Single locus affects embryonic segment polarity and multiple aspects of an adult evolutionary novelty
© Saenko et al. 2010
Received: 30 June 2010
Accepted: 26 August 2010
Published: 26 August 2010
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© Saenko et al. 2010
Received: 30 June 2010
Accepted: 26 August 2010
Published: 26 August 2010
The characterization of the molecular changes that underlie the origin and diversification of morphological novelties is a key challenge in evolutionary developmental biology. The evolution of such traits is thought to rely largely on co-option of a toolkit of conserved developmental genes that typically perform multiple functions. Mutations that affect both a universal developmental process and the formation of a novelty might shed light onto the genetics of traits not represented in model systems. Here we describe three pleiotropic mutations with large effects on a novel trait, butterfly eyespots, and on a conserved stage of embryogenesis, segment polarity.
We show that three mutations affecting eyespot size and/or colour composition in Bicyclus anynana butterflies occurred in the same locus, and that two of them are embryonic recessive lethal. Using surgical manipulations and analysis of gene expression patterns in developing wings, we demonstrate that the effects on eyespot morphology are due to changes in the epidermal response component of eyespot induction. Our analysis of morphology and of gene expression in mutant embryos shows that they have a typical segment polarity phenotype, consistent with the mutant locus encoding a negative regulator of Wingless signalling.
This study characterizes the segregation and developmental effects of alleles at a single locus that controls the morphology of a lineage-specific trait (butterfly eyespots) and a conserved process (embryonic segment polarity and, specifically, the regulation of Wingless signalling). Because no gene with such function was found in the orthologous, highly syntenic genomic regions of two other lepidopterans, we hypothesize that our locus is a yet undescribed, possibly lineage-specific, negative regulator of the conserved Wnt/Wg pathway. Moreover, the fact that this locus interferes with multiple aspects of eyespot morphology and maps to a genomic region containing key wing pattern loci in different other butterfly species suggests it might correspond to a 'hotspot' locus in the diversification of this novel trait.
The origin and diversification of novel morphological traits, such as angiosperm flowers, bird feathers, insect wings, or beetle horns, have always fascinated biologists and laymen alike and are currently a key theme in evolutionary developmental biology [1, 2]. Novelties arise during the evolution of a lineage and perform new functions within its ecology . While the ecological and evolutionary factors that promote the diversification of such traits have been studied for some time (e.g., [4, 5]), it is only more recently that the genetic and developmental mechanisms underlying their formation have become the focus of attention. Morphological novelties seem to arise largely through redeployment, or co-option, of conserved developmental toolkit genes (e.g., [6–9]), though recent studies suggest that taxonomically restricted genes might also be important .
Lepidoptera (the insect order of butterflies and moths) provide several examples of lineage-restricted traits that presumably evolved by co-option of genes or gene networks shared across insects. Signalling pathways and enzymes involved in the development of insect wings, sensory bristles or visual pigments have been implicated in the development [11, 12] and coloration [13, 14] of the scales that cover lepidopteran wings, and in the formation of particular pattern elements, the eyespots (reviewed in ). Eyespots have emerged as a promising model to investigate how novel characters arise [16–18], and which genes and components of pattern induction are modified in relation to phenotypic variation [19–22]. Pattern elements made of concentric rings of different colours are present on the wings of many butterflies and moths and show extreme intra- and interspecific variation in colour, size, shape and number [23, 24]. Their adaptive role in predator avoidance [25–27] and mate choice [28, 29] has been demonstrated in different species, and the mechanistic basis of eyespot diversity is now the focus of active research in evolutionary developmental biology [15, 30].
Classical surgical manipulations of developing wings, such as tissue transplantation and damage, showed that eyespot centres, called foci, act as organizers of pattern formation [31, 32]. In pupal wings, the focal cells presumably produce one or more diffusible morphogens, and the neighbouring epidermal cells respond to these signals in a concentration-dependent manner and become fated to synthesize particular pigments [23, 33]. A number of conserved genes and signalling pathways with known functions in insect wing development have been implicated in the determination of eyespot foci [16, 34] and colour rings [17, 35]. Little, however, is known about which particular genes contribute to variation in eyespot morphology  and how these affect the signal/response components of eyespot induction.
