Metamorphosis in Craniiformea revisited: Novocrania anomala shows delayed development of the ventral valve

Anatomy of the coelomic system in Novocrania anomala (Brachiopoda, Craniiformea) and relationships within brachiopods

Brachiopoda is a relict phylum of marine benthic animals that have not been adequately studied with modern microscopy methods. Microscopic study may provide useful information on the evolution of the brachiopod body plan and brachiopod phylogeny. Understanding the organisation of the coelomic system is important because of its role in body form and compartmentalisation. Most brachiopods are considered to have a bipartite coelomic system; the only known exception is Lingulida, which have a tripartite coelomic system. In the present study, we provide the first complete 3D reconstruction of the coelomic system in the craniide brachiopod Novocrania anomala (Müller, 1776). Its coelomic system consists of the following five main parts, which are entirely separated from each other: 1) a pair of large brachial canals; 2) a complex system of paired small brachial canals and a perioesophageal coelom; 3) frontal coelomic chambers; 4) a main trunk coelom, which includes several semi-detached muscular chambers and mantle sinuses; and 5) a pair of posterior adductors chambers. These results indicate that the coelomic system of N. anomala (and perhaps of other craniides) is complex and cannot be considered to be bipartite or tripartite. The frontmost part of the coelomic system is represented by a pair of frontal chambers, which are considered to be a part of the lophophore but which are derived from dorsal mantle fold extensions and thus may be a part of the trunk coelomic system. A number of similarities were discovered between craniiformean and rhynchonelliformean coelomic systems, including the prominent dorsal projections of the large brachial canals and the morphological features of the perioesophageal coelom. The complex subdivision of the N. anomala trunk coelom is explained by the location and function of muscles, and by the location of several mesenteries.

Hox gene expression in postmetamorphic juveniles of the brachiopod Terebratalia transversa

BACKGROUND

Hox genes encode a family of homeodomain containing transcription factors that are clustered together on chromosomes of many Bilateria. Some bilaterian lineages express these genes during embryogenesis in spatial and/or temporal order according to their arrangement in the cluster, a phenomenon referred to as collinearity. Expression of Hox genes is well studied during embryonic and larval development of numerous species; however, relatively few studies focus on the comparison of pre- and postmetamorphic expression of Hox genes in animals with biphasic life cycle. Recently, the expression of Hox genes was described for embryos and larvae of Terebratalia transversa, a rhynchonelliformean brachiopod, which possesses distinct metamorphosis from planktonic larvae to sessile juveniles. During premetamorphic development, T. transversa does not exhibit spatial collinearity and several of its Hox genes are recruited for the morphogenesis of novel structures. In our study, we determined the expression of Hox genes in postmetamorphic juveniles of T. transversa in order to examine metamorphosis-related changes of expression patterns and to test whether Hox genes are expressed in the spatially collinear way in the postmetamorphic juveniles.

RESULTS

Hox genes are expressed in a spatially non-collinear manner in juveniles, generally showing similar patterns as ones observed in competent larvae: genes labial and post1 are expressed in chaetae-related structures, sex combs reduced in the shell-forming epithelium, whereas lox5 and lox4 in dorso-posterior epidermis. After metamorphosis, expression of genes proboscipedia, hox3, deformed and antennapedia becomes restricted to, respectively, shell musculature, prospective hinge rudiments and pedicle musculature and epidermis.

CONCLUSIONS

All developmental stages of T. transversa, including postmetamorphic juveniles, exhibit a spatial non-collinear Hox genes expression with only minor changes observed between pre- and postmetamorphic stages. Our results are concordant with morphological observation that metamorphosis in rhynchonelliformean brachiopods, despite being rapid, is rather gradual. The most drastic changes in Hox gene expression patterns observed during metamorphosis could be explained by the inversion of the mantle lobe, which relocates some of the more posterior larval structures into the anterior edge of the juveniles. Co-option of Hox genes for the morphogenesis of novel structures is even more pronounced in postmetamorphic brachiopods when compared to larvae.

Earliest ontogeny of early Cambrian acrotretoid brachiopods - first evidence for metamorphosis and its implications

Background

Our understanding of the ontogeny of Palaeozoic brachiopods has changed significantly during the last two decades. However, the micromorphic acrotretoids have received relatively little attention, resulting in a poor knowledge of their ontogeny, origin and earliest evolution. The uniquely well preserved early Cambrian fossil records in South China provide a great new opportunity to investigate the phylogenetically important ontogeny of the earliest acrotretoid brachiopods, and give new details of the dramatic changes in anatomy of acrotretoid brachiopods during the transition from planktotrophic larvae to filter feeding sedentary juveniles.

