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. 2020 Feb 10;11:2.
doi: 10.1186/s13227-020-0148-z. eCollection 2020.

Hox Gene Expression During Development of the Phoronid Phoronopsis harmeri

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Free PMC article

Hox Gene Expression During Development of the Phoronid Phoronopsis harmeri

Ludwik Gąsiorowski et al. Evodevo. .
Free PMC article

Abstract

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.

Keywords: Biphasic life cycle; Body plan; Brain; Head; Indirect development; Intercalation; Life history evolution; Lophophorata; Lox2; Spiralia.

Conflict of interest statement

Competing interestsThe authors declare that they have no competing interests.

Figures

Fig. 1
Fig. 1
Phoronopsis harmeri: morphology of the anterior part of living animal (a) and scheme of its life cycle (b). Hox clusters organization and Hox genes complement in various Spiralia (c), based on [16, 89, 107]. Metasomal sac and adult trunk originating from it are marked in red in b. Gene antp from Phoronis australis (marked with asterisk) was originally described as lox2 (see text for discussion). For Phoronopsis harmeri and Novocrania anomala only the Hox complement is available (data on cluster organization are missing). The vertical bars in C indicate boundaries of the particular scaffolds of the split Hox clusters. Abbreviations: dt digestive tract, lp lophophore, ms metasomal sac, te larval tentacles, tr adult trunk
Fig. 2
Fig. 2
Bayesian phylogeny under JTT + I + G substitution model of the amino acid sequences of spiralian Hox genes homeodomains, including phoronid sequences. Genes from Ph. harmeri are marked in red. Posterior probability values are shown for important clades. Full species names and sequences accession number are provided in Additional file 1: Table S1
Fig. 3
Fig. 3
Development of Phoronopsis harmeri. Blastula, 12 hpf (A, A’); gastrula, 24 hpf (B, B’); early larva, 42 hpf (C, C’); pre-tentacle larva, 56 hpf (D, D’); actinotrochae: 3 dpf (E, E’), 5 dpf (F, F’) and 7 dpf (G, G’). For each developmental stage the left panel shows embryo or larvae in dorso-ventral view and right panel (marked as’) in lateral view with ventral to the left; anterior is to the top on all panels. Scalebars 50 μm. am anterior mesoderm, ao apical organ, ar archenteron wall, bc blastocoel, bp blastopore, es esophagus, mg midgut, mo mouth opening, ms metasomal sac, oh oral hood, pd proctodeum, pm posterior mesoderm, pn protonephridium, pr protonephridial rudiment, rt tentacle rudiment, st stomach, te tentacle, tt telotroch
Fig. 4
Fig. 4
Whole-mount in situ hybridization of each Hox gene during larval development of Phoronopsis harmeri. Name of each hybridized gene is shown on the left, while developmental stages are indicated on the top. All the stages are presented with anterior to the top. Larvae on panels a, c, e, g and i are in dorso-ventral view, whereas larvae on panels b, d, f, h and j in lateral view with ventral to the left. The black line indicates the onset of expression of each Hox gene based on in situ hybridization data. Black arrowheads indicate expression in the metasomal sac, blue arrowheads expression in the protonephridia, red arrowheads expression in the mesoderm, green arrowheads expression in the telotroch and magenta arrowheads expression in the digestive system. The detailed expression patterns are described in the text. Photographs are not to scale
Fig. 5
Fig. 5
Details of the expression of some of the Hox genes in the actinotrocha larvae of Phoronopsis harmeri. Expression of the Hox genes in the metasomal sac of 8-tentacle actinotrochae (af) and schematic interpretation of those expression patterns (a’f’). Expression of pb in the 8-tentacle actinotrocha (g). Expression of lox5 in the left mesoderm of late 6-tentacle (h) and 8-tentacle actinotrocha (i). Expression of lox4 in the digestive system of 8-tentacle actinotrocha (j). Scale bars 25 μm. ms metasomal sac, iw intestinal wall. Blue arrowheads indicate expression in the protonephridia, red arrowheads expression in the mesoderm and magenta arrowhead expression in the digestive system
Fig. 6
Fig. 6
Expression of head-specific genes in early larva (ae, h, i, np, v, w) and 8-tentacle stage actinotrocha (f, g, jm, qu, x, y) of Phoronopsis harmeri and comparison of expression of head-specific and Hox genes in both larval stages (z). For each panel the name of hybridized gene is shown in the white box above micrographs. Entire larvae in the dorso–ventral (a, d, f, h, j, n, o, q, v, x) and lateral (b, c, e, g, i, k, p, r, w, y) views. Details of expression in 8-tentacle stage larvae in oral hood and anterior body region (l), hood musculature (m), apical organ, preoral coelom and rim of the hood (s) and tips of the tentacles (t, u). Black arrowheads point to the particular expression domains (see text for details), while asterisks indicate unspecific background staining. Scalebars 25 μm

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