Captive populations of the lab model Bicyclus anynana harbour different types of genetic variation affecting eyespot patterns , including spontaneous mutations of large phenotypic effect , some of which have been assigned to linkage groups . Analysis of such mutations offers an opportunity to characterize the genetic and developmental bases of a novel trait not represented in classical model organisms (e.g., [17, 18, 38–41]). Of special interest are pleiotropic mutations that affect not only eyespots but also some other, conserved developmental processes. For example, veinless and Cyclops affect eyespot number and wing vein development, and Goldeneye disturbs eyespot colour composition and embryonic development [17, 18]. Here, we characterize the effects of different alleles of a single locus on various aspects of eyespot morphology and on embryonic segment polarity. Our analysis suggests that this locus is a potentially novel negative regulator of the Wnt/Wg signalling pathway and a candidate 'hotspot' for wing pattern diversification.
Segregation of aberrant embryos and adults in experimental crosses
No. adult progeny with eyespot phenotypes
χ 2 HOM [P]
χ 2 GF [P]
χ 2 HOM [P]
χ 2 GF [P]
BE × BE
Fr × Fr
Spr × Spr
BE2 × BE2
BE3 × BE3
BE × WT
Fr × WT
Spr × WT
BE3 × WT
BE × Fr
BE × Spr
BE3 × BE
The aberrant embryos in the BE × BE, Fr × Fr, or Spr × Spr crosses showed severe and very similar morphological defects, suggesting that all three mutations are alleles of the same locus. This was confirmed by complementation tests: crosses between BE and either Fr or Spr mutants yielded embryonic lethality in approximately one fourth of the offspring (23.9% of 4645 progeny in 33 families), with identical morphological aberrations as those found in the offspring of crosses between mutants of the same phenotype. In contrast, no embryonic lethality or aberrant morphology was observed in 1756 progeny of 13 crosses between BE and Goldeneye individuals. This shows that these two mutations occurred in different genes, which is also consistent with the fact that the embryonic phenotype caused by the Goldeneye allele (i.e., disturbed blastokinesis ) is different from the embryonic effects produced by the alleles underlying the BE, Fr or Spr phenotypes (see "Analysis of aberrant embryos reveals defects in segment polarity" section).
Dramatic effects on eyespot morphology in BE, Fr and Spr mutants seem to be caused by different dominant alleles at the same locus, each disturbing embryonic development in homozygotes. However, the inheritance mode of the Spr phenotype appears to be more complex. The effect of the underlying allele on eyespot colour composition but not size is recessive, since offspring from Spr × WT crosses either have 'wild-type' eyespots or large eyespots with 'normal' colour scheme (46.7 and 53.3%, respectively, in 1272 adult offspring from nine families; Table 1). The latter phenotype is indistinguishable from that of BE individuals and is hereafter referred to as BE2. A similar phenotype, hereafter called BE3, is found in the 'non-Spr' progeny from Spr × Spr crosses (34.8% of 440 adults in 13 families). Single-pair crosses were set up to determine whether enlarged eyespots in BE individuals and in the offspring of Spr × WT (BE2 individuals) and Spr × Spr crosses (BE3 individuals) were caused by the same or by different alleles (Table 1).
Similarly to BE × BE, crosses between two BE2 individuals yielded embryonic lethality in approximately one fourth of the progeny (22.9% of 1488 progeny in eight families) and enlarged eyespots in two thirds of the adults (64.8% of 429 adult offspring). These results are consistent with BE and BE2 phenotypes being due to the same embryonic lethal allele with a dominant effect on eyespot size. In contrast, only a small fraction of embryos from crosses between two BE3 individuals died before hatching and showed typical morphological defects (Table 1). This ratio was significantly lower than the expected 25% (χ 2 test of homogeneity among four families = 10.78, P < 0.05; χ 2 goodness-of-fit for each family = 53.05, 22.00, 29.37, 42.29, all with P < 0.05) and varied between 0 and 5.3%, perhaps due to modifier loci or incomplete penetrance. No embryonic lethality was observed in five of the seven crosses between BE3 and BE individuals. The fraction of aberrant embryos in the other two families (10.8 and 1.1%) was significantly lower than the 25% expected if the BE3 and BE phenotypes were produced by the same lethal allele (χ 2 test for homogeneity among seven families = 90.98; χ 2 goodness-of-fit in two families = 20.79 and 7.55; P < 0.01). This suggests that the mutation underlying the BE3 phenotype is a different allele. It has a mildly deleterious effect on embryogenesis (i.e., low incidence of embryonic mortality) and a recessive effect on eyespot size, since all offspring of the BE3 × WT crosses (i.e., BE3 heterozygotes) have 'wild-type' eyespots (Table 1).
