The retina of Manduca sexta: rhodopsin expression, the mosaic of green-, blue- and UV-sensitive photoreceptors, and regional specialization
1 Department of Biology, University of Massachusetts Boston, 100 Morrissey
Blvd, Boston, MA 02125-3393, USA,
2 MGH Cancer Center, Harvard Medical School, 13th Street, Charlestown, MA
02129, USA
3 Helen Wills Neuroscience Institute, University of California Berkeley, 145
Life Sciences Addition, Berkeley, CA 94720, USA
* Author for correspondence (e-mail: richard.white{at}umb.edu)
Accepted 25 June 2003
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Summary |
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Key words: Sphingidae, Manduca sexta, rhodopsin, retinal mosaic, regionalization, dorsal rim
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Introduction |
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The spectral sensitivity of foraging is consistent with our knowledge of
the visual pigments expressed in the retinal photoreceptors of
Manduca. Three rhodopsins - P520, P450 and P357 - have been
characterized by spectrophotometry, ERG spectral sensitivity and the cloning
of three opsin cDNAs (White et al.,
1983; Bennett and Brown,
1985
; Chase et al.,
1997
; Bennett et al.,
1997
). The G and UV photoreceptors expressing P520 and P357 were
identified by electron microscopy (EM) through the structural effects of
intense colored light, but the B cells expressing P450 were not found
(Cutler et al., 1995
). Here,
we confirm the identity of the G and UV receptors and identify the B receptors
by opsin immunocytochemistry. This approach has also provided a map of the
retinal mosaic of photoreceptors that refines our low-resolution ERG data. Our
aim is to examine more precisely the hypothesis that B receptors, predominant
in spontaneous foraging behavior, are localized to the ventral retina.
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Materials and methods |
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Antisera
Three Manduca opsin cDNA sequences were cloned from retinas and
identified by Chase et al.
(1997): MANOP1 encodes P520;
MANOP2, P357; MANOP3, P450. Rabbit anti-opsin antibodies were generated to the
three Manduca opsins. The amino terminus of MANOP2 with added His.Tag
sequence,
5'-TNFTQELYEIGPMAYPLKMISKDVAEHMLGWNIPEEHQDLVHDHWRNFPAVSKYWHTALALLYIFFTFAALVGHHHHHH-3',
was expressed in BL21 (DE3) NovaBlue Escherichia coli with the
expression plasmid vector pET-30a (Novagen, Madison, WI, USA). One hour after
induction, a peptide of appropriate molecular mass (9.8 kDa) appeared on
SDS-PAGE gels of total protein from culture samples. The expressed target
peptide was purified through a Novagen His. Bind column under denaturing
conditions, run on a 22% SDS-PAGE gel, and the target band was excised. Rabbit
antisera to the excised gel band were prepared by Charles River PharmServices
(Wilmington, MA, USA). As similar efforts to express P520 and P450 failed,
short synthesized peptides from amino-terminal segments were used to raise
antisera against these opsins: P520, 3'-DPHWYQFPPMNPLWH-5'; P450,
3'-EEHQDLVHDHWRNFPAVSK-5'. Peptides and antisera were provided by
Zymed Laboratories (San Francisco, CA, USA).
Antisera were assessed from western blots of retinal extracts subjected to SDS-PAGE electrophoresis. Dissected retinas were ground in 0.1 mol l-1 phosphate buffer, pH 7, with 1 mmol l-1 EDTA (Sigma, St Louis, MO, USA) and centrifuged at 16 000 g. The pellet was washed in phosphate buffer to remove solubilized screening pigment and extracted in SDS (Sigma). Following centrifugation, aliquot parts of supernatant were subjected to SDS-PAGE electrophoresis with pre-stained molecular mass standards (Bio-Rad, Hercules, CA, USA). Western blots (blocked with 5% bovine serum albumin) were prepared with the ABC reagents from Vector Laboratories (Burlingame, CA, USA). Blots were stained with 4-chloro-1-napthol in ethanol mixed with peroxide buffer (Sigma).
