Reciprocal transplantation of wing discs between a wing deficient mutant (fl) and wild type of the silkworm, Bombyx mori

The wingless mutant flügellos ( fl ) of the silkworm lacks all four wings. Although wing discs of the fl seem to develop normally until the fourth larval instar, wing morphogenesis stops after the fourth larval ecdysis, probably caused by aberrant expression of an unidentified factor, referred to as fl . To characterize factor fl , the wing discs dissected from the wild-type (WT) and fl larvae were transplanted into other larvae and developmental changes of the discs were examined. When the wing disc from a WT larva was transplanted into another WT larva and allowed to grow until emergence, a small wing appeared that was covered with scales. Thus, the transplanted wing discs can develop autonomously, form scales and evert from adult skin. The WT wing discs transplanted into the fl larvae also developed at a high rate. However, the fl wing discs transplanted into the WT larvae did not develop during the larval to pupal developmental stages. These data suggest that the fl gene product (factor fl) works in the wing disc cells during wing morphogenesis. Its function cannot be complemented by hemolymph in the WT larva. It is also implied that the level of humoral factors and hormones required for wing morphogenesis are normally maintained in the fl larva.     

Origin and evolution of animal life cycles

The ‘origin of larvae’ has been widely discussed over the years, almost invariably with the tacit understanding that larvae are secondary specializations of early stages in a holobenthic life cycle. Considerations of the origin and early radiation of the metazoan phyla have led to the conclusion that the ancestral animal (= metazoan) was a holopelagic organism, and that pelago-benthic life cycles evolved when adult stages of holopelagic ancestors became benthic, thereby changing their life style, including their feeding biology. The literature on the larval development and phylogeny of animal phyla is reviewed in an attempt to infer the ancestral life cycles of the major animal groups. The quite detailed understanding of larval evolution in some echinoderms indicates that ciliary filter-feeding was ancestral within the phylum, and that planktotrophy has been lost in many clades. Similarly, recent studies of the developmental biology of ascidians have demonstrated that a larval structure, such as the tail of the tadpole larva, can easily be lost, viz. through a change in only one gene. Conversely, the evolution of complex structures, such as the ciliary bands of trochophore larvae, must involve numerous genes and numerous adaptations. The following steps of early metazoan evolution have been inferred from the review. The holopelagic ancestor, blastaea, probably consisted mainly of choanocytes, which were the feeding organs of the organism. Sponges may have evolved when blastaea-like organisms settled and became reorganized with the choanocytes in collar chambers. The eumetazoan ancestor was probably the gastraea, as suggested previously by Haeckel. It was holopelagic and digestion of captured particles took place in the archenteron. Cnidarians and ctenophores are living representatives of this type of organization. The cnidarians have become pelago-benthic with the addition of a sessile, adult polyp stage; the pelagic gastraea-like planula larva is retained in almost all major groups, but only anthozoans have feeding larvae. Within the Bilateria, two major lines of evolution can be recognized: Protostomia and Deuterostomia. In protostomes, trochophores or similar types are found in most spiralian phyla; trochophore-like ciliary bands are found in some rotifers, whereas all other aschelminths lack ciliated larvae. It seems probable that the trochophore was the larval type of the ancestral, pelago-benthic spiralian and possible that it was ancestral in all protostomes. Most of the non-chordate deuterostome phyla have ciliary filter-feeding larvae of the dipleurula type, and this strongly indicates that the ancestral deuterostome had this type of larva.     

Who came first--larvae or adults? origins of bilaterian metazoan larvae

There is a classic controversy in zoology over whether the common ancestor of living bilaterian phyla was a benthic animal with a bilaterian body plan, or was a pelagic larva-like animal similar to what we see today in the primary larvae of indirect-developing bilaterians. We examine the current larva-like adult hypothesis, and present an alternate model for the evolution of complex life histories by intercalation of larval features into the ontogeny of an ancestral direct-developing bilaterian. This gradual accumulation of larval features results in a developmental regulatory program that produces a larva distinct in body plan from the adult. The evolution of a rapid and complete metamorphosis is made possible by the convergent evolution of set aside cells in the final stages of the emergence of indirect developing larval forms. Although convergences abound either hypothesis for the evolution of developmental pathways and life histories, the bilaterian first hypothesis is consistent with all stages of evolution of a complex life history being selectively advantageous, with the rapid evolution of larval forms, and with the frequent co-option of genes from the adult phase of the life cycle prevalent in the evolution of embryos and larvae.     

