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Many animals undergo indirect development, where their embryogenesis produces an intermediate life stage, or larva, that is often free-living and later metamorphoses into an adult. As their adult counterparts, larvae can have unique and diverse morphologies and occupy various ecological niches. Given their broad phylogenetic distribution, larvae have been central to hypotheses about animal evolution. However, the evolution of these intermediate forms and the developmental mechanisms diversifying animal life cycles are still debated. This review focuses on Spiralia, a large and diverse clade of bilaterally symmetrical animals with a fascinating array of larval forms, most notably the archetypical trochophore larva. We explore how classic research and modern advances have improved our understanding of spiralian larvae, their development, and evolution. Specifically, we examine three morphological features of spiralian larvae: the anterior neural system, the ciliary bands, and the posterior hyposphere. The combination of molecular and developmental evidence with modern high-throughput techniques, such as comparative genomics, single-cell transcriptomics, and epigenomics, is a promising strategy that will lead to new testable hypotheses about the mechanisms behind the evolution of larvae and life cycles in Spiralia and animals in general. We predict that the increasing number of available genomes for Spiralia and the optimization of genome-wide and single-cell approaches will unlock the study of many emerging spiralian taxa, transforming our views of the evolution of this animal group and their larvae.
Twelve fur seal pups, which had not nursed their mothers, were used in an infectivity experiment. Pups were exposed to parasitic 3rd-stage larvae of Uncinaria lucasi from belly tissues of fur seal bulls, bachelors, and pregnant cows, to determine maturation capability of the larvae. Hookworms were not recovered from the intestines of 3 pups receiving larvae from belly blubber of bulls, 6 pups receiving larvae from belly blubber of bachelors, and 1 nonexposed pup. Maturation of hookworms did occur in 2 pups exposed to larvae from a mixture of belly blubber, mammary tissue, and milk of pregnant cows. Parasitic 3rd-stage hookworm larvae from belly tissues of pregnant and "non-pregnant" fur seal cows averaged 938.1 and 802.1 micron long, and 34.1 and 31.5 micron wide, respectively; however, larvae from belly tissues of a fur seal bull, bachelors, 2-year-old males, male and female yearlings and pups, and Steller Sea Lion subadults averaged 640.5-732.0 micron long and 20.9-24.9 micron wide.
The bionomics of the free-living larvae of Dictyocaulus filaria on pasture, and the transmission of infection between lambs, were studied during different seasons of the year in North-East England. The rate of development of first-stage larvae to the third stage took 4-9 days in late spring and summer, 1 1/2-4 weeks in autumn and 5 1/2-7 weeks in winter. The proportion of first-stage larvae developing to the third stage ranged from 10-28% in autumn and winter, and 2-25% in spring and summer. The rate of mortality of the third-stage larvae was approximately logarithmic in nature, although the survival time was shorter in spring and summer than in autumn and winter. Third stage larvae were able to survive from autumn until the spring of the following year in sufficient numbers to perpetuate transmission but not to cause clinical disease. In a transmission study, the survival of the infective larvae on the experimental plot was poor in summer, but the larval population increased in the autumn and then declined slowly throughout the winter. Infection in the susceptible lambs was related to the level of infection on the herbage increasing in severity from early summer to late autumn. However, those lambs infected in the summer were resistant to the heavy autumn challenge of larvae on pasture.
Honeybees of the species Apis mellifera are important pollinators of crops and wild plants. Paenibacillus larvae, a spore-forming bacterium, is a problematic pathogen that causes American foulbrood (AFB) in honeybee larvae worldwide. In many countries, AFB is a notifiable disease, requiring the destruction of diseased colonies, resulting in economic loss that impacts beekeeping and agriculture. Disease onset starts with larval ingestion of P. larvae spores, which germinate into growing cells that proliferate in the larval gut, leading to larval death and eventually bee colony collapse. As infection progresses, P. larvae produce spores, reinitiating the disease cycle. Thus, growth, sporulation and germination underlie AFB. In this study, using various microbiological assays, quantitative cell biology methods, transmission electron microscopy and genomics, we sought to identify genetic and phenotypic characteristics associated with the predominant ERIC I and ERIC II genotypes of P. larvae during growth, sporulation and germination. Extending previous findings, our data identify genetic differences between ERIC I and ERIC II strains and some genetic variation between strains of the same ERIC type. Furthermore, we describe significant differences in cellular morphology during growth, differences in spore envelope structure and differences in germination efficiency between ERIC I and ERIC II genotypes. Collectively, our findings improve understanding of P. larvae biology and provide a foundation for developing genotype-specific disease management strategies for AFB.
