Expression of <Emphasis Type="Italic">defective proventriculus</Emphasis> during head capsule development is conserved in <Emphasis Type="Italic">Drosophila</Emphasis> and stalk-eyed flies (<Emphasis Type="Italic">Diopsidae</Emphasis>)

Hypercephaly, in the form of lateral extensions of the head capsule, is observed in several families of Diptera. A particularly exaggerated form is found in Diopsid stalk-eyed flies, in which both eyes and antennae are laterally displaced at the end of stalks. The processes of early development and specification of the head capsule in stalk-eyed flies are similar to those in Drosophila melanogaster. In Drosophila the homeobox gene ocelliless (oc) shows a mediolateral gradient of expression across the region of the eye-antennal imaginal disc that gives rise to the head capsule and specifies the development of different head structures. The genes and developmental mechanisms that subsequently define head shape in Drosophila and produce hypercephaly in stalk-eyed flies remain unclear. To address this, we performed an enhancer trap screen for Drosophila genes expressed in the same region as oc and identified the homeobox gene defective proventriculus (dve). In the eye-antennal imaginal disc, dve is coexpressed with oc in the region that gives rise to the head capsule and is active along the medial edge of the antennal disc and in the first antennal segment. Analyses of dve expression in mutant eye-antennal discs are consistent with it acting downstream of oc in the development of the head capsule. We confirm that orthologues of dve are present in a diverse panel of five stalk-eyed fly species and analyse patterns of dve sequence variation within the clade. Our results indicate that dve expression and sequence are both highly conserved in stalk-eyed flies.M. Carr and I. Hurley contributed equally to this work.     

Wing shape,wing size,and sexual dimorphism in eye‐span in stalk‐eyed flies (Diopsidae)

The eyes of stalk‐eyed flies (Diopsidae) are positioned at the end of rigid peduncles (‘stalks’) protruding laterally from the head. Eye‐stalk length varies within the family and, in some species, varies between males and females. Larger eye‐stalks in males result from sexual selection for longer stalks, a trait that increases male reproductive success. In the present study, we examined whether an increase in eye‐stalk length results in an adjustment of wing size and shape to deal with the burden of bearing an exaggerated ‘ornament’. We compared wing morphology among ten species of stalk‐eyed flies that differ in eye‐span and the degree of sexual dimorphism. Mass‐specific wing length differed between males and females in seven out of the ten species. Nondimensional wing shape parameters differed between the species (P < 0.001), but mostly did not differ between males and females of the same species. Dimorphism in eye‐span closely correlated with dimorphism in wing length (r = 0.89, P < 0.001) and the correlation remained significant (r = 0.81, P = 0.006) after correcting for phylogenetic relationships. Once corrected for phylogenetic relatedness, the mass‐specific wing length of males (but not females) was weakly correlated with mass‐specific eye‐span (r = 0.66, P = 0.042). We propose that the observed proportional increase in wing length associated with increased eye‐span can facilitate aerial manoeuverability, which would otherwise be handicapped by the elevated moment of inertia imposed by the wider head. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 860–871.     

The early stages of ommatidial development in the flour beetle Tribolium castaneum (Coleoptera; Tenebrionidae)

 Using electron microscopy, the first stages of ommatidial development in the flour beetle Tribolium castaneum were analysed in relation to the cellular architecture of the adult compound eye and were compared to the corresponding patterning process in the fruit fly Drosophila melanogaster. The ommatidia of the slightly horse-shoe shaped beetle compound eye contain six peripheral and two central retinula cells. The rhabdomere of the posteriorly located central photoreceptor cell is restricted to the distal half of the rhabdom whilst that of the anterior one is restricted to its proximal half. The development of the compound eye takes place in an external eye imaginal disc. Most stages of ommatidial development, as known from Drosophila, i.e. arc-like cell groups, five-cell clusters, immature eight-cell clusters and symmetrical eight-cell clusters, are very precisely conserved between the two species. Two major differences exist: 1. In Tribolium, the cone cell precursor cells synchronously join to the immature eight-cell cluster. As a consequence, the symmetrical eight-cell cluster immediately transforms into a four-cone-cell cluster. 2. The maturing ommatidia do not undergo rotation in Tribolium. Overall, no morphological indiation for an equator in the adult Tribolium compound eye could be found. Considering the strong evolutionary conservation of early ommatidial development, homology of photoreceptor cells of distantly related insects is proposed to be inferred from their ontogenetic origin. Received: 6 November 1995 / Accepted: 9 April 1996     