Immunocytochemistry
Retinas were fixed in phosphate-buffered 4% paraformaldehyde and cut into
various pieces along the anterior-posterior and dorsal-ventral axes. Retinal
pieces were oriented in paraffin blocks and sectioned at 6 µm. An ABC
peroxidase immunostaining protocol with VIP substrate (provided in kits from
Vector Laboratories) was used for localizing opsins to individual retinulae
and receptor cells (intense autofluorescence precluded fluorescent tags).
Digital images of sections were collected by Scion Image software from an
Olympus BX60 microscope. Localization of the three opsins was compared in
adjacent sections. During histological processing, retinas fractured randomly
along the tracheole palisades that separate the retinulae; the resulting
uniquely shaped blocks of retinulae greatly facilitated precise alignment of
adjacent sections.
Retinal morphology
Isolated retinas were fixed in cacodylate-buffered
glutaraldehyde-formaldehyde and processed according to standard procedures
(White and Bennett, 1989).
Thick sections were photographed in a Zeiss WL compound microscope, and thin
sections in a Philips 300 electron microscope. Whole retinas and hand-cut
sections were photographed in a Wild M5 stereomicroscope.
Rhabdomere volumes
Morphology and volumes of the rhabdomeres of the different classes of
Manduca photoreceptors were determined as follows. A single
tangential electron microscope section was chosen that extended from the
distal surface of a retina to below the proximal basement membrane near the
center of the retina. Low-magnification electron micrographs were taken of a
complete set of tangentially sectioned retinulae in two parallel rows, to
provide a composite of 21 profiles of retinulae distributed across the full
width of the retina (Fig. 1A).
In order to determine the plane of this tangential section across the retina,
a line was projected on a light micrograph of a longitudinal section from a
different retina that intersected 11 retinulae, the number in the longest row
of electron micrographs (Fig.
1B). From this, the actual depth of each profile could be
determined. The profiles were then treated as thick, virtual, serial
cross-sections of a single retinula from which rhabdomere volumes could be
estimated. Corresponding cells were identified from their positions in the
photoreceptor rosette in each sectioned retinula, and the areas of their
rhabdomeres were measured and appropriately adjusted downward to compensate
for profile elongation resulting from the tangential cut. To summarize, this
procedure enabled the reconstruction, from a single tangential section, of a
set of virtual serial sections, for which thickness dimensions could be
inferred and within which rhabdomere areas could be measured. Rhabdomere
volumes for each cell type were calculated from these values.
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Results |
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When the retina is exposed by cutting away the cornea and associated
pigment, it appears irregularly yellow to orange, except for a large,
distinctive dorsal rim area, which has a more transparent bluish appearance
(Fig. 2A). We will refer to the
main retina as the `yellow retina' to distinguish it from the dorsal rim.
Since the retinas of carotenoid-deprived moths are white, the yellow color
presumably results from carotenoids deposited in the retina
(Bennett and White, 1989).
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From its area, approximately 0.25 mm2, we estimate that the
dorsal rim contains about 1000 retinulae. Two factors may account for its
distinctive appearance. Perhaps it contains less carotenoid. In addition,
there is a difference in the tapetum that is responsible for the eye glow of
the dark-adapted eye (Banister and White,
1987). The tapetum is provided by tracheoles that branch into the
retina from a single tracheal cell that underlies each retinular unit. In the
yellow retina, the tracheole branches extend nearly to the surface of the
retina (Figs 2B,C,
3A,
4B), densely surrounding and
isolating each retinula from its neighbors. In the dorsal rim area, the
tracheoles terminate just above the basement membrane (Figs
2B,D,
4A). The blue cast of the
retina may come from underlying screening pigment made visible by the reduced
tracheal tapetum.