The Origin of Cladocera (Crustacea,Branchiopoda): A New Understanding of an Old Hypothesis

Comparative analysis of the ontogeny of representatives of two sister taxa (Cladocera and Cyclestherida) showed that the paedomorphic morphology of cladocerans (the small number of thoracic segments and segments of branches of antennae II, and the reduction of the carapace) was caused by the cessation of development of the somatic structures at early larval stages of ontogeny. It is demonstrated that this stop is not associated with the accelerated development of the reproductive system (progenesis), since it takes place long prior to the beginning of reproduction. In accordance with this fact, the past hypotheses that cladocerans evolved from the reproducing larvae of the ancestral form or that they are early maturing metanauplii should be recognized as erroneous. Cyclestheria. The origin of Cladocera from a Cyclestheria-like ancestor should be regarded as neotenic, taking into consideration the extended interpretation of this term.     

The larval stages of the sea urchin, Strongylocentrotus purpuratus

The adult body plan of Strongylocentrotus purpuratus is established within the imaginal rudiment during the larval stages. To facilitate the study of these stages, we have defined a larval staging scheme, which consists of seven stages: Stage I, four-arm stage; Stage II, eight-arm stage; Stage III, vestibular invagination stage; Stage IV, rudiment initiation stage; Stage V, pentagonal disc stage; Stage VI, advanced rudiment stage; and Stage VI, tube-foot protrusion stage. Each stage is characterized by significant morphological features observed for the first time at that stage. This scheme is intended as a guide for determining the degree of larval development, and for identifying larval and adult structures. Larval anatomy was visualized using light and confocal microscopy as required on living material, whole mount fixed specimens, and serial sections. Antibody staining to localize specific gene products was also used. Detailed analysis of these data has furthered our understanding of the morphogenesis of the rudiment, and has suggested provocative questions regarding the molecular basis for these events. We intend this work to be of use to investigators studying gene expression and morphogenesis in postembryonic larvae.     

Structure and development of setae on the thoracic limbs of the anostracan crustacean, Thamnocephalus platyurus

Setae are a prominent feature of arthropod limbs. In taxa where the limbs develop during the larval phase, developing setae are an integral part of the developing limb bud and their differentiation cannot easily be separated from the early patterning and formation of the overall limb. Here I describe the morphogenesis and adult setae in a branchiopod crustacean, the anostracan, Thamnocephalus platyurus. The majority of the setae on the limbs are non-innervated plumose setae that are formed from six underlying cells. Because branchiopods are often sampled in comparative studies of limb development, the details of the cellular morphogenesis of their limbs provide a necessary basis for studies of limb patterning.     

Skeletal morphogenesis in the urodele skull: II. Effect of developmental stage in thyroid hormone-induced remodeling

This study investigates the effect of developmental stage on thyroid hormone (TH)-mediated remodeling in the skeletal tissues of hemidactyliine plethodontid urodeles. Rate of morphogenesis was quantified in 17 metamorphic tissues for three different size-age classes of Eurycea bislineata larvae immersed in a metamorphic dosage of T₄. Extent of morphogenesis after a 3-week immersion was also quantified in these tissues plus four larval ones for the full size range of E. bislineata larvae and for less complete size ranges of E. wilderae, E. longicauda guttolineata, Gyrinophilus porphyriticus, and Pseudotriton ruber larvae. Although all tissues respond more slowly with decreasing size/age, two tissue-specific effects are evident in all species. Larval ossifications are less inducible than metamorphic ossifications, and progressive metamorphic events are more retarded and, in some cases, more prone to abnormal morphogenesis than regressive ones. The first effect agrees with the prediction that tissues that naturally remodel at metamorphosis are more responsive to a metamorphic dosage of TH than those that respond at a larval stage and lower TH. The second effect agrees with the prediction that progressive morphogenesis is more likely to be impaired at small size than regressive morphogenesis, although the frequent discrepancies between individuals of similar size implicate developmental age more than size in this effect. Collectively, these two effects provide only equivocal support for the hypothesis that direct development in plethodontids evolved via precocious TH activity. However, the unexpected transition from ceratobranchial replacement to ceratobranchial shortening in medium-sized larvae suggests that the former pathway requires a longer period of cell specification at low TH. Since ancestral plethodontids appear to have been distinguished by an exceptionally long larval period with exceptionally low TH activity, this developmental prerequisite may in turn be partly responsible for their singular evolution of ceratobranchial replacement. © 1995 Wiley-Liss, Inc.     