Fate map of the eye-antennal imaginal disc of the tumorous-head mutant of Drosophila melanogaster

In specific genetic backgrounds, a mutation in the tuh-3 gene results in the homeotic transformation of head structures to either leg disc derivatives or structures normally found in the extreme posterior end of wild-type animals. The origins of the homeotic structures were mapped to defined positions in the eye-antennal imaginal disc by transplanting abnormal regions of discs isolated from tuh-3 mutants into host mwh;e4 larvae. These metamorphosed implants were removed and differentiated structures were identified. Of 211 successfully recovered implants, 157 gave rise to homeotic tissue: abdominal tergite, male or female external genitalia and/or leg tissue. Transformations to abdominal tergite occurred primarily in cells taken from the eye region of the compound disc. Male and female genitalia arose most often in implants taken from the antennal portion of the disc, although some tissue taken from the lateral region of the eye disc also gave rise to external genitalia. Leg structures came exclusively from implants from the antennal region of the imaginal disc. These results suggest that cells from within specific regions of the eye-antennal compound disc are constrained in their developmental potential. An obvious constraint observed with this mutation is a dorsal/ventral one: Cells from the eye disc, a dorsal structure, primarily gave rise to other dorsal structures, abdominal tergite tissue. Cells from the antennal disc, a ventrally derived structure, primarily gave rise to other ventral structures including genital tissue and distal leg.     

Fate map of the eye-antennal imaginal disc in the stalk-eyed fly <Emphasis Type="Italic">Cyrtodiopsis dalmanni</Emphasis>

Hypercephaly, in the form of lateral extensions of the head capsule, is observed in several families of Diptera. A particularly extreme form is found in diopsid stalk-eyed flies, in which both eyes and antennae are laterally displaced at the end of eyestalks. We have studied the developmental basis of this exaggerated morphology in Cyrtodiopsis dalmanni. Diopsid eye-antennal imaginal discs are divided into anterior and posterior portions, which are joined by a narrow "disc-stalk" of intervening tissue. We established a fate map for this disc by cutting it into fragments and culturing them in vivo by injecting them into host larvae. The adult eye and dorsal head capsule structures, including the eyestalk and the ocelli, are derived from the posterior portion of the disc, while ventral adult structures such as the antenna and the palpus are derived from the anterior portion of the disc. Thus both posterior and anterior disc portions give rise to structures that are widely separated in the adult head. Moreover, structures that are adjacent in the adult are derived from different regions of the disc. These results confirm and extend previous conclusions about regional identity in diopsid eye-antennal discs that were based on the analysis of molecular markers.     

Induction of reproductive diapause via perception of photoperiod through the compound eyes in the adult blow fly, Protophormia terraenovae

Covering and surgical removal of the compound eyes were performed to localize photoreceptors for photoperiodic responses in the blow fly Protophormia terraenovae (Diptera, Calliphoridae). Adult females showed a long-day photoperiodic response to control reproductive diapause. When the compound eyes were bilaterally covered with silver paint, diapause incidence increased under diapause-preventing conditions, i.e., a long-day photoperiod and constant light, as though flies were kept under constant darkness. Neither silver painting on a medial region of the head capsule nor control painting in which both compound eyes were painted in a clear solvent caused significant effects on diapause incidence. Unilateral painting of the compound eye caused an increment of diapause incidence under constant light but no effects under a long-day photoperiod. When the compound eyes were bilaterally removed, all the flies developed their ovaries both under a long-day and a short-day photoperiod. Unilateral removal of the compound eye also caused ovarian development under a short-day photoperiod, whereas removal of one antennal lobe or all ocelli caused no effects on diapause incidence. Since P. terraenovae completely lost responsiveness to photoperiod after blinding of the compound eyes, it is likely that this fly perceives photoperiod through its compound eyes. Accepted: 18 February 1997     