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Each retinula is made up of a small proximal cell and seven or eight
elongate photoreceptor cells that span the full depth of the retina.
Morphological details of retinulae in the yellow retina and the dorsal rim
area are presented in Fig. 5.
The elongate cells can be distinguished by characteristic morphologies and
their positions relative to the axes of the hexagonal lattice of the compound
eye (Cutler et al., 1995).
There are one or two dv cells (either the dorsal or ventral member of
the pair may be missing: Carlson et al.,
1967
; Cutler et al.,
1995
) oriented in the dorsal-ventral axis of the retina; two
ap cells in the anterior-posterior axis; and four oblique ob
cells. In the yellow retina, the rhabdomeres of dv cells are
restricted to the distal half of the retinula. The rhabdomeres of the
ap and ob cells extend most of the length of the retinula
but have distinctive rhabdomere morphologies
(Fig. 3). The proximal
pr cell lies at the center of the retinula just above the basement
membrane with its rhabdomere on either side in the anterior-posterior axis.
Electron micrographs of Manduca retinulae can be found in Cutler et
al. (1995
) and earlier papers
cited therein. Rhabdomere volumes (Fig.
3) were estimated for each morphological class of receptor, as
outlined in Materials and methods, from the set of 21 retinulae shown in
Fig. 1: dv cell, 1574
µm3; ap cell, 1063 µm3; ob cell,
964 µm3; pr cell, 315 µm3.
The same set of elongate receptors can be recognized in dorsal rim retinulae from their orientations; however, only the ventral dv cell is present. Rhabdomere organization is also distinctive: microvilli are oriented orthogonally; those of the ob and narrow dv cells parallel to the dorsal-ventral axis of the eye, those of the ap cells parallel to the anterior-posterior axis (Fig. 5B). From a limited EM study, we found that this structure is preserved down the full length of the retinula. We have not determined whether or not a pr cell is present.
Assessment of anti-opsin antisera
Fig. 6 shows western blots
of the three rhodopsin antisera. There is a major band in each rhodopsin lane
at approximately 37 kDa, as expected
(Bennett and White, 1989;
Chase et al., 1997
). Antisera
immunostained specific rhabdomeres in retinal sections with little background.
Rhabdomeres were not stained above background in control sections processed
without primary or secondary antibodies.
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Localization of opsins to receptor cells
In longitudinal sections of the yellow retina, anti-P520 was seen to stain
rhabdoms from just below the distal surface of the retina to just above the
basement membrane (Fig. 7A),
where the rhabdomeres of the ap, ob and pr cells are found.
Anti-P450 and anti-P357 stained the rhabdom distally, where the rhabdomeres of
the dv cells are located (Fig.
7B,C).
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Cross-sections provided precise identification of immunolabeled cell types.
Fig. 8 compares staining for
P520 and P357 in adjacent sections from about 70 µm below the retinal
surface. Anti-P357-stained rhabdomeres oriented in the dorsal-ventral axis of
the retina (Fig. 8B), whereas
anti-P520-stained rhabdomeres oriented on either side
(Fig. 8A). Comparison of this
pattern with the EM image of a retinula at a similar depth in the retina
(Fig. 5A; 70 µm) confirms
that P357 is expressed by dv cells and P520 by ap and
ob cells. However, Fig.
8B also indicates that that many dv cells express neither
P357 nor P520. Adjacent sections stained with anti-P357 or anti-P450
(Fig. 9) show that dv
cells express either P357 or P450. No instances were found in which more than
one rhodopsin was expressed in the same cell, as reported in the butterfly
Papilio xuthus (Kitamoto et al.,
1998). To summarize, ap and ob cells are
green-sensitive (G) receptors expressing P520; dv cells are either
blue-sensitive (B) or UV-sensitive (UV) receptors expressing P450 or P357,
respectively.
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Close examination of longitudinally sectioned retinulae strongly suggested that the proximal pr cell also expresses P520, but its small rhabdomere cannot be distinguished with certainty in the light microscope. It certainly does not express either P357 or P450.