Using Drosophila melanogaster, the relative sensitivities of various larval stages to the toxic effects of growth on media supplemented with either 0.44 or 0.88 ppm aflatoxin B1 (AFB1) were determined. Two strains of fruit flies were tested: strain A-11 which is relatively resistant to AFB1 induced toxicity, and strain A-9 which is quite sensitive. Eggs, mid-first, mid-second and early-, mid- and late-third instar larvae were transferred onto AFB1 media and allowed to complete larval and pupal development and eclose as adults. At the 0.44 ppm concentration, strain A-11 showed no effect, while only first instar larvae of strain A-9 showed significant mortality rates for first instar larvae, but the A-9 larvae die at higher rates than the A-11 larvae. In addition, second and third instar larvae of strain A-9 show significant mortality rates when grown at 0.88 ppm AFB1, while these stages are not affected in strain A-11.
The authors have been studying the bacterial diseases of the brood for a fairly long time. American and European foul brood has been studied with the highest attention. Cultures of Bacillus larvae (White, 1906) were examined both in freshly isolated strains and in collection strains of this micro-organism. In cases of foul brood, the pathological material was found to contain not only the typical rods of B. larvae but also immobile spiral forms which are usually referred to in literature as fragments or developmental forms of B. larvae. These spiral forms were found to constitute spindle-shaped formations in the culture of B. larvae; the multiplication of these spindles depends on the presence of the rods of B. larvae and their development and reproduction can be observed on wet gelatine agar in a Petri dish turned upside down under a normal microscope (10 X 10 magnification). In the combined liquid medium, used in the experiments, these formations disintegrate into immobile spirals; if re-cultivated on a solid medium they re-assume their spindle shape with transverse meridian arrangement (in different amounts). Staining for proving the presence of nucleic acids does not eliminate the possibility of these formations being separate micro-organisms which cannot be stained by current staining methods but can be represented by the contrast method according to Burri, or by silvering according to Klein. The authors succeeded to separate these micro-organisms, but without the rods of B. larvae the colonies of these formations are feeble.
Larvae of the yellow mealworm, Tenebrio molitor L., Gembloux strain, race F, were reared on diets of 17 cultivars of wheat and 29 cultivars of barley, prepared for determination of digestible energy with mice, for 4 weeks at 27 +/- 0.25 degrees C and 65 +/- 5% relative humidity. Values for percentage crude protein of tissues of larvae fed wheat cultivars were significantly and positively correlated with values for digestible energy as determined with mice. These values were not correlated for larvae fed barley cultivars; however, values for per cent dry matter content of larvae were significantly and positively correlated with values for digestible energy determined with mice. This apparent discrepancy is explained on the basis of the chemical constitution of barley and the availability of amino acids of barley to the larvae. Use of larvae of Tenebrio molitor to indicate the digestible energy of cereal grains is feasible, provided that the proper parameter is chosen. Nevertheless, use of this biological method seems more suitable for evaluation of protein quality and of amino acid availability than for a measure of digestible energy of feeds.
Toxocara larval excretions and secretions collected from in vitro culture were used as antigen in passive haemagglutination and soluble antigen fluorescent antibody tests for the diagnosis of visceral larva migrans in experimental animals and man. Antibody to toxocaral secretions was detected in rabbits within 13 days of light Toxocara infection (ten larvae per kg) and within four days of heavy infection (10(4) larvae). Antibody was not detected following infection with 10(4) Ascaris suum larvae. In human sera, antibody was detected at low titre in 1% of 100 healthy adults and in 2% of 50 children. High titres were observed in one third of 170 patients with suspected visceral larva migrans and in 23 of 27 such patients presenting with an eosinophilia greater than 20%. In 25 patients with ocular lesions of an undiagnosed nature, four showed significant levels of anti-Toxocara antibody.