Size matters,but so does shape: quantifying complex shape changes in a sexually selected trait in stalk‐eyed flies (Diptera: Diopsidae)

The elaborate morphologies of sexually selected ornaments are ubiquitous across the animal kingdom. In studies investigating these traits, ornament size is frequently the focus, and empirical evidence supports its positive correlation with fitness. Yet shape plays an important role and, surprisingly, is often overlooked. Shape frequently changes with size, influenced by biomechanical, developmental, or performance constraints. Therefore, shape can provide additional insights into the morphological differences between individuals and the potential limits on sexual trait exaggeration. Here, we used landmark‐based geometric morphometric methods on a sexually dimorphic species of stalk‐eyed fly (Teleopsis dalmanni) to examine patterns of sexual shape dimorphism. Our analyses reveal a significant difference in head shape between the sexes, with males exhibiting smaller eye bulbs, thinner stalks, and smaller heads than females. Additionally, as eyestalk length increases within each sex, a similar pattern of shape change was observed as that observed between sexes. This pattern of shape change may be the result of constraints acting against further ornament exaggeration, and we suggest that this may significantly impact the whole‐organism performance in stalk‐eyed flies. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 104–113.     

Developmental changes of the antenna and its neurons in the silkworm,Bombyx mori,with special regard to larval-pupal transformation

The larval antenna of Bombyx mori has 13 sensilla and about 52 sensory neurons in its distal portion. The axons form two nerve cords which unite in the cranial hemocoel to supply the brain as the olfactory nerve. The antennal imaginal disc, which is a thick pseudostratified epithelium continuous with the antennal epidermis, thickens markedly during the 5th instar by rapid cell proliferation. At the prepupal stage cell proliferation ceases and the disc everts to form a large pupal antenna. Simultaneously, an extensive cell rearrangement occurs in the antennal epidermis and the disc tissue becomes much thinner because of the abrupt expansion of antennal surface area. The two larval nerve cords thin down markedly by degeneration of axons, but they do not disintegrate totally even after the onset of pupation. The epidermis of the larval antenna forms the distal portion of the pupal antenna, while the imaginal disc forms the more basal portion. Development to the adult antenna occurs almost immediately after the onset of pupation; many adult neurons appear in the simple epidermis facing toward the thick outer side of the newly formed pupal cuticle. By 12 hours after the onset of pupation, these neurons align themselves in many transverse rows which are the first sign of the adult antennal configuration. Addition of these neuronal axons to the once-thinned nerve cords causes resumed thickening of the cords during the first 24 hours and thereafter. Differentiation of adult sensilla begins in the next 24 hours and is almost completed at the third day of pupation, which requires a total of 10 days.     

The role of the peripodial membrane in the morphogenesis of the eye-antennal disc ofDrosophila melanogaster

Summary The early morphogenesis of the eye-antennal disc ofDrosophila in response to 20-hydroxy ecdysone involves the curling of the eye anlagen dorsally over the antenna. During this process, the area of the peripodial membrane is substantially reduced. The peripodial membrane is taut at this stage, and if it is cut the curling of the disc cannot continue, and the eye anlagen returns to its original position within one minute of the operation. In contrast, cutting the columnar epithelium between the eye and antennal anlagen does not disrupt curling, but actually facilitates it. During curling, the cells of the peripodial membrane appear healthy, and exhibit basal extensions. We suggest that the curling of the eye is mediated by the conversion of cuboidal peripodial membrane cells into pseudostratified columnar epithelium at the edges of the peripodial membrane. Subsequently, cells of the peripodial membrane secrete first a pupal cuticle, and then an imaginal cuticle.     