Neither P520 nor P450 was expressed in dorsal rim retinulae. However, some of the ventral dv cells stained for P357 (Fig. 4A).
Regionalization
The pattern of expression of the three opsins was examined in samples from
all sectors of the retina. P520 was expressed uniformly in ap and
ob cells in all regions of the yellow retina but was not expressed in
the dorsal rim area. Regional differences in the expression of P357 and P450
in dv cells of the yellow retina are shown in Tables
1,
2 and
Fig. 10. The
immunocytochemical data summarized in Table
1 were gathered from retinas cut into quadrants along the
dorsal-ventral and anterior-posterior axes. As no anterior-posterior
differences were found, the data for dorsal quadrants are combined, as are
those from ventral quadrants. The dorsal and ventral densities of dv
cells expressing P357 were similar: approximately 60 cells per 100 retinulae.
However, cells expressing P450 were much more abundant in ventral retinas:
approximately 140 receptors per 100 ventral retinulae compared with
approximately 35 receptors per 100 dorsal retinulae.
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Detailed maps of the retinal mosaic were assembled from three sets of exceptionally well-stained adjacent sections from three different retinas showing large patches of retinulae from dorsal, central and ventral regions (Fig. 10; Table 2). Fig. 10C shows a portion of a larger ventral patch; almost all retinulae in this region had two dv cells, with B cells that expressed P450 predominantly. 69% of the dv cells were B receptors, giving a density of 137 B cells and 62 UV cells per 100 retinulae.
Fig. 10B shows the dorsal patch, which included retinulae from both the yellow retina and dorsal rim. Here, only one dv cell was stained in most retinulae at the dorsal edge of the yellow retina, with 66% of stained cells expressing P357. In addition, a number of retinulae showed no stained dv cells. The densities of B and UV receptors were 33 and 64 cells per 100 retinulae, respectively.
The patch from the middle of the retina (Fig. 10D,E) showed that a distinct equatorial border separates the predominantly UV-sensitive dorsal and predominantly blue-sensitive ventral halves. Below this border, most retinulae have two dv cells, 66% of which express P450. The densities of B and UV receptors were 118 and 60 cells per 100 retinulae, respectively. On the dorsal side of the border, half of the retinulae had only one dv cell, and 74% expressed P357. The densities of B and UV receptors were 38 and 111 cells per 100 retinulae, respectively.
In the portion of the dorsal rim area shown in Fig. 10B, about one-third of 124 retinulae showed dv cells that expressed P357, while none expressed P450. 65% of 2125 retinulae from five larger dorsal rim samples (data not shown) contained a cell expressing P357. As all dorsal rim retinulae surveyed in electron micrographs contained a ventral dv cell, the lack of P357 expression does not reflect the absence of dv cells from unstained retinulae. None of the dorsal rim ap and ob cells was stained by antisera to any of the three opsins.
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Discussion |
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The distinctive features of Manduca dorsal rim retinulae indicate
that they function, as in other insects, for perception of polarized light
(Kolb, 1986;
Labhart and Meyer, 1999
;
Labhart et al., 1992
,
2001
): rhabdoms have
orthogonally oriented microvilli for analyzing the plane of polarization, and
retinulae lack mutually isolating features such as screening pigment or
tracheole sheaths to enable large visual fields. The dorsal rim area of
Manduca is remarkable for its large size, encompassing about 1000
retinulae. It seems likely that this is an adaptation for the unique
behavioral ecology of such crepuscular/nocturnal sphingids; they are strong
flyers that navigate over long distances under dim light
(Janzen, 1984
;
Haber and Frankie, 1989
).
Similar dorsal rim areas have been reported for other moth species [the
sphingid Deilephila elpenor, the noctuids Spodoptera exempta
and Plusia gamma (Meinecke,
1981
) and the saturnid Anthera polyphemus
(Anton-Erxleben and Langer,
1988
)].