Compared to WT embryos of the same age, homozygotes for each of the lethal alleles displayed severe and similar abnormalities. They were much shorter and thicker than WT embryos (Figure 1b). The typical 3 thoracic and 10 abdominal segments were all present, as was clear from the number of thoracic and abdominal appendages, but the segments were compressed and their borders poorly defined. Dorsal closure was not completed in approximately 30% of the embryos. The thoracic legs and the mouthparts (arrows and arrowheads in Figure 1b) were short; some embryos were missing one leg while one of the remaining legs was branched. We observed variation in timing of death among mutants. Typically, embryos from Spr × Spr crosses died before the stage when bristles appear (~70% of developmental time, DT ), and looked more compacted, while embryos from crosses between BE or Fr butterflies were fully sclerotized and had melanized head capsules, and died at approximately 90% DT. The severe shortening of segments and poorly defined segment boundaries observed in all mutant embryos suggest that the mutations affect the structure of each segment, rather than their establishment, possibly by interfering with the normal function of segment polarity genes.
These results show that the lethal alleles at the BFS locus do not affect the specification of segment number or the establishment of en and wg expression domains in embryonic segments, but rather disturb their correct maintenance and generate a typical segment polarity phenotype, i.e., the 'replacement' of the anterior part of each segment by a mirror image duplication of the posterior part . In Drosophila melanogaster, identical embryonic defects are caused by loss-of-function mutations, or experimental knockdown, of one of the negative regulators of Wg signalling; zeste-white 3 , naked cuticle , axin , APC2 , or Bili .
The formation of eyespots in developing pupal wings involves the production and diffusion of a morphogen from the cells at the center (stage called 'focal signalling') and the response of the surrounding epidermal cells [31, 32]. Different levels of the signal induce expression of different regulatory genes, e.g., Distall-less (Dll) and spalt (sal) in the inner disc, and en in the outer ring , which then directly or indirectly control biosynthesis of different pigments. Changes in the focal signal (e.g., morphogen concentration, stability or diffusion coefficient) have been shown to explain most of the variation in eyespot size between artificial selection lines , while the sensitivity levels of the epidermal cells to that signal determine eyespot colour composition [20, 51] and, to a lesser extent, eyespot size [21, 50]. Genetic correlations between eyespot size and colour composition are typically low [50–52], suggesting that different sets of genes determine these two aspects of eyespot morphology, and their underlying components of pattern induction.
Our study of embryonic defects and of en/wg expression in the target B. anynana mutants suggests that the BFS locus is a negative regulator of the Wnt/Wg signalling pathway. Analysis of eyespot formation in Spr wings further suggests that the product of this gene acts upstream of eyespot ring patterning transcription factors during the 'response to focal signal' stage. The Wg morphogen, implicated in the evolution of wing pigmentation spots in Drosophila , has been proposed as a candidate focal signal in butterflies . It is produced by eyespot centres in early pupal wings of B. anynana , and is known to upregulate en and Dll in insect embryos and imaginal wing discs, respectively [45, 55]. In butterfly wings, Wg could act as the inducer of the circular Dll and en domains corresponding to eyespot rings in adults . Mutations in a negative regulator of the Wnt/Wg pathway, as could be the BFS gene, would affect the response of epidermal cells to this signal, altering en and Dll expression and therefore the distribution of black and gold scales. Molecular identification and functional characterization of the BFS locus, including the analysis of expression of key components of Wg signalling in both normal and altered eyespot development, will be necessary to assess its role in the regulation of this signalling pathway.
