Current Biology Vol 16 No 9
wall. We propose that the high
speed of discharge is due to the
release of energy stored in the
stretched configuration of the
collagen polymer of the capsule
wall. How the ejection of the stylets
is initiated during exocytosis and
after fusion pore formation remains
to be shown.
This work was supported by the DFG
and Hamamatsu Photonics Germany. We
thank Uwe Denzer for his generous help.
Supplemental data including Experimen-
tal Procedures are available at http://
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1Institute of Zoology, University of
Frankfurt, Theodor-Stern-Kai 7,
60590 Frankfurt am Main, Germany.
2Department for Applied Physics, LM
University Munich, 80799 München,
Germany. 3Department of Biology,
Darmstadt University of Technology,
Schnittpahnstrasse 10, 64287
Darmstadt, Germany. 4Heidelberg
Institute of Zoology, University of
Heidelberg, 69120 Heidelberg, Germany.
dim-light vision in
the last common
ancestor of the
Samantha M. Mohun2,
Simon R. Harris3,
James O. McInerney1
and Mark Wilkinson2
Animal vision is mediated
through pigments belonging
exclusively to the opsin family.
These are members of the
family that bind retinal . Based
on function and phylogenetic
relationship, vertebrate visual
opsins can be clustered in five
groups: Rhod photoreceptors
(Rh1), Rh1-like (Rh2), Short Wave
Sensitive (SWS1), SWS1- like
(SWS2), and Long (LWS) or
Medium (MWS) Wave Length
Sensitive (LWS/ MWS). Rh1 is
used for seeing under dim light
conditions (scotopic vision),
while the others permit full
colour (photopic) vision in
bright light [2–5]. Opsins have
diversified by a series of gene
duplications, and the inferred
order of these duplications
indicates that photopic vision
predated scotopic vision in
Assuming that the jawless
vertebrates (Agnatha) are
monophyletic , the broad
distribution of opsins associated
with photopic vision indicates
that these — and thus the
capacity for photopic vision —
were present in the last
common ancestor of all living
vertebrates [2–5]. However, it is
still unclear whether ‘true’ (i.e.,
Rh1-mediated ) dim- light
vision predated the split
between Agnatha and the jawed
vertebrates (Gnathostomata), or
whether it is an apomorphy of
the Gnathostomata [2–5,7].
Solving this question has
implications and, assuming
that orthologous opsins have
inherited a common ancestral
function, it depends on the
correct classification (phylogeny
based orthologous clustering) of
the few agnathan Rh sequences
available to date.
Based on a Neighbour
Joining (NJ) analysis,
Yokoyama  suggested
that the two agnathan Rh
sequences available at that
time represented Rh1 opsins.
This would imply that the
was potentially capable of
Jawed Vertebrates Rh1
Jawless Vertebrates Rh1
Jawed Vertebrates Rh2
Jawless Vertebrates RhB
Figure 1. Phylogenetic relationships among vertebrate visual opsins.
The left tree shows a summary of the relationships obtained from the quartet puz-
zling and the minimum evolution analyses (Supplemental data). Numbers at the nodes
represent quartet-puzzling support values. The right tree shows a summary of the
relationships obtained in the ML analyses (PHYML, SPR-PHYML and SPR), in the
Bayesian analysis, and in the equally and differentially weighted parsimony analyses
(Supplemental data). Numbers at the nodes represent the bootstrap support values
obtained in the PHYML analysis (above) and posterior probabilities (below). The red
star represents the gene duplication resulting in the origin of Rh1.
Magazine Download full-text
true dim-light vision.