Sequential Analysis Reveals Behavioral Differences Underlying Female‐Biased Predation Risk in Stalk‐Eyed Flies

Stalk‐eyed flies are classic models of how sexual selection can drive morphological and behavioral elaboration. Exaggerated ornaments born by stalk‐eyed flies could impose locomotor costs and increase susceptibility to predation; however, a previous study determined that behavior, not eye span, was the major influence on predation risk. Despite the importance of behavior, relatively little is known about how these flies avoid and deter predators. We created an ethogram of behaviors and used it to score individual interactions of male and female Teleopsis dalmanni paired with an actively foraging, generalist arachnid predator (Phidippus audax). Sequential analysis was employed to identify temporal patterns in behavior and determine how males and females differ in their approaches to avoiding predation. Our results indicate that males and females significantly differ when specific behaviors were employed. Patterns in the behavioral transitions suggest that males are more aggressive than females and are more likely to approach a predator to jab, abdomen bob, or display. Males elicited more retreat responses from the predator, whereas females elicited more attacks. Although the behavioral repertoires of male and female stalk‐eyed flies are indistinguishable, their uses of the behaviors differ, particularly the sequential order of presentation, suggesting a strong sex difference in anti‐predatory behavior.     

Functional stability of the aristaless gene in appendage tip formation during evolution

The appendages of an insect are subdivided into distinct segments or podomeres. Many genes responsible for the regionalization of the growing limb into subdomains have been isolated from Drosophila. So far, only one gene is known in the leg that is solely required for specifying the distal-most pattern element—the pretarsal claw. In Drosophila, the gene aristaless is expressed in the centre of the antennal and leg imaginal disc that represents the most distal position of appendages, and in a proximal region. When Drosophila aristaless function is impaired, antennae and legs develop without their distal-most structures—the arista and the claw. We describe here the analysis of aristaless in the beetle Tribolium—an insect that shows a different, more ancestral mode of appendage formation than Drosophila. In Tribolium, appendages grow out continuously during embryogenesis, and no imaginal discs are formed. Tribolium aristaless (Tc-al) expression starts midway during appendage elongation, and is seen in a distal and a proximal position of head and trunk appendages. At the end of embryogenesis, Tc-al is seen in four expression domains in the leg, in the dorsal epidermis, and ventrally in every segment in lateral groups of cells, presumably the histoblasts. Like in the Drosophila adult, Tc-al is required in the larva for the formation of the most distal structures of the leg and the antenna as revealed by RNAi experiments. We conclude that aristaless is evolutionarily robust, meaning that it has retained its expressional and functional characteristics, although a heterochronic change of the process of appendage elongation took place towards the evolution of the highly derived diptera.Edited by D. Tautz     

Occurrence and genetics of black‐eyed migratory locusts,Locusta migratoria (Orthoptera: Acrididae)

Black‐eyed Locusta migratoria appeared in albino locusts as a result of crossing between a short‐winged strain originating from Tsushima Island, Japan, and an albino strain originating from Okinawa Island. The black eye trait was recessive to the white eye trait because the crosses between black‐ and white‐eyed albino locusts produced only individuals with white eyes in the F₁ generation. In the F₂ generation, black‐ and white‐eyed individuals appeared in a ratio of 1:3, indicating that the black eye trait was controlled by a simple Mendelian unit. The black eye trait showed no genetic association with other traits including wing morph, adult body dimensions and classical morphometric ratios such as hind femur length / head width and forewing length / hind femur length.     