FURTHER STUDY RE-EXAMINING WHETHER LARVAL EPIDERMIS CAN OMIT THE SECRETION OF A PUPAL CUTICLE, USING THE SILKWORM, BOMBYX MORI

When larval tissue is exposed to a hormonal milieu lacking juvenile hormone, adult characters appear directly, omitting the pupal stage, in some insects but not in others, including Bombyx mori. An attempt was made to induce omission of pupal characters in this species by varying the stage of the larval epidermis to be tested. Pieces of larval integument taken from fourth- and fifth-instar larvae of various stages were transplanted to developing adults. Although the number of cuticle layers and the types of cuticle produced differed depending on the age of the donors, none of the pieces omitted secreting the pupal cuticle. It is concluded that the larval epidermis cannot omit secreting pupal cuticle, and that a transition of tissue competence may play an important part in the sequential appearance of larval, pupal, and adult characters.     

Genetic Analysis of Two Allelic Temperature-Sensitive Mutants of DROSOPHILA MELANOGASTER Both of Which Are Zygotic and Maternal-Effect Lethals

After fertilization, the development of a zygote depends upon both gene products synthesized by its maternal parent and gene products synthesized by the zygote itself. To analyze genetically the relative contributions of these two sources of gene products, several laboratories have been isolating two classes of mutants of Drosophila melanogaster: maternal-effect lethals and zygotic lethals. This report concerns the analysis of two temperature-sensitive mutants, OX736hs and PC025hs, which were isolated as alleles of a small-disc mutant, l(3)1902. These alleles are not only zygotic lethals, but also maternal-effect lethals. They have temperature-sensitive periods during larval life and during oogenesis. Mutant larvae exposed continuously to restrictive temperature have small discs. One-or two-day exposures to the restrictive temperature administered during the third larval instar lead to a homeotic transformation of the midlegs and hindlegs to the pattern characteristic of the forelegs. Mutant females exposed to the restrictive temperature during oogenesis produce eggs that can develop until gastrulation, but do not hatch.--The existence of these mutants, and one that was recently described by another group, implies that there may be a class of genes, heretofore unrecognized, whose products are synthesized during oogenesis, are essential for embryogenesis and are also synthesized during larval stages within imaginal disc cells.     

Circadian synchronization and rhythmicity in larval photoperception-defective mutants of Drosophila

A single light episode during the first larval stage can set the phase of adult Drosophila activity rhythms, showing that a light-sensitive circadian clock is functional in larvae and is capable of keeping time throughout development. These behavioral data are supported by the finding that neurons expressing clock proteins already exist in the larval brain and appear to be connected to the larval visual system. To define the photoreceptive pathways of the larval clock, the authors investigated circadian synchronization during larval stages in various visual systems and/or cryptochrome-defective strains. They show that adult activity rhythms cannot be entrained by light applied to larvae lacking both cryptochrome and the visual system, although such rhythms were entrained by larval stage-restricted temperature cycles. Larvae lacking either pathway alone were light entrainable, but the phase of the resulting adult rhythm was advanced relative to wild-type flies. Unexpectedly, adult behavioral rhythms of the glass60j and norpAP24 visual system mutants that were entrained in the same conditions were found to be severely impaired, in contrast to those of the wild type. Extension of the entrainment until the adult stage restored close to wild-type behavioral rhythms in the mutants. The results show that both cryptochrome and the larval visual system participate to circadian photoreception in larvae and that mutations affecting the visual system can impair behavioral rhythmicity.     

A Novel Technique for Identifying the Instar of Field-Collected Insect Larvae

Many field studies of insects have focused on the adult stage alone, likely because immature stages are unknown in most insect species. Molecular species identification (e.g., DNA barcoding) has helped ascertain the immature stages of many insects, but larval developmental stages (instars) cannot be identified. The identification of the growth stages of collected individuals is indispensable from both ecological and taxonomic perspectives. Using a larval–adult body size relationship across species, I present a novel technique for identifying the instar of field-collected insect larvae that are identified by molecular species identification technique. This method is based on the assumption that classification functions derived from discriminant analyses, performed with larval instar as a response variable and adult and larval body sizes as explanatory variables, can be used to determine the instar of a given larval specimen that was not included in the original data set, even at the species level. This size relationship has been demonstrated in larval instars for many insects (Dyar’s rule), but no attempt has been made to include the adult stage. Analysis of a test data set derived from the beetle family Carabidae (Coleoptera) showed that classification functions obtained from data sets derived from related species had a correct classification rate of 81–100%. Given that no reliable method has been established to identify the instar of field-collected insect larvae, these values may have sufficient accuracy as an analytical method for field-collected samples. The chief advantage of this technique is that the instar can be identified even when only one specimen is available per species if classification functions are determined for groups to which the focal species belongs. Similar classification functions should be created for other insect groups. By using those functions together with molecular species identification, future studies could include larval stages as well as adults.     

