Why the term “larva” is ambiguous, or what makes a larva?

Haug, Joachim T.

doi:10.1111/azo.12283

“…According to the first hypothesis, the specific actinotrocha larva would represent an evolutionary novelty in the life cycle of phoronids, which was intercalated in the phoronid lineage and that is why it is not patterned by an ancestral Hox gene system. Such an idea is supported by the fact, that the actinotrocha body plan does not bear obvious homology to those of any other spiralian larvae [80,[115][116][117]. Additionally, similar to the case of pilidium, most of the larval tissues are lost during the drastic metamorphosis event and the larval A-P axis is not aligned with the juvenile one [60,72,77,81,82].…”

Section: Hox Gene Expression and The Nature Of Actinotrocha Larvaementioning

confidence: 99%

Hox gene expression during development of the phoronid Phoronopsis harmeri

Gąsiorowski

¹

,

Hejnol

²

2020

EvoDevo

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Background: Phoronida is a small group of marine worm-like suspension feeders, which together with brachiopods and bryozoans form the clade Lophophorata. Although their development is well studied on the morphological level, data regarding gene expression during this process are scarce and restricted to the analysis of relatively few transcription factors. Here, we present a description of the expression patterns of Hox genes during the embryonic and larval development of the phoronid Phoronopsis harmeri. Results:We identified sequences of eight Hox genes in the transcriptome of Ph. harmeri and determined their expression pattern during embryonic and larval development using whole mount in situ hybridization. We found that none of the Hox genes is expressed during embryonic development. Instead their expression is initiated in the later developmental stages, when the larval body is already formed. In the investigated initial larval stages the Hox genes are expressed in the non-collinear manner in the posterior body of the larvae: in the telotroch and the structures that represent rudiments of the adult worm. Additionally, we found that certain head-specific transcription factors are expressed in the oral hood, apical organ, preoral coelom, digestive system and developing larval tentacles, anterior to the Hox-expressing territories. Conclusions:The lack of Hox gene expression during early development of Ph. harmeri indicates that the larval body develops without positional information from the Hox patterning system. Such phenomenon might be a consequence of the evolutionary intercalation of the larval form into an ancestral life cycle of phoronids. The observed Hox gene expression can also be a consequence of the actinotrocha representing a "head larva", which is composed of the most anterior body region that is devoid of Hox gene expression. Such interpretation is further supported by the expression of head-specific transcription factors. This implies that the Hox patterning system is used for the positional information of the trunk rudiments and is, therefore, delayed to the later larval stages. We propose that a new body form was intercalated to the phoronid life cycle by precocious development of the anterior structures or by delayed development of the trunk rudiment in the ancestral phoronid larva.

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“…According to the first hypothesis, the actinotrocha larva might represent an evolutionary novelty in the life cycle of phoronids, which was intercalated in the phoronid lineage and that is why it is not patterned by an ancestral Hox gene system. Such idea is supported by the fact, that the actinotrocha does not bear obvious homology to any other spiralian larvae (80,111,112). Moreover, the actinotrocha is lacking in Ph.…”

Section: Hox Gene Expression and The Nature Of Actinotrocha Larvaementioning

confidence: 99%

Hox gene expression during the development of the phoronidPhoronopsis harmeri

Gąsiorowski

¹

,

Hejnol

²

2019

Preprint

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Background: Phoronida is a small group of marine worm-like suspension feeders, which together with brachiopods and bryozoans form the clade Lophophorata. Although their development is well studied on the morphological level, data regarding gene expression during this process are scarce and restricted to the analysis of relatively few transcription factors. Here we present a description of the expression patterns of Hox genes during the embryonic and larval development of the phoronid Phoronopsis harmeri. Results:We identified sequences of 8 Hox genes in the transcriptome of P. harmeri and determined their expression pattern during embryonic and larval development using whole mount in situ hybridization. We found that none of the Hox genes is expressed during embryonic development. Instead their expression is initiated in the later developmental stages, when the larval body is already formed. The Hox genes are expressed in the metasomal sac, posterior mesoderm and junction between midgut and hindgut -structures that represent rudiments of the adult worm, which emerges through the process of drastic

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“…Haug's criteria can be used to explore the developmental space that is occupied by holometabolan insects. At one end of the spectrum, one may expect species with immature stages that lack discrete features that distinguish them from the corresponding adults (i.e., species without larvae), but such species are apparently not known from insects so that all holometabolan species have at least morpho-larvae sensu lato [1]. Similarly, rare or missing in Holometabola are eco-larvae sensu strictu because the dispersal stage of endopterygote insects is the adults.…”

Section: Introductionmentioning

confidence: 99%

Contribution to understanding the evolution of holometaboly: Transformation of internal head structures during the metamorphosis in the green lacewing Chrysopa pallens (Neuroptera: Chrysopidae)

Y

¹

,

Wang

²

,

Gao

³

et al. 2020

Preprint

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Abstract Background：Metamorphosis remains one of the most complicated and poorly understood processes in insects. This is particularly so for the very dynamic transformations that take place within the pupal sheath of holometabolous insects. Only few studies address these transformations especially with regard to cranial structures of those holometabolous species where the larval and adult forms have a similar diet. It thus remains unclear to what extent the internal structures undergo histolysis and rebuilding. Here, the development of the brain and skeleto-muscular system of the head of Chrysopa pallens (Rambur, 1838) is studied. This species is a predator of aphids in the larval and adult stage.Results：We used micro-computed-tomography (µ-CT) to study the transformations of the larval, prepupal and pupal head within the cocoon. We first assessed the morphological differences and similarities between the stages. We then determined the point in time when the compound eyes appear and describe the re-orientation of the head capsule which transforms the prognathous larva into a hypognathous adult. The internal head muscles are distinctly more slender in larvae than adults. In addition, the adults have a significantly larger brain which is likely needed for the processing of the signals obtained by the adults’ vastly expanded sensory organs that are presumably needed for dispersal and mating. Our study shows that the histolysis and modification of the inner muscles and skeletal elements take place within the prepupa. The central nervous system persists throughout metamorphosis but its morphology changes significantly. Conclusion：Our study reveals that not only the inner structures, but also the outer morphology continues to change after the final larval moult. The adult cuticle and internal structures form gradually within the cocoon. The histolysis and rebuilding begins with the skeletal elements and is followed by changes in the central nervous system before it concludes with modifications of the musculature. This order of events is likely ancestral for Holometabola because it is also known from Hymenoptera, Diptera, Mecoptera, and Coleoptera.

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Why the term “larva” is ambiguous, or what makes a larva?

Cited by 49 publications

References 100 publications

Hox gene expression during development of the phoronid Phoronopsis harmeri

Hox gene expression during development of the phoronid Phoronopsis harmeri

Hox gene expression during the development of the phoronidPhoronopsis harmeri

Contribution to understanding the evolution of holometaboly: Transformation of internal head structures during the metamorphosis in the green lacewing Chrysopa pallens (Neuroptera: Chrysopidae)

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