The La Crosse (LAC) virus infection rate of Aedes triseriatus larvae that ingest LAC virus does not appear to be increased by concomitant infection of larvae by the gregarine parasite, Ascocystis barretti. Infection rates ranged only from 0--2.6% in adult Ae. triseriatus reared from groups of A. barretti-infected larvae that had ingested LAC virus (California encephalitis group) at dosages of 2.0--7.7 log10 SMICLD50/ml. Females resulting from orally infected larvae transmitted LAC virus to suckling mice. Larvae that were infected with A. barretti and devoured carcasses of adult mosquitoes containing 4.7 log10 SMICLD50/ml failed to become infected. A. barretti spores developing in transovarially infected mosquitoes did not harbor LAC virus; thus, A. barretti does not appear to be a mechanism for virus dispersal.
In order to find out factors affecting the presence of two generations of O. ovis in the southern parts of its distribution area there were conducted regular measurements of the body length of the Ist instar (3718 specimens) and weight of the IInd and IIId instar larvae (1460 specimens). These observations were carried out in two herds of sheep in low-lying and foothill zones of Aserbaijan. Statistical analysis has shown the following: a) the growth inhibition, which is especially distinct in autumn-spring generation, takes place in the Ist instar larvae 1.76-2.20 mm long inhabiting the walls of the nasal cavity and concha (their average body length at hatching is 1.08 plus or minus 0.004 mm); the inhibition is associated with interpopulation relations and apparently does not depend on the date of its beginning and can last from 6 to 7 months; c) after the growth resumption the development continues uninterruptedly up to the moulting; the inhibition is also possible at the beginning of the 2nd instar and then the development proceeds without any intervals up to the complete maturation of larvae. The total number of the Ist instar larvae in the bot fly populations is two times higher than that of the IInd and III-instars. This points to a high mortality of larvae during the Ist instar developmental period.
Eosinophil and/or neutrophil leukocytes appear to have important roles in host defense against invasive, migratory helminth infestations, but the mechanisms of larval killing by leukocytes are uncertain. This study examines killing of newborn (migratory phase) larvae of Trichinella spiralis during incubation with granule preparations of human eosinophils or neutrophils and generators of hydrogen peroxide (glucose-glucose oxidase) (G-GO) or superoxide and hydrogen peroxide (xanthine-xanthine oxidase). Larvae were killed by either hydrogen peroxide-generating system in a concentration-dependent manner. Direct enumeration of surviving larvae after incubation in microtiter wells containing the appropriate reagents was used in assess larval killing. Verification of the microplate assay was demonstrated by complete loss of larval ability to incorporate [(3)H]deoxyglucose and loss of infectivity after incubation in comparable concentrations of G-GO. Larvae were highly sensitive to oxidative products; significant killing occurred after incubation with 0.12 mU glucose oxidase and complete killing occurred with 0.5 mU. Comparable killing of bacteria required over 60 mU glucose oxidase. At 5 mU glucose oxidase, killing was complete after 6 h of incubation. Killing by G-GO was inhibited by catalase but not by boiled catalase or superoxide dismutase and was enhanced by azide. Addition of peroxidase in granule pellet preparations of eosinophils or neutrophils did not enhance killing by G-GO. These data indicate a remarkable susceptibility of newborn larvae of T. spiralis to the hydrogen peroxide generated by neutrophil and eosinophil leukocytes.
Infection rates ranged from 0-2.1% in adults of Aedes triseriaus reared from groups of larvae that had ingested La Crosse (LAC) virus (Clifornia encephalitis group) at dosages of 7.0-8.3 log 10 SMICLD50/ml. Females form orally infected larvae transmitted the virus to suckling mice. Larvae that devoured carcasses of transovarially infected larvae containing 3.0 log 10 SMICLD 50/ml failed to become infected. Ingestion by larvae of infected carcasses appears, therefore, to be unimportant as a method of horizontal amplification of LAC virus.
Lignin is a crucial defense phytochemical against phytophagous insects. Cinnamoyl-CoA reductase (CCR) is a key enzyme in lignin biosynthesis. In this study, transgenic Populus davidiana × P. bolleana overexpressing the PdbCCR gene were generated via Agrobacterium-mediated transformation. Successful integration of PdbCCR into the poplar genome was confirmed by PCR amplification and quantitative reverse transcription PCR (qRT-PCR). The lignin content in the transgenic poplar leaves was significantly higher than that in the wild poplar, and after L. dispar larvae fed on the transgenic poplar, the CCR activity was clearly induced. The L. dispar larvae grew slowly after feeding on transgenic poplar and the laccase, cellulase and three detoxifying enzymes were induced compared with larvae after feeding on wild-type poplar. The bioassay further revealed that transgenic poplar plants overexpressing PdbCCR showed a high level of resistance to L. dispar larvae. These results confirmed that PdbCCR is a candidate gene for breeding insect resistant poplar.