Gross and fine structure of the antennal circulatory organ in cockroaches (Blattodea,Insecta)

The antennal circulatory organ of Periplaneta americana and Blaberus craniifer was investigated by light and electron microscopy. This organ consists of two pulsatile ampullae located near the antennal base which are interconnected by a large transverse muscle and associated blood vessels which run into the antennae. Diastole is caused simultaneously in both ampullae by the transverse muscle. Systole is produced passively by the elasticity of the wall of the ampullae and minute accessory tendons. Both elastic structures contain fine unbanded extracellular filaments. The antennal vessels possess two distinct regions: a proximal convoluted region lying within the hemocoel of the head and a narrower distal region running through the antenna and opening near the antennal apex. The length of the proximal portion increases markedly during ontogeny in correlation with the growing antenna. Its wall consists of a high-prismatic epithelium ensheathed by a thick layer of collagen fibrils. The structure of the wall cells is comparable to that found in some salt transporting epithelia: it shows a polar organization with basal infoldings, a large number of mitochondria, and typical arrangement of the junctions or mitochondrial-scalariform junctional complexes. The possible physiological function of this epithelium in ionic or osmoregulation of the hemolymph entering the antenna is discussed. The wall of the distal vessel region consists of a flat single-layered epithelium and seems to be specialized only for delivery of hemolymph to antennae. The structure and function of the antennal heart in cockroaches is compared to that found in other insects.     

Opposing interactions between homothorax and Lobe define the ventral eye margin of Drosophila eye

Patterning in multi-cellular organisms involves progressive restriction of cell fates by generation of boundaries to divide an organ primordium into smaller fields. We have employed the Drosophila eye model to understand the genetic circuitry responsible for defining the boundary between the eye and the head cuticle on the ventral margin. The default state of the early eye is ventral and depends on the function of Lobe (L) and the Notch ligand Serrate (Ser). We identified homothorax (hth) as a strong enhancer of the L mutant phenotype of loss of ventral eye. Hth is a MEIS class gene with a highly conserved Meis-Hth (MH) domain and a homeodomain (HD). Hth is known to bind Extradenticle (Exd) via its MH domain for its nuclear translocation. Loss-of-function of hth, a negative regulator of eye, results in ectopic ventral eye enlargements. This phenotype is complementary to the L mutant phenotype of loss-of-ventral eye. However, if L and hth interact during ventral eye development remains unknown. Here we show that (i) L acts antagonistically to hth, (ii) Hth is upregulated in the L mutant background, and (iii) MH domain of Hth is required for its genetic interaction with L, while its homeodomain is not, (iv) in L mutant background ventral eye suppression function of Hth involves novel MH domain-dependent factor(s), and (v) nuclear localization of Exd is not sufficient to mediate the Hth function in the L mutant background. Further, Exd is not a critical rate-limiting factor for the Hth function. Thus, optimum levels of L and Hth are required to define the boundary between the developing eye and head cuticle on the ventral margin.     

Effects of ornamentation and phylogeny on the evolution of wing shape in stalk‐eyed flies (Diopsidae)

Exaggerated male ornaments are predicted to be costly to their bearers, but these negative effects may be offset by the correlated evolution of compensatory traits. However, when locomotor systems, such as wings in flying species, evolve to decrease such costs, it remains unclear whether functional changes across related species are achieved via the same morphological route or via alternate changes that have similar function. We conducted a comparative analysis of wing shape in relation to eye‐stalk elongation across 24 species of stalk‐eyed flies, using geometric morphometrics to determine how species with increased eye span, a sexually selected trait, have modified wing morphology as a compensatory mechanism. Using traditional and phylogenetically informed multivariate analyses of shape in combination with phenotypic trajectory analysis, we found a strong phylogenetic signal in wing shape. However, dimorphic species possessed shifted wing veins with the result of lengthening and narrowing wings compared to monomorphic species. Dimorphic species also had changes that seem unrelated to wing size, but instead may govern wing flexion. Nevertheless, the lack of a uniform, compensatory pattern suggests that stalk‐eyed flies used alternative modifications in wing structure to increase wing area and aspect ratio, thus taking divergent morphological routes to compensate for exaggerated eye stalks.     

