Eye stalks or no eye stalks: a structural comparison of pupal development in the stalk-eyed fly Cyrtodiopsis and in Drosophila

Proximal fate marker homothorax marks the lateral extension of stalk-eyed fly Cyrtodopsis whitei

The placement of eyes on insect head is an important evolutionary trait. The stalk-eyed fly, Cyrtodopsis whitei, exhibits a hypercephaly phenotype where compound eyes are located on lateral extension from the head while the antennal segments are placed inwardly on this stalk. This stalk-eyed phenotype is characteristic of the family Diopsidae in the Diptera order and dramatically deviates from other dipterans, such as Drosophila. Like other insects, the adult eye and antenna of stalk-eyed fly develop from a complex eye-antennal imaginal disc. We analyzed the markers involved in proximo-distal (PD) axis of the developing eye imaginal disc of the stalk-eyed flies. We used homothorax (hth) and distalless (dll), two highly conserved genes as the marker for proximal and distal fate, respectively. We found that lateral extensions between eye and antennal field of the stalk-eyed fly's eye-antennal imaginal disc exhibit robust Hth expression. Hth marks the head specific fate in the eye- and proximal fate in the antenna-disc. Thus, the proximal fate marker Hth expression evolves in the stalk-eyed flies to generate lateral extensions for the placement of the eye on the head. Moreover, during pupal eye metamorphosis, the lateral extension folds back on itself to place the antenna inside and the adult compound eye on the distal tip. Interestingly, the compound eye in other insects does not have a prominent PD axis as observed in the stalk-eyed fly.

The complexity of mating decisions in stalk-eyed flies

All too often, studies of sexual selection focus exclusively on the responses in one sex, on single traits, typically those that are exaggerated and strongly sexually dimorphic. They ignore a range of less obvious traits and behavior, in both sexes, involved in the interactions leading to mate choice. To remedy this imbalance, we analyze a textbook example of sexual selection in the stalk-eyed fly (Diasemopsis meigenii). We studied several traits in a novel, insightful, and efficient experimental design, examining 2,400 male-female pairs in a "round-robin" array, where each female was tested against multiple males and vice versa. In D. meigenii, females exhibit strong mate preference for males with highly exaggerated eyespan, and so we deliberately constrained variation in male eyespan to reveal the importance of other traits. Males performing more precopulatory behavior were more likely to attempt to mate with females and be accepted by them. However, behavior was not a necessary part of courtship, as it was absent from over almost half the interactions. Males with larger reproductive organs (testes and accessory glands) did not make more mating attempts, but there was a strong tendency for females to accept mating attempts from such males. How females detect differences in male reproductive organ size remains unclear. In addition, females with larger eyespan, an indicator of size and fecundity, attracted more mating attempts from males, but this trait did not alter female acceptance. Genetic variation among males had a strong influence on male mating attempts and female acceptance, both via the traits we studied and other unmeasured attributes. These findings demonstrate the importance of assaying multiple traits in males and females, rather than focusing solely on prominent and exaggerated sexually dimorphic traits. The approach allows a more complete understanding of the complex mating decisions made by both males and females.

Arthropod eyes: The early Cambrian fossil record and divergent evolution of visual systems

Four types of eyes serve the visual neuropils of extant arthropods: compound retinas composed of adjacent facets; a visual surface populated by spaced eyelets; a smooth transparent cuticle providing inwardly directed lens cylinders; and single-lens eyes. The first type is a characteristic of pancrustaceans, the eyes of which comprise lenses arranged as hexagonal or rectilinear arrays, each lens crowning 8-9 photoreceptor neurons. Except for Scutigeromorpha, the second type typifies Myriapoda whose relatively large eyelets surmount numerous photoreceptive rhabdoms stacked together as tiers. Scutigeromorph eyes are facetted, each lens crowning some dozen photoreceptor neurons of a modified apposition-type eye. Extant chelicerate eyes are single-lensed except in xiphosurans, whose lateral eyes comprise a cuticle with a smooth outer surface and an inner one providing regular arrays of lens cylinders. This account discusses whether these disparate eye types speak for or against divergence from one ancestral eye type. Previous considerations of eye evolution, focusing on the eyes of trilobites and on facet proliferation in xiphosurans and myriapods, have proposed that the mode of development of eyes in those taxa is distinct from that of pancrustaceans and is the plesiomorphic condition from which facetted eyes have evolved. But the recent discovery of enormous regularly facetted compound eyes belonging to early Cambrian radiodontans suggests that high-resolution facetted eyes with superior optics may be the ground pattern organization for arthropods, predating the evolution of arthrodization and jointed post-protocerebral appendages. Here we provide evidence that compound eye organization in stem-group euarthropods of the Cambrian can be understood in terms of eye morphologies diverging from this ancestral radiodontan-type ground pattern. We show that in certain Cambrian groups apposition eyes relate to fixed or mobile eyestalks, whereas other groups reveal concomitant evolution of sessile eyes equipped with optics typical of extant xiphosurans. Observations of fossil material, including that of trilobites and eurypterids, support the proposition that the ancestral compound eye was the apposition type. Cambrian arthropods include possible precursors of mandibulate eyes. The latter are the modified compound eyes, now sessile, and their underlying optic lobes exemplified by scutigeromorph chilopods, and the mobile stalked compound eyes and more elaborate optic lobes typifying Pancrustacea. Radical divergence from an ancestral apposition type is demonstrated by the evolution of chelicerate eyes, from doublet sessile-eyed stem-group taxa to special apposition eyes of xiphosurans, the compound eyes of eurypterids, and single-lens eyes of arachnids. Different eye types are discussed with respect to possible modes of life of the extinct species that possessed them, comparing these to extant counterparts and the types of visual centers the eyes might have served.

