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The Molecular Operational Taxonomic
Unit or MOTU
Mark Blaxter, IEB, University of Edinburgh, 1999
By sequencing an informative segment of DNA it is
possible to define "molecular operational
taxonomic units" (MOTU). To be useful, the
segment of DNA must be known to be orthologous between species (as
paralogues will define gene rather than organismal trees), and the
segment must encompass sufficient variability to allow discrimination
between MOTU useful to the research program, or defined in other
(for example biological or morphological) ways.
In the MOTU concept therefore, taxa can be
identified through sequence identity. Identity in sequence need not
correspond to identity of OTU as measured by other models (biological
or morphological): identity in sequence could mean "the same taxon"
or "there is insufficient variation to define distinct taxa". The
same operational problem plagues other (biological or morphological)
methods of defining taxa.
Differences in sequences between specimens can arise
in two ways. The first is that the differences are part of the
natural, within-OTU variation, or are due to sequencing
(methodological) errors. The second is that the differences are
related to a distinction between taxa.
It is thus necessary (as with other methods,
biological or morphological) to use heuristics for MOTU distinction
based on known error rates in measurement, and perceived levels of
difference that distinguish "useful" OTU.
For MOTU, these measures can be made explicit. For
example, from known, accepted taxa within a particular group, the
level of between-taxa within-group variation can be measured.
Multiple resequencing of a single taxon will yield an observational
error rate. The comparison between the between-taxon difference rate
and the within-taxon variation and error rates will define the
accuracy and specificity of the MOTU measurement. Given that it is
clear from many gene sequences that different higher taxonomic groups
can differ markedly in their background and adaptive substitution
rates, and that different sized populations might be expected to
harbour different levels of within-taxon variation (also dependent on
the population's evolutionary history), it may be necessary to define
different heuristics for MOTU designation depending on the higher
taxon studied.
The benefit of the MOTU is that data can be obtained
from single specimens, often without compromising parallel or
subsequent morphological identification, that morphologically
indistinguishable taxa can be separated without the need for live
material, and that a single technique is applicable to all taxa. Thus
a long and partial training in a particular (sub-) group is not
necessary, and recourse can be made to published monographs and keys
to understand the known biological properties of the identified MOTU
and their close relatives. All stages/morphs of taxa are amenable to
study, as the method depends on genotype, not phenotype.
In addition, the MOTU data, the sequences, are
suited to exhaustive and model-driven phylogenetic analyses to derive
independent and testable hypotheses of OTU interrelatedness.
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