In a previous study, we mapped the BFS locus to approximately a 1.3 Mb interval on B. anynana chromosome 17, which has a high level of synteny with its ortholog in Bombyx mori , the model lepidopteran with the fully sequenced and annotated genome. Curiously, the orthologous chromosome in the butterfly Heliconius melpomene carries colour pattern loci implicated in intra- and interspecific divergence . Inspection of gene content in the genomic regions of B. mori (nscaf2829 ) and H. melpomene , orthologous to the BFS interval, revealed that they do not contain any of the candidate genes generated via the comparison of embryonic phenotype to Drosophila mutants (namely, Axin, zeste-white 3, APC2, naked cuticle, and Bili), or any other gene with a known role in the regulation of Wg signalling. The high levels of synteny between these lepidopterans  suggest that the implicated interval in B. anynana, even though its exact genetic content is still unknown, probably also does not contain any of the described negative regulators of the Wnt/Wg pathway. This pathway is crucial for embryonic development and numerous other processes, and is remarkably conserved and extensively studied (reviewed in [58–60]). Nevertheless, sophisticated new genetic and genomic approaches continue to identify new Wnt/Wg regulators [61, 62], some of which are taxon-restricted (e.g., WTX ). The BFS locus could be another, yet unknown, negative regulator of this pathway, potentially specific to this lepidopteran lineage.
Analysis of mutant alleles of large effect is a powerful approach for exploring the developmental basis of novel traits, and especially valuable for studies in nonmodel organisms with limited genetic resources. This work describes three pleiotropic mutations in a single locus, presumably a negative regulator of Wg signalling. The product of this gene is involved in the formation of eyespots, a butterfly-specific novelty, and a relatively conserved process of embryonic development [64–66]. Different alleles at this locus produce similar effects on embryonic segment polarity, but disturb different aspects of eyespot morphology. This locus maps to a genomic interval whose orthologous regions in two other, syntenic leidopterans do not contain any annotated genes with a known role in the regulation of the Wnt/Wg pathway. This suggests that the B. anynana BFS locus might be a yet undescribed negative regulator of Wg signalling.
Future work will shed light onto the exact nature and evolutionary significance of this interesting locus. Fine mapping can reveal whether it encodes a conserved or a derived regulator of Wg signalling and to what extent it is related to colour pattern loci in other butterflies. Analysis of its contribution to intra- and interspecific variation in eyespot morphology can address its role in the evolutionary diversification of butterfly wing patterns.
All butterfly stocks were at reared 27°C as in . The BE and Fr stocks were each set up from a single individual isolated from different laboratory populations in 1994 and 2007, respectively. The Spr stock was founded from a single individual isolated from the BE stock in 2003. All mutant stocks have been maintained with selection in favour of the mutant phenotype and, when necessary to avoid inbreeding depression, were outcrossed to the laboratory outbred WT stock.
Different crosses were set up to determine the mode of inheritance of the mutant alleles (Table 1). To test whether the mutations behave as embryonic recessive lethal alleles, butterflies from each mutant stock were crossed to individuals of the same phenotype and to WT stock butterflies. Crosses between butterflies of different mutant phenotypes were performed to determine whether the mutant alleles occur in the same locus (complementation tests). Eggs were collected from individual mating pairs, hatched larvae were reared through to adulthood, and the eclosed butterflies were frozen and scored for eyespot phenotype. Unhatched eggs were counted, dechorionated in 50% bleach solution for 1 min, rinsed with water, and fixed in 4% formaldehyde solution in 1 × phosphate-buffered saline (PBS). Embryos were dissected under a light microscope. Unfertilized eggs, identified as those lacking large blastoderm cells, were excluded from the analysis.
χ2 tests were used to compare observed proportions of mutant embryos and adults to those expected if the alleles underlying the BE, Fr and Spr phenotypes were embryonic recessive lethal with dominant effect on eyespot pattern. First, we tested whether the ratios varied among families using the χ2 test of homogeneity. If ratios were not significantly different, the numbers of progeny were pooled over all families belonging to the same cross type and mutant phenotype; these overall frequencies were then tested against the expected ratios using the χ2 goodness-of-fit test. If the χ2 homogeneity test revealed significant variation among families, the frequencies of aberrant embryos and mutant adults were tested against the expected values separately in each of the families (the case for only BE3).