Siphonariid development: Quintessential euthyneuran larva with a mantle fold innovation (Gastropoda; Panpulmonata)

Recent phylogenetic revisions of euthyneuran gastropods (“opisthobranchs” and “pulmonates”) suggest that clades with a planktotrophic larva, the ancestral life history for euthyneurans, are more widely distributed along the trunk of the euthyneuran tree than previously realized. There is some indication that the planktotrophic larva of euthyneurans has distinctive features, but information to date has come mainly from traditional “opisthobranch” groups. Much less is known about planktotrophic “pulmonate” larvae. If planktotrophic larvae of “pulmonates” share unique traits with those of “opisthobranchs,” then a distinctive euthyneuran larval-type has been the developmental starting template for a spectacular amount of evolved morphological and ecological disparity among adult euthyneurans. We studied development of a siphonariid by preparing sections of larval and postmetamorphic stages for histological and ultrastructural analysis, together with 3D reconstructions and data from immunolabeling of the larval apical sensory organ. We also sought a developmental explanation for the unusual arrangement of shell-attached, dorso-ventral muscles relative to the mantle cavity of adult siphonariids. Adult siphonariids (“false limpets”) have a patelliform shell but their C-shaped shell muscle partially embraces a central mantle cavity, which is different from the arrangement of these components in patellogastropods (“true limpets”). It is not obvious how shell muscles extending into the foot become placed anterior to the mantle cavity during siphonariid development from a veliger larva. We found that planktotrophic larvae of Siphonaria denticulata are extremely similar to previously described, planktotrophic “opisthobranch” larvae. To emphasize this point, we update a list of distinctive characteristics of planktotrophic euthyneuran larvae, which can anchor future studies on the impressive evolvability of this larval-type. We also describe how premetamorphic and postmetamorphic morphogenesis of larval mantle fold tissue creates the unusual arrangement of shell-muscles and mantle cavity in siphonariids. This result adds to the known postmetamorphic evolutionary innovations involving mantle fold tissue among euthyneurans.     

A novel ontogenetic pathway in hybrid embryos between species with different modes of development

To investigate the bases for evolutionary changes in developmental mode, we fertilized eggs of a direct-developing sea urchin, Heliocidaris erythrogramma, with sperm from a closely related species, H. tuberculata, that undergoes indirect development via a feeding larva. The resulting hybrids completed development to form juvenile adult sea urchins. Hybrids exhibited restoration of feeding larval structures and paternal gene expression that have been lost in the evolution of the direct-developing maternal species. However, the developmental outcome of the hybrids was not a simple reversion to the paternal pluteus larval form. An unexpected result was that the ontogeny of the hybrids was distinct from either parental species. Early hybrid larvae exhibited a novel morphology similar to that of the dipleurula-type larva typical of other classes of echinoderms and considered to represent the ancestral echinoderm larval form. In the hybrid developmental program, therefore, both recent and ancient ancestral features were restored. That is, the hybrids exhibited features of the pluteus larval form that is present in both the paternal species and in the immediate common ancestor of the two species, but they also exhibited general developmental features of very distantly related echinoderms. Thus in the hybrids, the interaction of two genomes that normally encode two disparate developmental modes produces a novel but harmonious ontongeny.     

Nervous system development of the sea cucumber Stichopus japonicus

The nervous system development of the sea cucumber Stichopus japonicus was investigated to explore the development of the bilateral larval nervous system into the pentaradial adult form typical of echinoderms. The first nerve cells were detected in the apical region of epidermis in the late gastrula. In the auricularia larvae, nerve tracts were seen along the ciliary band. There was a pair of bilateral apical ganglia consisted of serotonergic nerve cells lined along the ciliary bands. During the transition to the doliolaria larvae, the nerve tracts rearranged together with the ciliary bands, but they were not segmented and remained continuous. The doliolaria larvae possessed nerves along the ciliary rings but strongly retained the features of auricularia larvae nerve pattern. The adult nervous system began to develop inside the doliolaria larvae before the larval nervous system disappears. None of the larval nervous system was observed to be incorporated into the adult nervous system with immunohistochemistry. Since S. japonicus are known to possess an ancestral mode of development for echinoderms, these results suggest that the larval nervous system and the adult nervous system were probably formed independently in the last common ancestor of echinoderms.     

