"Alien" wasps and evolution of development

Growing an Embryo from a Single Cell: A Hurdle in Animal Life

A requirement that an animal be able to feed to grow constrains how a cell can grow into an animal, and it forces an alternation between growth (increase in mass) and proliferation (increase in cell number). A growth-only phase that transforms a stem cell of ordinary proportions into a huge cell, the oocyte, requires dramatic adaptations to help a nucleus direct a 10(5)-fold expansion of cytoplasmic volume. Proliferation without growth transforms the huge egg into an embryo while still accommodating an impotent nucleus overwhelmed by the voluminous cytoplasm. This growth program characterizes animals that deposit their eggs externally, but it is changed in mammals and in endoparasites. In these organisms, development in a nutritive environment releases the growth constraint, but growth of cells before gastrulation requires a new program to sustain pluripotency during this growth.

Teratocytes and their functions in parasitoids

Some endoparasitoid wasps (Hymenoptera) produce teratocytes, which are a type of cell that is released into host insects when wasp eggs hatch. In this short review I first summarize the different taxa of wasps that produce teratocytes, the embryonic origin of these cells, and key features of teratocyte growth. Then I discuss the known or hypothesized functions of teratocytes, and the range of teratocyte gene products that have been identified including recent transcriptome and proteome data.

Reversion of developmental mode in insects: evolution from long germband to short germband in the polyembrionic wasp Macrocentrus cingulum Brischke

Germband size in insects has played a central role in our understanding of insect patterning mechanisms and their evolution. The polarity of evolutionary change in insect patterning has been viewed so far as the unidirectional shift from the ancestral short germband patterning of basal hemimetabolous insects to the long germband patterning observed in most modern Holometabola. However, some orders of holometabolic insects display both short and long germband development, though the absence of a clear phylogenetic context does not permit definite conclusions on the polarity of change. Derived hymenoptera, that is, bees and wasps, represent a classical textbook example of long germband development. Yet, in some wasps putative short germband development has been described correlating with lifestyle changes, namely with evolution of endoparasitism and polyembryony. To address the potential reversion from long to short germband, we focused on the family Braconidae, which displays ancestral long germband development, and examined the derived polyembryonic braconid Macrocentrus cingulum. Using SEM analysis of M. cingulum embryogenesis coupled with analyses of embryonic patterning markers, we show that this wasp evolved short germband embryogenesis secondarily, in a way that is reminiscent of embryogenesis in the beetle Tribolium castaneum. This work shows that the evolution of germband size in insects is a reversible process that may correlate with other life-history traits and suggests broader implications on the mechanisms and evolvability of insect development.

A new embryonic pattern in parasitic wasps: divergence in early development may not be associated with lifestyle

Comparative embryogenesis of Encarsia formosa and Encarsia pergandiella (Hymenoptera Aphelinidae), two endoparasitoids of whiteflies (Hemiptera Aleyrodidae), revealed two strongly diverging developmental patterns. Indeed, the centrolecithal anhydropic egg of E. formosa developed through a superficial cleavage, as it occurs in Nasonia vitripennis, Apis mellifera, and Drosophila melanogaster. In contrast, the alecithal hydropic egg of E. pergandiella developed through holoblastic cleavage within a specialized extra-embryonic membrane (EEM). Since this developmental pattern evolved independently in several lineages of hymenopteran endoparasitoids, departures from the superficial cleavage mode have been argued to be strongly canalized in response to a shift from ecto- to endoparasitic lifestyle. Coexistence of both developmental patterns in two congeneric species suggests that alterations of early embryonic development may not be correlated with lifestyle. In addition, embryogenesis of E. pergandiella exhibited the following developmental novelties compared to other species possessing a hydropic egg: (i) polar body derivatives early acquired a cytoskeletal boundary prior to any other cellularization event; (ii) cellularization was asynchronous, starting with an early differentiation of a single apical blastomere at the end of the third cleavage; (iii) appearance of cytoskeletal boundaries of embryo blastomeres occurred between the third and fourth cleavages; (iv) the EEM originated through asynchronous participation of three separate lineages of cleavage nuclei, one of which associated with the polar body derivatives in a syncytium. Our results confirm a scenario of high plasticity in the early developmental strategies of hymenopteran endoparasitoids.

