Expression of the pupal determinant broad during metamorphic and neotenic development of the strepsipteran Xenos vesparum Rossi.

Derived members of the endoparasitic order Strepsiptera have acquired an extreme form of sexual dimorphism whereby males undergo metamorphosis and exist as free-living adults while females remain larviform, reaching sexual maturity within their hosts. Expression of the transcription factor, broad (br) has been shown to be required for pupal development in insects in which both sexes progress through metamorphosis. A surge of br expression appears in the last larval instar, as the epidermis begins pupal development. Here we ask if br is also up-regulated in the last larval instar of male Xenos vesparum Rossi (Stylopidae), and whether such expression is lost in neotenic larviform females. We clone three isoforms of br from X. vesparum (Xv'br), and show that they share greatest similarity to the Z1, Z3 and Z4 isoforms of other insect species. By monitoring Xv'br expression throughout development, we detect elevated levels of total br expression and the Xv'Z1, Xv'Z3, and Xv'Z4 isoforms in the last larval instar of males, but not females. By focusing on Xv'br expression in individual samples, we show that the levels of Xv'BTB and Xv'Z3 in the last larval instar of males are bimodal, with some males expressing 3X greater levels of Xv'br than fourth instar femlaes. Taken together, these data suggest that neoteny (and endoparasitism) in females of Strepsiptera Stylopidia could be linked to the suppression of pupal determination. Our work identifies a difference in metamorphic gene expression that is associated with neoteny, and thus provides insights into the relationship between metamorphic and neotenic development.

Pupariation site preference within and between Drosophila sibling species.

Holometabolous insects pass through a sedentary pupal stage and often choose a location for pupation that is different from the site of larval feeding. We have characterized a difference in pupariation site choice within and between sibling species of Drosophila. We found that, in nature, Drosophila sechellia pupariate within their host fruit, Morinda citrifolia, and that they perform this behavior in laboratory assays. In contrast, in the laboratory, geographically diverse strains of Drosophila simulans vary in their pupariation site preference; D. simulans lines from the ancestral range in southeast Africa pupariate on fruit, or a fruit substitute, whereas populations from Europe or the New World select sites off of fruit. We explored the genetic basis for the evolved preference in puariation site preference by performing quantitative trait locus mapping within and between species. We found that the interspecific difference is controlled largely by loci on chromosomes X and II. In contrast, variation between two strains of D. simulans appears to be highly polygenic, with the majority of phenotypic effects due to loci on chromosome III. These data address the genetic basis of how new traits arise as species diverge and populations disperse.

Morphological evolution caused by many subtle-effect substitutions in regulatory DNA.

Morphology evolves often through changes in developmental genes, but the causal mutations, and their effects, remain largely unknown. The evolution of naked cuticle on larvae of Drosophila sechellia resulted from changes in five transcriptional enhancers of shavenbaby (svb), a transcript of the ovo locus that encodes a transcription factor that governs morphogenesis of microtrichiae, hereafter called 'trichomes'. Here we show that the function of one of these enhancers evolved through multiple single-nucleotide substitutions that altered both the timing and level of svb expression. The consequences of these nucleotide substitutions on larval morphology were quantified with a novel functional assay. We found that each substitution had a relatively small phenotypic effect, and that many nucleotide changes account for this large morphological difference. In addition, we observed that the substitutions had non-additive effects. These data provide unprecedented resolution of the phenotypic effects of substitutions and show how individual nucleotide changes in a transcriptional enhancer have caused morphological evolution.

The nuclear receptor E75A has a novel pair-rule-like function in patterning the milkweed bug, Oncopeltus fasciatus.

Genetic studies of the fruit fly Drosophila have revealed a hierarchy of segmentation genes (maternal, gap, pair-rule and HOX) that subdivide the syncytial blastoderm into sequentially finer-scale coordinates. Within this hierarchy, the pair-rule genes translate gradients of information into periodic stripes of expression. How pair-rule genes function during the progressive mode of segmentation seen in short and intermediate-germ insects is an ongoing question. Here we report that the nuclear receptor Of'E75A is expressed with double segment periodicity in the head and thorax. In the abdomen, Of'E75A is expressed in a unique pattern during posterior elongation, and briefly resembles a sequence that is typical of pair-rule genes. Depletion of Of'E75A mRNA caused loss of a subset of odd-numbered parasegments, as well as parasegment 6. Because these parasegments straddle segment boundaries, we observe fusions between adjacent segments. Finally, expression of Of'E75A in the blastoderm requires even-skipped, which is a gap gene in Oncopeltus. These data show that the function of Of'E75A during embryogenesis shares many properties with canonical pair-rule genes in other insects. They further suggest that parasegment specification may occur through irregular and episodic pair-rule-like activity.

The role of the pupal determinant broad during embryonic development of a direct-developing insect.