Eggs were collected at different times after laying, dechorionated in 50% bleach solution for 1 min, rinsed with water, fixed in 10% formaldehyde in PBS/50 mM EGTA for 30 min, dehydrated in methanol in four steps and stored at -20°C until use. For scanning electron microscopy studies, 10-20 fixed embryos of each mutant phenotype were rehydrated in PBS in four steps and mounted on specimen holders, air-dried, coated with gold and viewed in a JEOL JSM 6400 scanning electron microscope.
For the analysis of gene expression patterns, 50-100 embryos were examined for each mutant line per developmental stage, defined according to the system developed for M. sexta . Fixed embryos were rehydrated in PBS, blocked in PBT (5 mg/ml bovine serum albumin in PBS) for 2 hr, incubated with primary antibody overnight at 4°C (anti-Engrailed 4F11 ), washed 10 times in PBS with 0.1% Triton X-100 and incubated for 2 hr at 4°C in a 1:200 dilution of the secondary antibody (Alexa Fluor 488; Molecular Probes). After 10 washes in PBS, embryos were incubated in 100% glycerol for 1 hr and mounted on glass slides. Images were collected on a Bio-Rad MRC 1024 ES laser scanning confocal microscope.
A 315-bp fragment of B. anynana wingless gene (AY218276) was amplified from embryonic cDNA with primers 5'-GTCATGATGCCCAATACCG and 5'-GCAGTTGCATCGTTCCACTA and cloned into pCRII-TOPO dual-promoter vector using the TOPO TA cloning kit (Invitrogen). Plasmids were isolated with QIAprep Spin Miniprep Kit (Qiagen) and used as template for PCR reactions with vector primers M13F and M13R. The amplified products were cleaned with Wizard SV Gel and PCR Clean-Up System (Promega) and used for SP6 or T7 transcription. Sense and antisense digoxygenin-labeled riboprobes were synthesized using SP6 and T7 RNA polymerases and DIG RNA labeling mix (Roche Applied Science). The probes were run on an agarose gel and measured with NanoDrop spectrophotometer (Thermo Scientific) to verify their quality and concentration. In situ hybridization with sense and antisense probes was performed at 55°C for 48 hr following the protocol described previously . The probes were detected with NBT/BCIP (Roche Applied Science). Stained embryos were mounted on slides and photographed with a Leica DC 200 digital camera attached to a Leica MZ 125 microscope.
Dorsal surface of left pupal forewings was damaged at 12-18 h (± 30 min) after pupation, when ectopic eyespots are produced most often . Wings were pierced with a finely sharpened tungsten needle (cat. no. 501317; World Precision Instruments) at a site next to the small anterior eyespot, approximately halfway between the wing margin and the normal location of the eyespots. Operated pupae were returned to 27°C; adults were frozen soon after emergence, and their wings were photographed with a Leica DC 200 digital camera attached to a Leica MZ 125 microscope.
Pupal wings of WT and Spr individuals were stained at 16-18 h after pupation according to the protocol described previously  with antibodies against Engrailed (4F11 , 1:50 dilution), Distal-less (, 1:200 dilution) and Spalt (, 1:500 dilution). Alexa Fluor 488 and Texas Red (Molecular Probes) were used as secondary antibodies in 1:200 dilutions.
We thank Sean Carroll, Nipam Patel, and Rosa Barrio for the antibodies they kindly provided; Michael Akam for fruitful discussions and suggestions during the early stages of this work; Maurijn van der Zee for discussions and critical comments on earlier versions of the manuscript; Gerda Lamers for help with the confocal microscopy; Dirk Gassmann for scanning electron microscope images; Oskar Brattström for the image of Bicyclus taenias; and two anonymous reviewers for useful suggestions. This work was supported by grants to Patrícia Beldade from the Dutch Science Foundation NWO (VIDI 864.08.010) and the Portuguese Foundation for Science and Technology FCT (PTDC/BIA-BDE/65295/2006).
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