Alternatively, Collin and
co- workers [3–5,7] suggested
that the vertebrate cenancestor
did not possess an Rh1 gene
and could not have been capable
of true dim-light vision. This
conclusion was based on the
isolation and NJ analysis of LWS/
MWS, SWS1, SWS2, and two Rh
sequences from the southern
lamprey Geotria australis. Collin
et al.  included also the other
available agnathan Rh sequences
(i.e., a putative Lampetra and
Petromyzon Rh1 sequences) in
their analysis and recovered all
four agnathan Rh sequences as
the monophyletic sister group
of the gnathostomatan Rh1 and
Rh2. This implies that agnathan
Rh opsins originated from a
gene duplication predating
that giving rise to Rh1 and
Rh2, such that agnathans have
neither an Rh1 nor an Rh2 gene
[3–5,7]. They named the new
agnathan- specific opsins they
isolated in G. australis RhA
and RhB, and reassigned the
Lampetra and the Petromyzon
Rh1 to RhA , thus concluding
that the cenancestor did not
possess the Rh1 gene and thus
no true scotopic vision.
Here, we analyse the
amino acid sequences of 47
representative vertebrate visual
opsins (LWS, SWS1, SWS2, Rh1,
Rh2, RhA, and RhB) using a
variety of phylogenetic methods
to test the alternative hypotheses
that true dim-light vision evolved
in the stem vertebrate lineage,
or within the Gnathostomata.
We analysed the data with
minimum evolution, equally and
differentially weighted maximum
parsimony, Bayesian analysis
and Maximum Likelihood while
testing for long branch attraction
artefacts using the method of
Pisani  (see Supplemental Data
available with this article online).
Our analyses show that a clade
containing RhA plus RhB is never
recovered (Figure 1). Rather,
Likelihood strongly supports the
agnathan RhA being orthologous
with the gnathostome Rh1 and,
albeit less strongly, identifies
the agnathan RhB as an Rh2
(Figure 1A). Standard Maximum
Likelihood analyses and the
Bayesian analysis provide very
high support for the orthology of
RhA and Rh1 (Figure 1B), but not
for the orthology of RhB and Rh2.
Minimum evolution supports the
tree shown in Figure 1A, while the
maximum parsimony analyses
support that in Figure 1B.
Pisani’s method  for countering
long branch attraction does not
change the inferred relationships
of RhA or RhB. Although the
data do not allow us to choose
between the alternative trees
in Figure 1, the approximately
unbiased test  unequivocally
shows that these trees explain
the data significantly better than
the tree of Collin et al. 
( p = 0.0002).
Based on these results, we
can confidently conclude in
accordance with  that the
last common ancestor of the
vertebrates possessed an
Rh1 gene, which is thus much
older then suggested in many
recent studies and reviews
[3–5,7]. The function of Rh1 in
agnathans is not yet known,
but assuming its function in the
vertebrate cenancestor was
not dramatically different from
its scotopic function in most
vertebrates, this implies that
both photopic and scotopic
vision evolved in the stem
vertebrate lineage and must have
been in place in the Cambrian by
about 522–518 Ma [10,11].
Early vertebrate evolution
probably took place in a
brightly lit environment and
thus the earliest stem vertebrates
were probably diurnal and
inhabited shallow waters.
However, at some stage after
photopic vision was
already in place, scotopic
vision also evolved in the stem
vertebrate lineage, which
implies that a behavioural or
ecological shift — perhaps a
move into deeper water or to
nocturnality — occurred in an
ancestral vertebrate. What
drove this shift can only
be conjectured, such as
the emergence of large
macrophagous predators ,
and is certainly a topic for
This work was supported by an MRF
grant to MW, a Marie Curie Intra European
Fellowship (contract Number MEIF-CT-
2005-01002) to DP and a BBSRC stu-
dentship (BBS/S/K/2003/10085) to SMM.
We thank David Hunt, Philip Donoghue,
Thomas Keane, Jennifer Commins and
four referees for advice and comments
on earlier drafts.
Supplemental data including Experimen-
tal Procedures are available at http://
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question of genetic promiscuity. Curr.
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1Bioinformatics Laboratory, The
National University of Ireland,
Maynooth, Ireland. 2Department
of Zoology, The Natural History
Museum. Cromwell Road, SW7 5BD,
London, UK. 3School of Biology and
Psychology, Division of Biology,
University of Newcastle-upon-Tyne,
NE1 7RU, UK.