Spectral classes of photoreceptors
The classification of Manduca photoreceptors depends on the proper
assignment of the cDNA sequences used to generate anti-opsin antisera. As the
opsin-encoding cDNAs isolated from Manduca retinas have not been
expressed, their initial identification was based mainly on similarities to
other arthropod opsin sequences (Chase et
al., 1997). Our conclusion that MANOP1 encodes P520, MANOP3, P450
and MANOP2, P357 has been strengthened by the subsequent isolation of more
insect opsin sequences (Briscoe,
1998
,
2000
,
2001
;
Briscoe and Chittka, 2001
) and
confirmed by the results presented here. We have verified, in particular, the
assignment of the similar MANOP2 and MANOP3 sequences to the P357 and P450
rhodopsins, respectively. Antisera to an expressed fragment of MANOP2 mark the
distal rhabdomeres of dv cells that were previously identified as UV
receptors in Manduca (Cutler et
al., 1995
) and in the similar retina of the sphingid
Deilephila (Schlecht et al.,
1978
; Schlecht,
1979
) through the morphological effects of light. Furthermore, we
show that MANOP3-expressing dv cells are concentrated in the ventral
retina where B cells are localized
(Bennett et al., 1997
),
whereas MANOP2 expression, like UV sensitivity, is more uniformly
distributed.
Our conclusions for Manduca are corroborated by similar analyses
of opsin expression in the compound eyes of the butterflies Papilio
xuthus (Kitamoto et al.,
1998,
2000
) and Vanessa
cardui (Briscoe et al.,
2003
). Receptors corresponding to Manduca dv cells
express homologous blue- and UV-sensitive rhodopsins; those corresponding to
ap and ob receptors express P520 homologs. Although the
small pr cell in the Manduca retina cannot be clearly
distinguished by light microscopy, it also appears to express P520. This
conclusion is strengthened by the expression of P520 homologs in the
corresponding cells of Papilio and Vanessa.
Retinal mosaic
The division of the Manduca retina into distinct dorsal and
ventral domains seems a common, perhaps basic, feature in the differentiation,
morphology and function of the insect compound eye
(White, 1961;
Stavenga, 1992
;
Wolff and Ready, 1993
;
Kitamoto et al., 1998
;
Briscoe et al., 2003
).
Nearly all retinulae near the ventral margin of the eye have two dv cells (Table 2; Fig. 10C). Retinulae showing only one stained cell are frequent in the dorsal half of the retina. We cannot tell with light microscopy if they actually have only a single dv cell, but electron microscopy (micrographs not shown) qualitatively supports this inference. However, it is possible that some of these retinulae may include two dv cells, one of which expresses an as yet unidentified rhodopsin. This suspicion arises because some retinulae, especially dorsally (Table 2; Fig. 10B), stain for neither P450 nor P357, and, as deduced below for the dorsal rim area, we clearly have not identified all the rhodopsins of the Manduca retina. Although retinulae lacking both dv cells have not been detected in electron micrographs, they easily would have been missed. Retinulae with one stained dv cell become more frequent towards the dorsal edge of the ventral domain and increase across the dorsal domain. In the equatorial region, stained dorsal and ventral dv receptors are randomly distributed among one-celled retinulae. Towards the dorsal edge of the dorsal domain, more than 90% are ventral cells, the same asymmetry seen in adjacent dorsal rim retinulae (Fig. 10B).
The density of B and UV receptors (against a uniform background of G
receptors), expressed as number of cells per 100 retinulae, is shown in
Table 1: in the dorsal retina,
35 B and 63 UV; ventrally, 139 B and 57 UV. B cells dominate the ventral
domain and are reduced, but not missing, as suggested by Bennett et al.