Building a Beetle: How Larval Environment Leads to Adult Performance in a Horned Beetle

The link between the expression of the signals used by male animals in contests with the traits which determine success in those contests is poorly understood. This is particularly true in holometabolous insects such as horned beetles where signal expression is determined during metamorphosis and is fixed during adulthood, whereas performance is influenced by post-eclosion feeding. We used path analysis to investigate the relationships between larval and adult nutrition, horn and body size and fitness-related traits such as strength and testes mass in the horned beetle Euoniticellus intermedius. In males weight gain post-eclosion had a central role in determining both testes mass and strength. Weight gain was unaffected by adult nutrition but was strongly correlated with by horn length, itself determined by larval resource availability, indicating strong indirect effects of larval nutrition on the adult beetle's ability to assimilate food and grow tissues. Female strength was predicted by a simple path diagram where strength was determined by eclosion weight, itself determined by larval nutrition: weight gain post-eclosion was not a predictor of strength in this sex. Based on earlier findings we discuss the insulin-like signalling pathway as a possible mechanism by which larval nutrition could affect adult weight gain and thence traits such as strength.

Sex-biased gene expression during head development in a sexually dimorphic stalk-eyed fly

Stalk-eyed flies (family Diopsidae) are a model system for studying sexual selection due to the elongated and sexually dimorphic eye-stalks found in many species. These flies are of additional interest because their X chromosome is derived largely from an autosomal arm in other flies. To identify candidate genes required for development of dimorphic eyestalks and investigate how sex-biased expression arose on the novel X, we compared gene expression between males and females using oligonucleotide microarrays and RNA from developing eyestalk tissue or adult heads in the dimorphic diopsid, Teleopsis dalmanni. Microarray analysis revealed sex-biased expression for 26% of 3,748 genes expressed in eye-antennal imaginal discs and concordant sex-biased expression for 86 genes in adult heads. Overall, 415 female-biased and 482 male-biased genes were associated with dimorphic eyestalk development but not differential expression in the adult head. Functional analysis revealed that male-biased genes are disproportionately associated with growth and mitochondrial function while female-biased genes are associated with cell differentiation and patterning or are novel transcripts. With regard to chromosomal effects, dosage compensation occurs by elevated expression of X-linked genes in males. Genes with female-biased expression were more common on the X and less common on autosomes than expected, while male-biased genes exhibited no chromosomal pattern. Rates of protein evolution were lower for female-biased genes but higher for genes that moved on or off the novel X chromosome. These findings cannot be due to meiotic sex chromosome inactivation or by constraints associated with dosage compensation. Instead, they could be consistent with sexual conflict in which female-biased genes on the novel X act primarily to reduce eyespan in females while other genes increase eyespan in both sexes. Additional information on sex-biased gene expression in other tissues and related sexually monomorphic species could confirm this interpretation.

Length polymorphism and head shape association among genes with polyglutamine repeats in the stalk-eyed fly, Teleopsis dalmanni

BACKGROUND

Polymorphisms of single amino acid repeats (SARPs) are a potential source of genetic variation for rapidly evolving morphological traits. Here, we characterize variation in and test for an association between SARPs and head shape, a trait under strong sexual selection, in the stalk-eyed fly, Teleopsis dalmanni. Using an annotated expressed sequence tag database developed from eye-antennal imaginal disc tissues in T. dalmanni we identified 98 genes containing nine or more consecutive copies of a single amino acid. We then quantify variation in length and allelic diversity for 32 codon and 15 noncodon repeat regions in a large outbred population. We also assessed the frequency with which amino acid repeats are either gained or lost by identifying sequence similarities between T. dalmanni SARP loci and their orthologs in Drosophila melanogaster. Finally, to identify SARP containing genes that may influence head development we conducted a two-generation association study after assortatively mating for extreme relative eyespan.

