An updated staging system for cephalochordate development: one table suits them all

Abstract

Background:The chordates are divided into three subphyla: Vertebrata, Tunicata and Cephalochordata. Phylogenetically, the Cephalochordata, more commonly known as lancelets or amphioxus, constitute the sister group of Vertebrata plus Tunicata. Due to their phylogenetic position and their conserved morphology and genome architecture, lancelets are important models for understanding the evolutionary history of chordates. Lancelets are small, marine filter-feeders, and the few dozen species that have so far been described have been grouped into three genera: Branchiostoma, Epigonichthys and Asymmetron.Given their relevance for addressing questions about the evolutionary diversification of chordates, lancelets have been the subjects of study by generations of scientists, with the first descriptions of adult anatomy and developmental morphology dating back to the 19 th century. Today, several different lancelet species are used as laboratory models, predominantly for developmental, molecular and genomic studies. It is thus very surprising that there is currently no universal staging system and no unambiguous nomenclature for developing lancelets. Results:We illustrated the development of the European amphioxus (Branchiostoma lanceolatum) using confocal microscopy and compiled a streamlined developmental staging system, from fertilization through larval life, with an unambiguous stage nomenclature. By tracing growth curves of the European amphioxus reared at different temperatures, we were able to show that our staging system permits the easy conversion of any developmental time into a defined 3 stage name. Furthermore, comparisons of embryos and larvae from the European amphioxus (B. lanceolatum), the Florida amphioxus (B. floridae), the Chinese amphioxus (B. belcheri), the Japanese amphioxus (B. japonicum) and the Bahamas lancelet (Asymmetron lucayanum) demonstrated that our staging system can readily be applied to other lancelet species.Conclusions: Here, we propose an updated staging and nomenclature system for lancelets. Although the detailed staging description was carried out on developing B. lanceolatum, comparisons with other lancelet species strongly suggest that both staging and nomenclature are applicable to all extant lancelets.We thus believe that this description of embryonic and larval development can be of great use for the scientific community and hope that it will become the new standard for defining and naming developing lancelets. KeywordsAmphioxus, Asymmetron lucayanum, Branchiostoma belcheri, Branchiostoma floridae, Branchiostoma japonicum, Branchiostoma lanceolatum, Confocal Microscopy, Embryonic and Larval Development, Evolution and Development, Lancelet 4

The Ontology of the Amphioxus Anatomy and Life Cycle (AMPHX)

“…The AMPHX development ontology is based on in vivo observations [see Carvalho et al (2021) for a morphological description of the different developmental stages] and is also inspired by the staging system proposed by Hirakow and Kajita (1990 , 1991 , 1994) , which we have expanded by adding and redefining some developmental stages. AMPHX thus presents amphioxus development subdivided into ten different periods from zygote to adult ( Figure 1 and Table 1 ).…”

“…The final gastrula stage, the gastrula-G6 stage, corresponds to a bottle-shaped gastrula. At this stage, the neuroectoderm starts to flatten [for a morphological description of these stages see Carvalho et al (2021) ].…”

“…Juveniles range in size from 5–7 mm to 3 cm and adults from 3 to 8 cm. Illustrations from zygote to larva periods have been adapted from Carvalho et al (2021) .…”

“…These studies are mainly carried out on three or four species of the genus Branchiostoma , but recently also on a species of the genus Asymmetron ( Holland and Holland, 2010 ). For this reason, and in order to standardize the results obtained in different species, we established a universal staging system ( Carvalho et al, 2021 ) as well as an ontology of the development and anatomy of this chordate subphylum. This ontology, AMPHX, is the first one developed for a cephalochordate and follows the example of ontologies previously established for other chordate species ( Segerdell et al, 2008 ; Manni et al, 2014 ; Van Slyke et al, 2014 ; Hotta et al, 2020 ).…”

Lamprey Lecticans Link New Vertebrate Genes to the Origin and Elaboration of Vertebrate Tissues

“…In support of this scenario, development in invertebrate deuterostomes is largely, or completely epithelial. Indeed, the invertebrate with the most vertebrate-like body plan, amphioxus, develops without any discernible mesenchyme [100,101].…”