Metamorphosis is one of the most common, yet dramatic of life history strategies. In insects, complete metamorphosis with morphologically distinct larval stages arose from hemimetabolous ancestors that were more direct developing. Over the past century, several ideas have emerged that suggest the holometabolous pupa is developmentally homologous to the embryonic stages of the hemimetabolous ancestor. Other theories consider the pupal stage to be a modification of a hemimetabolous nymph. To address this question, we have isolated an ortholog of the pupal determinant, broad (br), from the hemimetabolous milkweed bug and examined its role during embryonic development. We show that Oncopeltus fasciatus br (Of'br) is expressed in two phases. The first occurs during germ band invagination and segmentation when Of'br is expressed ubiquitously in the embryonic tissues. The second phase of Of'br expression appears during the pronymphal phase of embryogenesis and persists through nymphal differentiation to decline just before hatching. Knock-down of Of'br transcripts results in defects that range from posterior truncations in the least-affected phenotypes to completely fragmented embryonic tissues in the most severe cases. Analysis of the patterning genes engrailed and hunchback reveal loss of segments and a failure in neural differentiation after Of'br depletion. Finally, we show that br is constitutively expressed during embyrogenesis of the ametabolous firebrat, Thermobia domestica. This suggests that br expression is prominent during embryonic development of ametabolous and hemimetabolous insects but was lost with the emergence of the completely metamorphosing insects.

The pupal specifier broad directs progressive morphogenesis in a direct-developing insect.

A key regulatory gene in metamorphosing (holometabolous) insect life histories is the transcription factor broad (br), which specifies pupal development. To determine the role of br in a direct-developing (hemimetabolous) insect that lacks a pupal stage, we cloned br from the milkweed bug, Oncopeltus fasciatus (Of'br). We find that, unlike metamorphosing insects, in which br expression is restricted to the larval-pupal transition, Of'br mRNA is expressed during embryonic development and is maintained at each nymphal molt but then disappears at the molt to the adult. Induction of a supernumerary nymphal stage with a juvenile hormone (JH) mimic prevented the disappearance of br mRNA. In contrast, induction of a precocious adult molt by application of precocene II to third-stage nymphs caused a loss of br mRNA at the precocious adult molt. Thus, JH is necessary to maintain br expression during the nymphal stages. Injection of Of'br dsRNA into either early third- or fourth-stage nymphs caused a repetition of stage-specific pigmentation patterns and prevented the normal anisometric growth of the wing pads without affecting isometric growth or molting. Therefore, br is necessary for the mutable (heteromorphic) changes that occur during hemimetabolous development. Our results suggest that metamorphosis in insects arose as expression of br, which conveys competence for change, became restricted to one postembryonic instar. After this shift in br expression, the progressive changes that occur within the nymphal series in basal insects became compressed to the one short period of morphogenesis seen in the larva-to-pupa transition of holometabolous insects.

Imperfect eggs and oviform nymphs: a history of ideas about the origins of insect metamorphosis.

The problem of insect metamorphosis has inspired naturalists for centuries. One question that often arises is why some insects, such as butterflies and bees, undergo a fairly radical metamorphosis while others, such as crickets and lice, do not. Even before the concept of homology emerged scientists speculated which stage found in more direct-developing insects would correspond with the pupal stage of metamorphosing insects. William Harvey (1651) considered the pupal stage to be a continuation of embryonic events, calling it a "second egg." Since then variations of this idea have emerged over the centuries of scientific research and have been supported by a wide variety of methods and rationales. This review will follow those ideas and the ideas that emerged in opposition to them to the present state of the field.

Juvenile hormone acts at embryonic molts and induces the nymphal cuticle in the direct-developing cricket.

During embryogenesis of hemimetabolous insects, the sesquiterpenoid hormone, juvenile hormone (JH), appears late in embryogenesis coincident with formation of the first nymphal cuticle. We tested the role of embryonic JH by treating cricket embryos with JH III, or the JH-mimic (JHM) pyriproxifen, during early embryogenesis. We found two discrete windows of JH sensitivity. The first occurs during the formation of the first (E1) embryonic cuticle. Treatment with JHM prior to this molt produced small embryos that failed to complete the movements of katatrepsis. Embryos treated after the E1 molt but before the second embryonic (pronymphal) molt completed katatrepsis but then failed to complete dorsal closure and precociously formed nymphal, rather than pronymphal characters. This second sensitivity window was further assessed by treating embryos with low doses of JH III prior to the pronymphal molt. With low doses, mosaic cuticles were formed, bearing features of both the pronymphal and nymphal stages. The nymphal characters varied in their sensitivity to JH III, due at least in part to differences in the timing of their sensitivity windows. Unexpectedly, many of the JH III-treated embryos with mosaic and precocious nymphal cuticles made a second nymphal cuticle and successfully hatched. JH treatment also affected the growth of the embryos. By focusing on the developing limb, we found that the effect of JH upon growth was asymmetric, with distal segments more affected than proximal ones, but this was not reflected in misexpression of Distal-less or Bric-a-brac, which are involved in proximal-distal patterning of the limb.