RESULTS

We found that glutamine repeats occur more often than expected by amino acid abundance among 3,400 head development genes in T. dalmanni and D. melanogaster. Furthermore, glutamine repeats occur disproportionately in transcription factors. Loci with glutamine repeats exhibit heterozygosities and allelic diversities that do not differ from noncoding dinucleotide microsatellites, including greater variation among X-linked than autosomal regions. In the majority of cases, repeat tracts did not overlap between T. dalmanni and D. melanogaster indicating that large glutamine repeats are gained or lost frequently during Dipteran evolution. Analysis of covariance reveals a significant effect of parental genotype on mean progeny eyespan, with body length as a covariate, at six SARP loci [CG33692, ptip, band4.1 inhibitor LRP interactor, corto, 3531953:1, and ecdysone-induced protein 75B (Eip75B)]. Mixed model analysis of covariance using the eyespan of siblings segregating for repeat length variation confirms that significant genotype-phenotype associations exist for at least one sex at five of these loci and for one gene, CG33692, longer repeats were associated with longer relative eyespan in both sexes.

CONCLUSION

Among genes expressed during head development in stalk-eyed flies, long codon repeats typically contain glutamine, occur in transcription factors and exhibit high levels of heterozygosity. Furthermore, the presence of significant associations within families between repeat length and head shape indicates that six genes, or genes linked to them, contribute genetic variation to the development of this extremely sexually dimorphic trait.

Genomic analysis of a sexually-selected character: EST sequencing and microarray analysis of eye-antennal imaginal discs in the stalk-eyed fly Teleopsis dalmanni (Diopsidae)

BACKGROUND

Many species of stalk-eyed flies (Diopsidae) possess highly-exaggerated, sexually dimorphic eye-stalks that play an important role in the mating system of these flies. Eye-stalks are increasingly being used as a model system for studying sexual selection, but little is known about the genetic mechanisms producing variation in these ornamental traits. Therefore, we constructed an EST database of genes expressed in the developing eye-antennal imaginal disc of the highly dimorphic species Teleopsis dalmanni. We used this set of genes to construct microarray slides and compare patterns of gene expression between lines of flies with divergent eyespan.

RESULTS

We generated 33,229 high-quality ESTs from three non-normalized libraries made from the developing eye-stalk tissue at different developmental stages. EST assembly and annotation produced a total of 7,066 clusters comprising 3,424 unique genes with significant sequence similarity to a protein in either Drosophila melanogaster or Anopheles gambiae. Comparisons of the transcript profiles at different stages reveal a developmental shift in relative expression from genes involved in anatomical structure formation, transcription, and cell proliferation at the larval stage to genes involved in neurological processes and cuticle production during the pupal stages. Based on alignments of the EST fragments to homologous sequences in Drosophila and Anopheles, we identified 20 putative gene duplication events in T. dalmanni and numerous genes undergoing significantly faster rates of evolution in T. dalmanni relative to the other Dipteran species. Microarray experiments identified over 350 genes with significant differential expression between flies from lines selected for high and low relative eyespan but did not reveal any primary biological process or pathway that is driving the expression differences.

CONCLUSION

The catalogue of genes identified in the EST database provides a valuable framework for a comprehensive examination of the genetic basis of eye-stalk variation. Several candidate genes, such as crooked legs, cdc2, CG31917 and CG11577, emerge from the analysis of gene duplication, protein evolution and microarray gene expression. Additional comparisons of expression profiles between, for example, males and females, and species that differ in eye-stalk sexual dimorphism, are now enabled by these resources.

Male sexual ornament size is positively associated with reproductive morphology and enhanced fertility in the stalk-eyed fly Teleopsis dalmanni

BACKGROUND

Exaggerated male ornaments and displays often evolve in species where males only provide females with ejaculates during reproduction. Although "good genes" arguments are typically invoked to explain this phenomenon, a simpler alternative is possible if variation in male reproductive quality (e.g. sperm number, ejaculate content, mating rate) is an important determinant of female reproductive success. The "phenotype-linked fertility hypothesis" states that female preference for male ornaments or displays has been selected to ensure higher levels of fertility and has driven the evolution of exaggerated male traits. Females of the stalk-eyed fly Teleopsis dalmanni must mate frequently to maintain high levels of fertility and prefer to mate with males exhibiting large eyespan, a condition-dependent sexual ornament. If eyespan indicates male reproductive quality, females could directly increase their reproductive success by mating with males with large eyespan. Here we investigate whether male eyespan indicates accessory gland and testis length, and then ask whether mating with large eyespan males affects female fertility.