(1997), from the dorsal
domain. UV cells, like G cells, are fairly evenly distributed across the
retina except for a region of higher density just above the equator. There is
no obvious pattern in the local distribution of B and UV receptors in either
domain. The question of pattern can be examined quantitatively in the ventral
patch shown partially in Fig.
10C, where nearly all retinulae have two dv cells. B and
UV cells are randomly distributed among dorsal and ventral pairs, with 537
B/B, 449 B/UV and 121 UV/UV, i.e. a binomial distribution (P>0.2)
in which the frequency of B and UV cells is 0.69 and 0.31, respectively. The
details of the mosaic of dv receptors
(Table 2;
Fig. 10) suggest two
mechanisms that might control elaboration during retinal differentiation.
First, differing relative strengths in each domain of determinants that,
acting randomly on nascent dv cells, specify the alternative
expression of P450 or P357. Secondly, the deletion or developmental arrest of
some dv cells, an action that is prominent in the dorsal domain and
is graded from dorsal to ventral. The combined operation of these mechanisms
could result in the observed features of the mosaic. First, a high density of
B receptors in the ventral domain and a low density of B receptors in the
dorsal domain resulting from the different balances between determinants in
each domain. Second, a fairly uniform density of UV receptors across both
domains, but with a region of higher density towards the equatorial margin of
the dorsal domain, resulting from the gradient of dv cell deletion.
It will be informative to map the retina during its pupal differentiation.
In previous studies, we estimated the relative proportions of the three
visual pigments in the Manduca retina from measurements of rhodopsin
absorption and ERG spectral sensitivities. The most recent, and we believe
most accurate, estimates were based on fitting rhodopsin nomograms to ERG
spectral sensitivity measurements from dorsal and ventral regions of the
retina (Bennett et al., 1997).
From the data presented here, we can now estimate these values in a completely
different way, under the different assumptions described in the Appendix, by
combining receptor cell densities and rhabdomere volumes.
In the dorsal domain, the ratio of the three rhodopsins (P520:P450:P357)
ranges from 80:7:13 to 73:7:20; in the ventral domain, it ranges from 67:23:10
to 69:20:10. These values may be compared with those derived from the ERG
spectral sensitivity curves: dorsally, 88:0:12; ventrally, 62:19:19
(Bennett et al., 1997). The
similarity of rhodopsin ratios yielded by these different methods strengthens
our conclusions.
The photoreceptors in the retina of the butterfly Vanessa cardui
express three rhodopsins homologous to the three Manduca visual
pigments. The disposition of photoreceptors in the Vanessa retina is
also similar to that seen in Manduca: blue-sensitive cells are
concentrated in the ventral half (Briscoe
et al., 2003). Briscoe et al. suggested that the retinas of
Vanessa and Manduca may be closer to the ancestral
lepidopteran retina than that of Papilio, in which six opsins are
provided by gene duplication (Briscoe,
1998
,
1999
,
2000
,
2001
). In the same vein, we
suggest that the similar patterns of regionalization seen in Manduca
and Vanessa may be closer to the ancestral organization of the
lepidopteran retina than the more elaborately heterogeneous retinas of
butterflies in which opsin gene duplication and chromatic filtering by
screening pigments provide additional red- and violet-sensitive receptors
(Arikawa and Stavenga, 1997
;
Arikawa et al.,
1999a
,b
;
Qui et al., 2002; Stavenga et al.,
2001
; Stavenga,
2002a
,b
).
Rhodopsins of the dorsal rim
Only about one-third of the dv cells in the dorsal rim area
express P357, and the remaining cells express none of the three opsins that we
have characterized from cDNA sequences. There must be one or more opsins
expressed in the Manduca retina that remain unidentified.