Results

Well preserved specimens of the earliest Cambrian acrotretoid brachiopods Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis (Cambrian Series 2, Shuijingtuo Formation, Three Gorges area, South China) provide new insights into early acrotretoid ontogeny, and have significance for elucidating the poorly understood early phylogeny of the linguliform brachiopods. A more comprehensive understanding of the applied terminology based on new observation, especially in definition of the major growth stages (embryo, planktotrophic larva, post-metamorphically sessile juvenile and adult), is established. The so-called acrotretoid “larval shell” of both valves of Eohadrotreta demonstrates evidence for metamorphosis (shedding of the larval setae and transitions of shell secretion), during the planktotrophic stage. Therefore, it is here termed the metamorphic shell. The inferred early acrotretoid larval body plan included a bivalved protegulum, secreted at the beginning of the pelagic stage, which later developed two pairs of larval dorsal setal sacs and anterior–posterior alignment of the gut during metamorphosis.

Conclusion

The primary larval body plan of acrotretoid Eohadrotreta is now known to have been shared with most early linguliforms and their relatives (including paterinates, siphonotretoids, early linguloids, the problematic mickwitziids, as well as many early rhynchonelliforms). It is suggested that this type of earliest ontogeny can be considered as plesiomorphic for the Brachiopoda and probably first evolved in stem group brachiopods with subsequent heterochronic changes.

An early Cambrian agglutinated tubular lophophorate with brachiopod characters

The morphological disparity of lophotrochozoan phyla makes it difficult to predict the morphology of the last common ancestor. Only fossils of stem groups can help discover the morphological transitions that occurred along the roots of these phyla. Here, we describe a tubular fossil Yuganotheca elegans gen. et sp. nov. from the Cambrian (Stage 3) Chengjiang Lagerstätte (Yunnan, China) that exhibits an unusual combination of phoronid, brachiopod and tommotiid (Cambrian problematica) characters, notably a pair of agglutinated valves, enclosing a horseshoe-shaped lophophore, supported by a lower bipartite tubular attachment structure with a long pedicle with coelomic space. The terminal bulb of the pedicle provided anchorage in soft sediment. The discovery has important implications for the early evolution of lophotrochozoans, suggesting rooting of brachiopods into the sessile lophotrochozoans and the origination of their bivalved bauplan preceding the biomineralization of shell valves in crown brachiopods.

Organization and metamorphic remodeling of the nervous system in juveniles of Phoronopsis harmeri (Phoronida): insights into evolution of the bilaterian nervous system

BACKGROUND

Metamorphic remodeling of the nervous system and its organization in juvenile may shed light on early steps of evolution and can be used as an important criterion for establishing the relationships among large groups of animals. The protostomian affiliation of phoronids does not still have certain morphological and embryological proofs. In addition, the relationship of phoronids and other former "lophophorates" is still uncertain. The resolving of these conflicts requires detailed information from poorly investigated members of phoronids, such as Phoronopsis harmeri.

RESULTS

During metamorphosis, the juvenile consumes the nerve elements of the larval hood. Two dorsolateral groups of larval perikarya remain and give rise to the dorsal ganglion, which appears as the "commissural brain". The juvenile inherits the main and minor tentacular nerve rings from the larva. Although the larval tentacles are directly inherited by the juvenile in P. harmeri, the ultrastructure and location of the definitive tentacular neurite bundles change greatly. Innervation of the juvenile lophophore exhibits a regular alternation of the intertentacular and abfrontal neurite bundles. The giant nerve fiber appears at early stage of metamorphosis and passes from the right group of dorsolateral perikarya to the left side of the body.

DISCUSSION

THE METAMORPHIC REMODELING OF THE PHORONID NERVOUS SYSTEM OCCURS IN TWO DIFFERENT WAYS: with complete or incomplete destruction of organ systems. The morphology of the lophophore seems similar to those of the former members of "Lophophorata", but its innervation differs greatly. These findings support the separation of bryozoans from Lophophorata and establish a need for new data on the organization of the brachiopod nervous system. The nervous system of the phoronid juvenile is organized as an epidermal nerve plexus but exhibits a nerve center in the anterior portion of the body. The simultaneous presence of both the apical organ and anlage of the cerebral ganglion in phoronids at the larval stage, and the reduction of the apical organ during metamorphosis support the Trochea theory and allow to suggest the presence of two nervous centers in the last common ancestor of the Bilateria. Phoronids retained some plesiomorphic traits and can be regarded as one of the most primitive groups of lophotrochozoans.