Placenta-like structure of the aphid endoparasitic wasp Aphidius ervi: a strategy of optimal resources acquisition

Aphidius ervi (Hymenoptera: Braconidae) is an entomophagous parasitoid known to be an effective parasitoid of several aphid species of economic importance. A reduction of its production cost during mass rearing for inundative release is needed to improve its use in biological control of pests. In these contexts, a careful analysis of its entire development phases within its host is needed. This paper shows that this parasitoid has some characteristics in its embryological development rather complex and different from most other reported insects, which can be phylogenetically very close. First, its yolkless egg allows a high fecundity of the female but force them to hatch from the egg shell rapidly to the host hemocoel. An early cellularisation allowing a rapid differentiation of a serosa membrane seems to confirm this hypothesis. The serosa wraps the developing embryo until the first instar larva stage and invades the host tissues by microvilli projections and form a placenta like structure able to divert host resources and allowing nutrition and respiration of embryo. Such interspecific invasion, at the cellular level, recalls mammal's trophoblasts that anchors maternal uterine wall and underlines the high adaptation of A. ervi to develop in the host body.

Caste-based differences in gene expression in the polyembryonic wasp Copidosoma floridanum

The polyembryonic parasitoid Copidosoma floridanum produces two larval castes, soldiers and reproductives, during development within its host. Soldier larvae defend the brood against competitors while reproductive larvae develop into adult wasps. As with other caste-forming insects, the distinct morphological and behavioral features of soldier and reproductive larvae likely involve differential gene expression. In this study we used a bi-directional suppression subtractive hybridization (SSH) approach to isolate differentially expressed genes from C. floridanum soldier and reproductive larvae. We isolated 230 novel expressed sequence tags (ESTs) from the two subtractions (114 soldier/116 reproductive ESTs). Among these ESTs were sequences with significant similarity to genes coding for serine proteinases, proteinase inhibitors, odorant-binding and chemosensory proteins, and cuticular proteins. Also, three novel genes were isolated that resemble one another in conceptual translation and share the cysteine spacing pattern of short scorpion toxins and insect defensins. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of 20 ESTs from the two libraries indicated that 85% were differentially expressed in one caste or the other. We conclude that our SSH strategy was effective in identifying a number of genes differentially expressed in soldier and reproductive larvae and that several of these genes will be useful in characterizing caste-specific gene networks in C. floridanum.

Transplantation of a polyembryonic wasp embryo: a technique for transferring endoparasitic embryo into the host egg

Concealed development of many animal embryos prevents examination of development and limits the application of embryo manipulation techniques aimed at understanding developmental processes. In embryos developing in utero, such as in mammals, it is necessary to dissect embryos from the mother and, upon manipulative intervention, to implant them back into the recipient. Parasitic wasps present a promising system for understanding the evolution of early developmental processes. In basal ectoparasitic species that lay eggs on the surface of the host, it is possible to adapt embryo manipulation techniques developed in Drosophila. However, their derived endoparasitic relatives, which exhibit various modifications of developmental programs, undergo concealed development within the host body. For example, the parasitic polyembryonic wasp Copidosoma floridanum oviposits an egg into the egg of the host moth Trichoplusia ni. The host larva emerges and the parasite undergoes development within the host body, preventing embryo manipulation as a means of examining developmental regulation. Here we present a protocol for embryo transfer that allows the transplantation of C. floridanum egg into the host egg. This approach opens a new avenue in the application of various embryo manipulation techniques aimed at understanding the evolution of embryogenesis in endoparasitic Hymenoptera. In addition, this approach has potential for the development of other tools in C. floridanum, such as transgenesis and reverse genetics, which can also be extended to other endoparasitic species.

