species exhibits several „growth forms“,
dependent on envrionmental conditions:
The Biological
Concept of „Species“
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Classical
definition: If two individuals can produce
viable offspring, they belong to one species.
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Problems:
Sexual reproduction is not observed in some algal groups and occurs rarely
in others; culture material may not be available or environmental conditions
to induce sexual reproduction are unknown
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Hybrids:
intergenetic as well as fertile hybrids from morphologically distinct species
occur in algae, particularily in kelp
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Morphological
species concept: one species can exhibit different
growth forms, physiological attributes may differ among morphologically
identical isolates
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Phylogenetic
species concept: address the genetic similarity
among specimens; also provide evolutionary information. Ideally, a species
should be monophyletic, i.e. have one common ancestor. Some species, however,
appear paraphyletic (groups of analyzed strains do not include all of the
descendants of a commong ancestor)
Molecular Sequences
versus Morphology
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Morphologically
deduced phylogeny can be misleading due to
parallel or convergent evolution (e.g. squid and human eyes)
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Molecular
sequeces can also undergo parallel and convergent
evolution; it is not possible to conclude that molecular phylogenies represent
the „ultimate truth“
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Sole
reliance upon molecular data excludes important
data
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Modern
approaches check molecular phylogeny against
morpholo-gical phylogenies, or two molecular phylogenies based on different
genetic information (SSU rRNA vs plastid DNA) for congruence
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Addressed
sequences: Ribosomal RNA, protein-coding genes,
internal spacer regions (ITS, nuclear-encoded ribosomal RNA), DNA fingerprinting
(randomly amplified polymorphic DNA, restriction fragment-length polymorphism),
microsatellite DNA, presence and location of introns
Phylogenetic
Trees
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The tree:
Statistical analyses (computer-based) produce dichotomous trees and group
similar species in close clusters; the length of the branches indicates
the genetic similarity (kinship) between branches
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Bootstrep
values indicate how solid a certain branch
is supported by the data; value should be >50, max 100
Application
of Phylogeny
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Revisit
species concept: molecular phylogenetic studies
showed that zooxanthellae in corals are more genetically diverse than thought;
evidence that prochlorophytes (Prochlorococcus), the smallest phytoplankton
(0.4 µm), are not a coherent cluster but strains are often closer
related to coccoid cyanobacteria than to other prochlorophyte strains
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Evolutionary
utility: assess monophyletic or polyphyletic
origin of algal groups or algal characteristics
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Ecologically
utility: diversity of plankton communities;
prove of N2-fixation by the filamentous cyanobacterium Trichodesmium
because its nifH gene is different from those of bacteria; which
organisms are capable of which physiological processes (presence/ absence
of the necessary gene)
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