Amphioxus, commonly referred to as lancelets, are benthic filter feeding chordates found throughout tropical and temperate coastal waters. These animals share many similarities to vertebrates in terms of size and structure.
We examined the expression of SoxB1 paralogs in amphioxi embryos and discovered they are present throughout neurulation. This suggests they may have a similar role in neurectoderm specification and neuron differentiation as their vertebrate and Drosophila homologs do.
Origins
They are small marine animals found mainly in warmer oceanic waters and less frequently in temperate waters. Based on morphological evidence and molecular similarities, they appear to be closely related to vertebrates.
They are distinctive in that they lack a mucus coat and possess only an epithelium (naked simple epithelium), unlike most other invertebrates which develop a cuticle layer on their skin. This trait stands alone among extant amphioxus clades and suggests it might have originated as an ancestral character within their lineage.
Amphioxus clades across North America exhibit benthic habits; they burrow into sand, gravel or shell deposits for shelter. Furthermore, some are tolerant to seagrass habitat.
The amphioxus life cycle is a planktonic one with ten developmental periods from zygote to adult. The earliest stage is the zygote stage followed by early embryonic and larval stages which are both planktonic; juvenile and adult stages are benthic in origin and typically measure between 80-100mm long while embryos in this age range between 3-8cm in length.
Amphioxus larvae show asymmetric development, especially around its primary gill openings and anus. Furthermore, amphioxus has asymmetrical gonad development; in genus Epigonichthys, the gonads develop on one side while Branchiostoma’s found both sides of its body.
An integral aspect of amphioxus embryo axial patterning is the presence of a notochord, which is present in all chordates but often absent or highly modified in other animal species. The organizer is a skeletal structure present in their embryo which assists with organizing its main body axes and has implicated in its evolution41.
Amphioxus is an important model organism for studying the evolutionary transition between invertebrates and vertebrates. Its basic morphology and genome make it a prime candidate for studying this process.
Furthermore, there’ evolution rate is rapid despite its slow divergence times3,4,5,10. This study suggests that amphioxus may be evolving at rates similar to vertebrates despite having much more recent origins3.
Amphioxus is an intricate organism with numerous adaptations that set it apart. These traits enable it to adapt and thrive in various environments – essential for its continued existence as a species.
Habitat
They undergo a developmental process known as metamorphosis, which involves changes to their body shape and physiological function. Similar to vertebrates, thyroid hormone (TH) regulates this process in amphioxus; it’s believed to be the driving force behind metamorphosis among chordates and amphioxus species.
In this study, we performed a phylogenetic analysis of amphioxus using 19-way concatenated multiple protein alignments based on 397 orthologous gene groups intersected by HaMSTR and BRH methods. This revealed four major clades: Bellisonia belcheri, Boronia japonicum, Lanceolatum amphioxus and Florideae; one species within each taxa (Asymmetron lucayanum); two outgroups: Acorn Worm and Sea Urchin
Our results indicated that they descended from a common ancestor during an era when the seas relatively closed off. This environment may have enabled amphioxus to adjust energy demands and production through adaptive evolution, leading to their speciation.
An analysis of pairwise p-distances between mitochondrial and nuclear gene sequences revealed that the divergence of amphioxus and vertebrates occurred during the Cretaceous (104-61 million years ago), supporting previous studies which noted a period of great extinction and rapid evolutionary change among cephalochordates (Kon et al. 2007).
Additionally, molecular dating of these species revealed branching events among clades to 240 Mya for B. belcheri, 240 Mya for B. floridae and 120 Mya for Asymmetron – Branchiostoma.
We then compared the diversity of amphioxus-specific domains to that of model vertebrates, identifying lineage-specific domains from B. belcheri and B. floridae as well as their functional enrichment in immune response, apoptosis and lipid metabolism/utilization. Our findings suggest that evolution of amphioxus-specific domains driven by multiple factors including pathogens and adaptation to changing environments.
Amphioxus requires clear oceanic water combined with sand sediment that has low organic content. This condition most often found in shallow coastal seawater at 8-16 m depth, providing an ideal filter-feeding habitat for them. Furthermore, they are tolerant to seagrass habitat conditions as well. These findings indicate that amphioxus has a diverse range of habitats and serves as an excellent model organism for studying its behavior, physiology, and development.
Feeding
Amphioxus are a taxon of about 30 species divided between two genera–Branchiostoma (also called amphioxus) and Epigonichthyes (also called asymmetron). These segmented coelomate deuterostomes feature gill slits and a notochord-like stiffening rod in their larval stage.
They are an important group to research as the last common ancestor of chordates, giving rise to vertebrates. They exhibit a slow evolutionary rate which is evident in their gene diversity and expression patterns.
Amphioxus harbors a diverse set of genes, which provide insight into the development and evolution of chordate morphology. Many of these genes discovered using amphioxus as a model system, while others have isolated from various other taxa.
For instance, the genes responsible for lateralizing the pharynx regulated by retinoic acid (RA), suggesting they evolved during the last common ancestor of chordates. Furthermore, several genes related to muscle development and regeneration have identified in amphioxus which may provide a more precise representation of these structures than has been achieved with other models such as flies.
Unfortunately, they lacks several essential genes required for myoblast fusion. These include Jam-B and C, Netrin/Neogenin, as well as NFAT.
Interesting, genes involved in dorsal formation such as the notochord and hollow nerve cord appear to have evolved through a different mechanism in amphioxus. This is evident from the distribution of maternal nodal mRNA within amphioxus embryos which asymmetrically distributes along its axial axis due to remodelling of its cytoskeleton.
These asymmetric gene distributions in the embryo likely stem from factors such as RNA binding protein (Rbp) and the retinoic acid-regulated signaling pathway present in amphioxus endoderm. We have demonstrated that invertebrate chordate amphioxus, RA signaling regulates AmphiHox1 expression in the middle third of this endoderm; knockdown of AmphiHox1 function with an antisense morpholino oligonucleotide leads to a reduction in notochord size.
Breeding
During the breeding season, male and female amphioxus release gametes into the water column. This period can last anywhere from 3-6 months depending on species, followed by a larval and post-metamorphic stage. During this stage, amphioxus live buried beneath fine sand on the seafloor and filter-feed by feeding off microorganisms present in the seawater.
It exhibits a remarkable simplicity, with an average body length of 15 cm (Li et al., 2001). Furthermore, their genome sequences are among the most complete in all of animal kingdom – making them a key model organism for studying chordate developmental biology from an evolutionary perspective.
Our phylogenetic analysis of amphioxus species and populations suggests the lineage diverged at 35.6 Ma (Supplementary Figure S2; Putnam et al., 2008). The most derived lineage among extant amphioxus is Epigonichthys, which has an atrophied metapleura as well as a slight leftward shift of its buccal opening (which is located at the end of a cavity separating it from the pharynx; see Figure 5).
Though the rate of amphioxus evolutionary transition has considered slow, there remains much to discover about its development and evolution. Furthermore, new genome sequences have recently become available, providing invaluable resources for comparative genomic studies at both inter- and intraspecies levels.
To understand the evolutionary relationships among extant amphioxus species, we conducted a detailed phylogenetic analysis using several molecular methods and an alignment of genes containing amino acid sequences from five amphioxus species and 11 vertebrate species. With these data, we could construct a phylogenetic tree and calculate branch lengths for each amphioxus species.
Our analysis indicates that the ancestor of modern amphioxus was a small LCA with a ciliated larva-like body and left-right asymmetry and unilateral gonads in its midventral region. It lived in shoals on sandy seafloors and may have undergone metamorphosis to acquire its more complex morphology.
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