RESULTS

Male eyespan was a better predictor of two key male reproductive traits--accessory gland and testis length--than was body size alone. This positive relationship held true over three levels of increasing environmental stress during the maturation of the adult accessory glands and testes. Furthermore, females housed with a large eyespan male exhibited higher levels of fertility than those with small eyespan males.

CONCLUSION

Male eyespan in stalk-eyed flies is subject to strong directional mate preference and is a reliable indicator of male reproductive quality--both because males with larger eyespan have bigger accessory glands and testes, and also as they confer higher fertility on females. Fertility enhancement may have arisen because males with larger eyespan mated more often and/or because they transferred more sperm or other substances per ejaculate. The need to ensure high levels of fertility could thus have been an important selective force in the coevolution of female preference and male eyespan in stalk-eyed flies. Our results support the phenotype-linked fertility hypothesis and suggest that it might be of general importance in explaining the evolution of exaggerated male ornaments and displays in species where males only provide females with ejaculates during reproduction.

Stalk-eyed flies (Diopsidae): modelling the evolution and development of an exaggerated sexual trait

Stalk-eyed flies of the family Diopsidae exhibit a unique form of hypercephaly, which has evolved under both natural and sexual selection. Male hypercephaly is used by female diopsids as an indicator of male quality. By choosing to mate with males expressing the most-exaggerated hypercephaly, females can benefit both from the enhanced fertility of these males and the transmission of other heritable advantages to their offspring. Stalk-eyed flies are close relatives of the model organism, Drosophila melanogaster. We have shown that similar genetic and cellular mechanisms regulate the initial development of the head capsule in fruitflies and diopsids. The great diversity of stalk-eyed fly species, exhibiting varying degrees of hypercephaly and sexual dimorphism, constitutes a major advantage for comparative studies of their development and evolution.

Juvenile hormone mediates a trade-off between primary and secondary sexual traits in stalk-eyed flies

Trade-offs between developing body parts may contribute to variation in allometric scaling relationships in a variety of taxa. Experimental evidence indicates that both circulating levels of juvenile hormone (JH) and sensitivities of developing body parts to JH can influence morphology in polyphenic insects. However, the extent to which JH may regulate both the development of traits that scale continuously with body size and trade-offs between these traits is largely unknown. Here, I present evidence that the JH analog methoprene applied to final instar larvae of a stalk-eyed fly (Cyrtodiopsis dalmanni) can induce males to produce larger eye-stalks relative to their body size. Examination of testis growth, sperm transfer, and egg maturation indicates that JH induces a trade-off between eye-span and gonad development in adult males, but not females. Age at sexual maturity was unaffected by larval JH applications to either sex. Collectively, these results are consistent with JH-mediated allocation of resources to eye-span at the expense of testes, and indicate potential costs for the production of an exaggerated trait.

The development of a long, coiled, optic nerve in the stalk-eyed fly Cyrtodiopsis whitei

In the stalk-eyed fly Cyrtodiopsis whitei (Diopsidae; Diptera), the relatively long optic nerve develops within the tight lumen of a very short eyestalk. Axonal growth is generally considered in terms of path finding, selective fasciculation, and towing. Physical forces that are necessary for axon lengthening are generated either by the growth cone or by the growth of surrounding tissues. Therefore, it is surprising to encounter a loosely coiled nerve apparently lacking any attachments that could allow for pull, or towing, of the nerve. In this study, we used histological sections and whole-mount preparations to confirm that the optic nerve of the stalk-eyed fly indeed elongates without the external application of tension to the nerve. Secondly, we examined the distribution of cytoskeletal elements and selected proteins that may be involved in axon extension. Staining against the vesicle fusion proteins SNAP-24 and SNAP-25 consistently results in stronger staining in the rapidly extending optic nerve than in a control nerve, suggesting a possible role of these proteins in the extension process. On a gross morphological level, SNAP-24/25 as well as the cytoskeletal elements actin and tubulin are uniformly distributed throughout the lengths of the growing nerve, suggesting that nerve elongation is distributed rather than localized. Finally, we identified glia as a possible source for tension within the nerve bundle. Glia proliferate rapidly in the optic nerve but not in the control nerve. Much work continues to focus on the growth of axons in culture, but this study is one of the few that considers the dynamics of nerve bundle extension as a whole.