BACKGROUND: Porphyrophora sophorae is a subterranean piercing-sucking scale insect that damages licorice (Glycyrrhiza uralensis) roots, but the molecular responses associated with larval root colonization remain insufficiently defined. METHODS: We compared non-parasitic larvae (NP) and root-colonizing larvae (RC) using six RNA-seq libraries, de novo transcriptome assembly, DESeq2-based differential expression analysis, GO/KEGG enrichment, annotation-based candidate gene screening, and RT-qPCR validation of selected genes. RESULTS: Sequencing yielded 260.91 million clean reads, and de novo assembly produced 60,794 non-redundant transcripts. DESeq2 identified 703 FDR-significant DEGs, including 49 upregulated and 654 downregulated genes in RC larvae. Upregulated genes were mainly associated with translation- and ribosome-related processes, whereas downregulated genes were enriched in mitochondrial, oxidation-reduction, energy metabolism, and oxidative phosphorylation-related functions. Annotation-based screening identified 75 FDR-significant candidate genes associated with chemosensation, defense-related responses, and energy metabolism, with mitochondrial energy metabolism-related genes forming the largest module. RT-qPCR validation based on the raw Ct data showed concordant expression directions for ten selected transcript targets. CONCLUSIONS: Root colonization in P. sophorae larvae was associated with coordinated transcriptional remodeling involving selective activation of translation-related processes, adjustment of mitochondrial energy metabolism, and changes in defense-related gene expression. These results provide candidate molecular targets for future functional studies of host contact, feeding establishment, and physiological adjustment in this subterranean scale insect.
For both visceral larva migrans and alveolar hydatid diseases, it is the feces of companion animals that are the primary source of human infection. Clearly, whatever is done to reduce this form of environmental contamination and the prevalence of the parasites' infective stages will help to protect the health of the human population in the United States and Canada, particularly the children. Very important are appropriate anthelmintic treatment programs, especially for T. canis, and proper control of close associations between pets and people. Probably the known incidence of visceral larva migrans in humans and the range of its known pathogenicity will increase as better diagnostic methods become available. In addition, other animal helminths, for instance Toxocara cati and Toxascaris leonina, may become more substantially documented as causes of human visceral larva migrans. Until more is known of the geographical distribution and prevalence of Echinococcus multilocularis among animals in North America, it will be difficult to assess accurately the future significance of alveolar hydatid disease in the continent's human population. Another important advance would be the development of anthelmintics that are effective for somatic second stage larvae of T. canis, and others effective for adult or larval Echinococcus species. Meanwhile it is a most important responsibility of the veterinary profession to educate its clients thoroughly concerning these dangerous helminths.
The intervals between the jetting of sheep with dieldrin or BHC, and the inducement of flystrike by implants of hybrid and homozygous BHC/dieldrin resistant larvae of Lucilia cuprina, were compared. Flystrikes soon established with homozygous resistant larvae, but considerable protection was afforded against hybrid larvae, the interval being longer with BHC than with dieldrin. Dieldrin was shown to have a residual effect on rr larvae in a mixed population 9 weeks after jetting. It is suggested that wide adoption of mulesing and mid-season's crutching would favour reversion to susceptibility in the blowfly and thus extend the field life of insecticides to which resistance has emerged.
We describe some flatworms (some in the genus Mesostoma) that kill mosquito larvae and may account for the variability in the population densities of Culex tarsalis and Anopheles freeborni in rice fields. When mosquito larvae brush against these worms, the larvae immediately become paralyzed and die. When C. tarsalis larvae are placed inside floating cages that exclude flatworms (50-micromter mesh), there is a fourfold increase in the their survival. Rice fields that have abundant mosquito populations lack flatworms. Most such fields have only recently been turned over to rice production, suggesting that the flatworms have difficulty dispersing to new fields but, once established, are able to overwinter and control mosquitoes for the subsequent years of rice production.