Caste formation in the polyembryonic wasp Copidosoma floridanum (Hymenoptera: Encyrtidae): in vivo and in vitro analysis

The polyembryonic wasp Copidosoma floridanum produces two morphologically distinct types of larvae in its host Trichoplusia ni. Reproductive larvae consume the host, pupate, and form adult wasps, whereas precocious larvae manipulate the sex ratios of the reproductive caste and defend the brood against interspecific competitors. The previous study indicated that morphogenesis of the reproductive caste was associated with a 9-day competency period, and that ecdysteroids of host origin were required for completion of embryogenesis. Here we investigated whether factors associated with the host environment mediate morphogenesis of precocious larvae and caste determination. Embryogenesis of precocious larvae was found to be synchronized with specific stages of the host first-fourth instars. However, development of precocious larvae did not depend on environmental factors specifically associated with these host stages. Elevation of the host juvenoid titer using the analogue methoprene induced T. ni to undergo a supernumerary sixth instar, but did not alter the proportion of wasp embryos that developed into precocious and reproductive larvae. In contrast, embryos competent to initiate morphogenesis developed into precocious larvae when transplanted into novel host stages such as pupae. Development of precocious larvae was arrested by ablation of the host's source of ecdysteroids, but could be rescued dose-dependently by injection of 20-hydroxyecdysone. In vitro rearing studies confirmed that completion of embryogenesis of the precocious caste required an exogenous pulse of 20-hydroxyecdysone. Combined with previous studies, our results indicate that embryos forming precocious and reproductive larvae acquire the competence to undergo morphogenesis at different times. However, we find no evidence to suggest that caste determination is mediated by environmental factors associated with a specific stage of the host.     

Effects of maternal diet on fecundity and larval development in the 'primitive' granivorous carabid Amara (Curtonotus) macronota

Granivory is one of the most specialized food habits in the Carabidae and has been reported for species from the tribes Zabrini and Harpalini. Most studies of carabid granivory have been conducted using specialized granivorous species, and few have examined primitive ones. This study examined effects of maternal diet on fecundity and larval development in Amara ( Curtonotus ) macronota (Solsky) (Coleoptera: Carabidae), a member of the most basal clade of the tribe Zabrini; a previous study indicated that larvae of this species are omnivores with a tendency toward carnivory. Three diet types, Tenebrio molitor L. larvae, mixed seeds [ Bidens frondosa L. (Asteraceae), Setaria spp., and Digitaria ciliaris (Retz.) Koeler (both Poaceae)], and T. molitor larvae + mixed seeds, were used as maternal diets, and larvae were reared on T. molitor larvae + mixed seeds (optimal diet) or T. molitor larvae (suboptimal diet). Fecundity differed significantly among treatments, with individuals fed the T. molitor larvae + mixed seeds diet having the highest fecundities and the mixed seeds diet producing the lowest values. Larval development [survival and duration of development through pre-overwintering stages (first and second instars)] was not significantly affected by either maternal or larval diets, but the addition of seeds to the maternal diet had a weak negative effect on larval survival. These results are in contrast with findings from specialized granivorous carabids, in which both adults and larvae performed best with pure-seed diets. Recent molecular phylogenies indicate that these specialized granivorous carabids belong to derived lineages, while A. macronota is the most basal clade of each tribe. These results are compatible with the hypothesis that carabid granivory has evolved gradually from ancestral carnivory, with omnivorous habits occurring as a transient state.     

Differential replication of ribosomal DNA during larval development in Calliphora (Diptera)

The hybridization of ribosomal RNA with DNA extracted from whole insects has been compared at six stages during the development of the blowfly Calliphora stygia. Total DNA from hatching larvae contains approximately 20% more rDNA than does total DNA from either mid-embryonic or adult insects. DNA from late third-instar larvae close to pupariation contains about 30% less rDNA than that from hatching larvae. The proportion of rDNA to total DNA in larvae of intermediate ages falls between these two extremes. Ribosomal RNA cistrons are thus replicated more rapidly than the remainder of the genome in the late embryonic or earliest larval stages, but more slowly during the later cycles of DNA synthesis in those larval tissues which develop high levels of polyteny.     

A three stage population model with cannibalism

A simple population model consisting of one adult and two larval stages with cannibalism or competition among the larval stages is presented. The solutions are found to be either periodic or of a steady state nature depending on the ratios of fertility and cannibalism among the larvae. Two similar cannibalism pressure functions are compared and the conditions that lead to steady or periodic solutions, or to extinction, are examined.