Biology Reference
In-Depth Information
concept (Biological species concept; sensu Mayr
1942
) but are recognized within
molecular phylogenies as genetically distinct, statistically robust, reciprocally
monophyletic clades (Evolutionary Significant Units, ESUs, sensu Moritz
1994
;
Wattier and Maggs
2001
; Provan et al.
2005
; Uwai et al.
2006a
; Stam et al.
2006
;
Andreakis et al.
2007a
; Verbruggen et al.
2007
). The condition of monophyly
represents a key assumption for biological classification of organisms; monophy-
letic clades herein include an organism and all its descendants. ESUs may correlate
with geographical distribution and common morphology. Overall, ESUs respect the
phylogenetic species concept and are characterized by distinct evolutionary
trajectories although the possibility of interbreeding is not excluded.
In the context of invasion biology, ESUs are intended as suitable taxonomic
units to elucidate invasions and represent terms such as strains, haplo-groups,
lineages, or varieties, conventionally used to express the level of genetic variation
and explain the phylogeographic patterns found in introduced seaweeds. The
number of uncovered ESUs can vary significantly between studies and largely
depends upon the resolution and the speed of “lineage sorting” of the molecular
marker used. This is because molecular marker systems are characterized by
variable levels of evolutionary speed, distinct genealogical trajectories, and/or
uneven intensities of selective pressure; these are all characteristics accounting
for the heterogeneity in a phylogenetic signal and its resolution. Therefore, inde-
pendent information for more than one DNA region and ideally from distinct
genomes (nuclear, mitochondrial, and plastid) is necessary to achieve consistent
results for delineating ESUs and inferring phylogenies at the sub-species level.
In the last decades, DNA barcoding has been largely used in plant and animal
systematics. The method involves sequencing and similarity analysis of a short
fragment of DNA from multiple specimens, corresponding to a highly conserved
region in the genome such as the subunit one of the mitochondrial gene cytochrome
c
oxidase (COI; Blaxter
2003
). However, information coming from a single,
extremely short DNA region, compared to the size of a genome, is far from being
reliable in modern phylogenetics. This is particularly true in (1) groups of
organisms with limited sequence diversity, (2) recently diverged species, (3)
hybrids (i.e., offspring of parents belonging to biologically different species, and
(4) pseudogenes (i.e., nonfunctional DNA sequences resembling functional genes).
The need to employ in a single assay more than one universal biological marker in
organism classification can today be satisfied by the increasingly growing number
of completely sequenced prokaryotic and eukaryotic genomes. At present,
techniques such as DNA microarrays or DNA chips represent the most powerful
way to gather as much information as possible from a single individual genome in a
single trial in a cost-effective way. In the future, the latter techniques and
improvements therein, in combination with advances in data processing and com-
putation power, will permit the inference of phylogenetic relationships among
taxonomically robust taxa, ideally at the whole-genome level based on the analysis
of their full set of genes (Garzon and Wong
2010
).