Neuro-behavioral implications
Dusk- and night-active hawkmoths like Manduca depend on their
well-developed olfactory and visual sensory systems to forage at
night-blooming `hawkmoth flowers' (White
et al., 1994; Raguso and
Willis, 2002
). The visual component of spontaneous foraging
behavior in Manduca is driven mainly by blue-sensitive receptors
(Cutler et al., 1995
). The
details of the retinal mosaic support our hypothesis
(Bennett et al., 1997
) that
the ventral retina plays a particular role in foraging. The spontaneous
feeding behavior of butterflies is also dominated by blue receptors (Scherer
and Kolb,
1987a
,b
),
which, as indicated above, are also concentrated in the ventral retina of the
butterfly Vanessa.
Arikawa and Uchiyama (1996)
proposed that dv and pr cells mediate color vision in the
butterfly Papilio xuthus. They pointed out that the axons of the
dv and pr cells in Lepidoptera project to the medulla as
long visual fibers, whereas ap and ob cells terminate in the
lamina (Ribi, 1987
;
Bandai et al., 1992
;
Shimohigashi and Tominaga,
1986
,
1991
,
1999
). Wavelength
discrimination in flies may be mediated by receptors giving rise to long
visual fibers (Strausfeld and Lee,
1991
). Although more recent analysis indicates that other
receptors in the Papilio retinula must be involved in wavelength
discrimination (Kelber, 1999
),
the original hypothesis of Arikawa and Uchiyama may be relevant to the visual
system of Manduca, especially if it represents a simpler ancestral
organization than that of Papilio. The spectral sensitivity of
spontaneous foraging in Manduca peaks in the blue, has a low shoulder
in the green and cuts off sharply at 400 nm
(Cutler et al., 1995
). The
addition of UV wavelengths to mock flowers or illuminated feeding stations
hinders foraging (White et al.,
1994
). These features of the spectral sensitivity of flower
visitation may arise from neuronal interactions in the medulla that combine a
large positive component from the blue-sensitive dv cells, a lesser
contribution from the small, green-sensitive pr cells and an
antagonistic influence from the UV-sensitive dv receptors.
Arikawa, Stavenga and associates
(Arikawa and Stavenga, 1997;
Arikawa et al., 1999b
;
Qiu et al., 2002
; Stavenga,
2002a
,b
)
have argued that the heterogeneous retinal organization of butterfly eyes is
associated with color vision; a sensory modality likely to be localized to the
ventral retina in some species of these diurnal nectar feeders
(Kinoshita et al., 1999
). We
have shown a similar heterogeneity in the retinal mosaic of Manduca.
Although Manduca's spontaneous foraging behavior demonstrates only
wavelength discrimination, is true color vision also a possibility? Perhaps,
because Kelber et al. (2002
)
have recently found that the hawkmoth Deilephila elpenor can employ
color vision for foraging under nocturnal light intensities. The fine-grained
map of photoreceptor distribution across the Manduca retina that we
have presented here will benefit further investigation into the remarkable
capacities of scotopic vision in hawkmoths.
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Appendix: calculation of ratios of P520, P450 and P357 in domains of the yellow retina |
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The volume of rhabdomeres in cells expressing P520
(V520) is the same in all domains of the main retina and
is the sum of the rhabdomere volumes from
Fig. 3 for the ap, ob
and pr cells (Vap, Vob and
Vpr, respectively). As calculated for a population of 100
retinulae:
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The corresponding volumes of rhabdomeres containing P450 and P357 retinulae
in a particular retinal domain are represented by the rhabdomere volume of a
dv cell (Vdv) from
Fig. 3 multiplied by the
relative numbers of dv cells
(Table 2) expressing P450 or
P357 in 100 retinulae of that domain. Thus, for the dorsal domain
(Fig. 10B):
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Rhodopsin ratios for each domain are estimated from the proportions of
rhabdomere volumes:
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Dorsal domain: P520:P450:P357=80:7:13. Dorsal domain just above the equator: P520:P450:P357=73:7:20.
Ventral domain just below the equator: P520:P450:P357=69:20:10.
Ventral domain: P520:P450:P357=67:23:10.
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Acknowledgments |
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References |
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