Sexual dimorphism in wing beat frequency in relation to eye span in stalk‐eyed flies (Diopsidae)

Although male ornaments may provide benefits to individuals bearing them, such structures may also entail fitness costs. Selection should favour aspects of the phenotype that act to reduce such costs, yet such compensatory traits are often ignored in studies of sexual selection. If a male ornament increases predation risk via reduced locomotor performance, then there may be selection for changes in morphological traits to compensate for behavioural or biomechanical changes in how individuals use their morphology (or both). We took a comparative approach aiming to test whether changes in wing beat frequency are evolutionarily correlated with increases in male ornamentation across stalk‐eyed fly species. Previous studies have shown that increased male eye span is evolutionarily correlated with increased wing size; thus, we tested whether there is additional compensation via increases in size‐adjusted wing beat frequency. The results obtained revealed that relative wing beat frequency is negatively related to relative eye span in males, and sexual dimorphism in wing beat frequency is negatively related to dimorphism in eye span. These findings, in addition to our finding that eye span dimorphism is positively related to aspect ratio dimorphism, suggest that male stalk‐eyed flies compensate primarily by increasing wing size and shape, which may then have resulted in the subsequent evolutionary reduction in wing beat frequency. Thus, exaggerated ornaments can result in evolutionary modifications in wing morphology, which in turn lead to adjustments in flapping kinematics, illustrating the tight envelope of trade‐offs when compensating for exaggerated ornaments. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 670–679.     

The hernandez and fernandez genes of Drosophila specify eye and antenna

The formation of different structures in Drosophila depends on the combined activities of selector genes and signaling pathways. For instance, the antenna requires the selector gene homothorax, which distinguishes between the leg and the antenna and can specify distal antenna if expressed ectopically. Similarly, the eye is formed by a group of "eye-specifying" genes, among them eyeless, which can direct eye development ectopically. We report here the characterization of the hernandez and fernandez genes, expressed in the antennal and eye primordia of the eye-antenna imaginal disc. The predicted proteins encoded by these two genes have 27% common amino acids and include a Pipsqueak domain. Reduced expression of either hernandez or fernandez mildly affects antenna and eye development, while the inactivation of both genes partially transforms distal antenna into leg. Ectopic expression of either of the two genes results in two different phenotypes: it can form distal antenna, activating genes like homothorax, spineless, and spalt, and it can promote eye development and activates eyeless. Reciprocally, eyeless can induce hernandez and fernandez expression, and homothorax and spineless can activate both hernandez and fernandez when ectopically expressed. The formation of eye by these genes seems to require Notch signaling, since the induction of ectopic eyes and the activation of eyeless by the hernandez gene are suppressed when the Notch function is compromised. Our results show that the hernandez and fernandez genes are required for antennal and eye development and are also able to specify eye or antenna ectopically.     

Timing of Wingless signalling distinguishes maxillary and antennal identities in Drosophila melanogaster

The Drosophila adult head mostly derives from the composite eye-antenna imaginal disc. The antennal disc gives rise to two adult olfactory organs: the antennae and maxillary palps. Here, we have analysed the regional specification of the maxillary palp within the antennal disc. We found that a maxillary field, defined by expression of the Hox gene Deformed, is established at about the same time as the eye and antennal fields during the L2 larval stage. The genetic program leading to maxillary regionalisation and identity is very similar to the antennal one, but is distinguished primarily by delayed prepupal expression of the ventral morphogen Wingless (Wg). We find that precociously expressing Wg in the larval maxillary field suffices to transform it towards antennal identity, whereas overexpressing Wg later in prepupae does not. These results thus indicate that temporal regulation of Wg is decisive to distinguishing maxillary and antennal organs. Wg normally acts upstream of the antennal selector spineless (ss) in maxillary development. However, mis-expression of Ss can prematurely activate wg via a positive-feedback loop leading to a maxillary-to-antenna transformation. We characterised: (1) the action of Wg through ss selector function in distinguishing maxillary from antenna; and (2) its direct contribution to identity choice.