A major role for zygotichunchbackin patterning theNasoniaembryo

Developmental genetic analysis has shown that embryos of the parasitoid wasp Nasonia vitripennis depend more on zygotic gene products to direct axial patterning than do Drosophila embryos. In Drosophila, anterior axial patterning is largely established by bicoid, a rapidly evolving maternal-effect gene, working with hunchback, which is expressed both maternally and zygotically. Here,we focus on a comparative analysis of Nasonia hunchback function and expression. We find that a lesion in Nasonia hunchback is responsible for the severe zygotic headless mutant phenotype, in which most head structures and the thorax are deleted, as are the three most posterior abdominal segments. This defines a major role for zygotic Nasonia hunchback in anterior patterning, more extensive than the functions described for hunchback in Drosophila or Tribolium. Despite the major zygotic role of Nasonia hunchback, we find that it is strongly expressed maternally, as well as zygotically. NasoniaHunchback embryonic expression appears to be generally conserved; however, the mRNA expression differs from that of Drosophila hunchback in the early blastoderm. We also find that the maternal hunchback message decays at an earlier developmental stage in Nasonia than in Drosophila, which could reduce the relative influence of maternal products in Nasonia embryos. Finally, we extend the comparisons of Nasonia and Drosophila hunchback mutant phenotypes, and propose that the more severe Nasonia hunchback mutant phenotype may be a consequence of differences in functionally overlapping regulatory circuitry.

Early blastomere determines embryo proliferation and caste fate in a polyembryonic wasp

Polyembryonic development is a unique mode of metazoan development in which a single zygote generates multiple embryos by clonal proliferation. The polyembryonic parasitic insect Copidosoma floridanum shows one of the most extreme cases of polyembryony, producing up to 2,000 embryos from a single egg. In addition, this wasp exhibits an unusual polyphenism, producing two morphologically distinct larval castes, termed precocious and reproductive, that develop clonally from the same zygote. This form of development seems incompatible with a model of insect development in which maternal pre-patterning of the egg specifies embryonic axial polarity. Here we show that maternal pre-patterning in the form of germ plasm creates cellular asymmetry at the four-cell stage embryo of Copidosoma that is perpetuated throughout development. Laser ablations of cells show that the cell inheriting the germ plasm regulates both the fate and proliferation of the reproductive caste. Thus, we have uncovered a new mechanism of caste specification, mediated by the regulatory capacity of a single cell. This study shows that the evolution of mammalian-like regulative development of an insect embryo relies on a novel cellular context that might ultimately enhance developmental plasticity.

Caste determination in a polyembryonic wasp involves inheritance of germ cells

Social insects are characterized by the development of castes in which some colony members reproduce whereas others function as altruistic helpers. The conditional switch controlling caste formation usually involves environmental stimuli that act on processes that regulate development of individuals. Unlike other social species, embryos of polyembryonic wasps develop clonally to produce large numbers of genetically identical offspring and two morphologically distinct castes. All embryos in a clone exist in an identical environment, the host, yet develop into either reproductive larvae that mature into adult wasps or soldier larvae whose function is defense. Here, we report that caste determination in Copidosoma floridanum involves inheritance of germ cells. Expression of a C. floridanum homolog (Cf-vas) of the germ cell marker Vasa indicated that the B(4) blastomere in four cell-stage embryos is specified as a primordial germ cell. Vas expression later in development further indicated that embryos developing into reproductive larvae possess primordial germ cells whereas embryos developing into soldier larvae do not. Ablation of the B(4) blastomere resulted in most broods containing only soldiers whereas ablation of other blastomeres produced broods containing both castes. These results indicate that soldier larvae are obligately sterile and reveal a previously unknown role for germ cells in caste formation.

Evo-devo aspects of classical and molecular data in a historical perspective

We discuss the interplay between evolution and development as reflected in data and concepts since about 1800. Darwin and his "continental apostle" Haeckel put the striking similarity between early vertebrate embryos in an evolutionary context. Haeckel's partly illicit generalizations discredited evolutionary thinking among early experimental embryologists who moreover noted riddles incompatible with contemporary concepts of homology and evolution. Relevant solutions were suggested by the more recent concept of ontogenetic networks that embody complex regulatory properties and genes with partly overlapping functions. Molecular data on development increasingly reveal evolutionary opportunism, for instance when a widespread signaling chain involved in primitive immune defense was apparently recruited later on for dorso-ventral axis determination in some evolutionarily advanced insect groups. Recently, Rickettsia-related bacteria colonizing many arthropod species were found to "manipulate" the first steps of host development to the advantage of their own propagation, but by ways that could also promote host speciation. Molecular genetics can now document evolutionary steps in ontogenetic networks. In the fruit fly for instance, the novel bicoid gene has superseded a crucial patterning function within a pre-existing network--a case of "molecular caenogenesis." The expression patterns of conserved genes that antagonistically determine dorso-ventral polarity support a literal revolution envisioned almost 200 years ago. This is the dorso-ventral inversion of the body plan in some metazoans--ascribed then to the Articulata, now to the Chordata. The final section posits that the opportunistic character of evolutionary innovations is detrimental to parsimony in development.

Hold the germ cells, I'm on duty

Germ cell segregation and gamete production are developmental problems that all sexually reproducing species must solve in order to survive. Many people are familiar with the complex social structures of some insect species, where specialised castes of adult insects perform specific tasks, one of which is usually to guard the sexually reproductive queen. The parasitic wasp Copidosoma floridanum adds another level of complexity to the caste system: a fertilised egg produces both sterile, short-lived "soldier" larvae and "reproductive" larvae that complete metamorphosis to produce sexually reproductive adults. How two morphologically and functionally distinct larval castes are produced by genetically identical groups of cells developing under the same environmental conditions is a baffling problem. A recent paper suggests that differential germ cell segregation during embryogenesis may be an event both necessary and sufficient for caste determination.(1)

Prokaryote and eukaryote evolvability

The concept of evolvability covers a broad spectrum of, often contradictory, ideas. At one end of the spectrum it is equivalent to the statement that evolution is possible, at the other end are untestable post hoc explanations, such as the suggestion that current evolutionary theory cannot explain the evolution of evolvability. We examine similarities and differences in eukaryote and prokaryote evolvability, and look for explanations that are compatible with a wide range of observations. Differences in genome organisation between eukaryotes and prokaryotes meets this criterion. The single origin of replication in prokaryote chromosomes (versus multiple origins in eukaryotes) accounts for many differences because the time to replicate a prokaryote genome limits its size (and the accumulation of junk DNA). Both prokaryotes and eukaryotes appear to switch from genetic stability to genetic change in response to stress. We examine a range of stress responses, and discuss how these impact on evolvability, particularly in unicellular organisms versus complex multicellular ones. Evolvability is also limited by environmental interactions (including competition) and we describe a model that places limits on potential evolvability. Examples are given of its application to predator competition and limits to lateral gene transfer. We suggest that unicellular organisms evolve largely through a process of metabolic change, resulting in biochemical diversity. Multicellular organisms evolve largely through morphological changes, not through extensive changes to cellular biochemistry.

The jewel wasp Nasonia: querying the genome with haplo-diploid genetics

The jewel wasp Nasonia vitripennis is considered the "Drosophila melanogaster of the Hymenoptera." This diminutive wasp offers insect geneticists a means for applying haplo-diploid genetics to the analysis of developmental processes. As in bees, haploid males develop from unfertilized eggs, while diploid females develop from fertilized eggs. Nasonia's advantageous combination of haplo-diploid genetics and ease of handling in the laboratory facilitates screening the entire genome for recessive mutations affecting a developmental process of interest. This approach is currently directed toward understanding the evolution of embryonic pattern formation by comparing Nasonia embryogenesis to that of Drosophila. Haplo-diploid genetics also facilitates developing molecular maps and mapping polygenic traits. Moreover, Nasonia embryos are also proving amenable to cell biological analysis. These capabilities are being exploited to understand a variety of behavioral, developmental, and evolutionary processes, ranging from cytoplasmic incompatibility to the evolution of wing morphology.

Conservation of the segmented germband stage: robustness or pleiotropy?

Gene expression patterns of the segment polarity genes in the extended and segmented germband stage are remarkably conserved among insects. To explain the conservation of these stages, two hypotheses have been proposed. One hypothesis states that the conservation reflects a high interactivity between modules, so that mutations would have several pleiotropic effects in other parts of the body, resulting in stabilizing selection against mutational variation. The other hypothesis states that the conservation is caused by robustness of the segment polarity network against mutational changes. When evaluating the empirical evidence for these hypotheses, we found strong support for pleiotropy and little evidence supporting robustness of the segment polarity network. This points to a key role for stabilizing selection in the conservation of these stages. Finally, we discuss the implications for robustness of organizers and long-term conservation in general.

Insect segmentation: Genes, stripes and segments in "Hoppers"

Recent work has revealed that orthologues of several segmentation genes are expressed in the grasshopper embryo, in patterns resembling those shown in Drosophila. This suggests that, despite great differences between the embryos, a hierarchy of gap/pair-rule/segment polarity gene function may be a shared and ancestral feature of insect segmentation.