Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida

Ultrastructural aspects of the 'statocyst' of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

The "statocyst" in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body ("statolith"), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.

Pax-Six-Eya-Dach network during amphioxus development: conservation in vitro but context specificity in vivo

The Drosophila retinal determination gene network occurs in animals generally as a Pax-Six-Eyes absent-Dachshund network (PSEDN). For amphioxus, we describe the complete network of nine PSEDN genes, four of which-AmphiSix1/2, AmphiSix4/5, AmphSix3/6, and AmphiEya-are characterized here for the first time. For amphioxus, in vitro interactions among the genes and proteins of the network resemble those of other animals, except for the absence of Dach-Eya binding. Amphioxus PSEDN genes are expressed in highly stage- and tissue-specific patterns (sometimes conspicuously correlated with the local intensity of cell proliferation) in the gastrular organizer, notochord, somites, anterior central nervous system, peripheral nervous system, pharyngeal endoderm, and the likely homolog of the vertebrate adenohypophysis. In this last tissue, the anterior region expresses all three amphioxus Six genes and is a zone of active cell proliferation, while the posterior region expresses only AmphiPax6 and is non-proliferative. In summary, the topologies of animal PSEDNs, although considerably more variable than originally proposed, are conserved enough to be recognizable among species and among developing tissues; this conservation may reflect indispensable involvement of PSEDNs during the critically important early phases of embryology (e.g. in the control of mitosis, apoptosis, and cell/tissue motility).

Axial patterning in cephalochordates and the evolution of the organizer

The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins (BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character.

Insights into spawning behavior and development of the european amphioxus (Branchiostoma lanceolatum)

The cephalochordate amphioxus (Branchiostoma sp.) is an important animal model for studying the evolution of chordate developmental mechanisms. Obtaining amphioxus embryos is a key step for these studies. It has been shown that an increase of 3-4 degrees C in water temperature triggers spawning of the European amphioxus (Branchiostoma lanceolatum) in captivity, however, very little is known about the natural spawning behavior of this species in the field. In this work, we have followed the spawning behavior of the European amphioxus during two spawning seasons (2004 and 2005), both in the field and in captivity. We show that animals in the field spawn approximately from mid-May through early July, but depending on the year, they show different patterns of spawning. Thus, even if temperature has a critical role in the induction of the spawning in captivity, it is not the major factor in the field. Moreover, we report some improvements on the methodology for inducing spawning in captivity (e.g. in maintenance, light cycle control and induction of spawning in a laboratory without running sea water system). These studies have important implications for amphioxus animal husbandry and for improving laboratory techniques to develop amphioxus as an experimental animal model.

Continuous culture of two lancelets and production of the second filial generations in the laboratory

The lancelet is considered to be a promising laboratory model animal. To establish laboratory colonies of lancelet, we collected parental lancelets of Branchiostoma belcheri and B. japonicum (previously named as B. belcheri tsingtauense) with fully developed gonads from Xiamen Rare Marine Creature Conservation Areas (Fujian, China) on dates just before their spawning in the field in 2005. Those parental lancelets spawned spontaneously in the laboratory and produced thousands upon thousands of fertilized eggs. After carefully hatching and maintaining for almost 1 year, we successfully obtained about 500 first generation (F1) adults of B. belcheri and 3,300 of B. japonicum. Part of those F1 lancelets ripened and spontaneously spawned in 2006, and several thousands of second generation (F2) individuals of both species were produced. The young F2 lancelets are growing in good condition and some of B. japonicum initiated gonad development in December, 2006. Our experience emphasizes that cleanness of settlement substratum and sufficient food supply are important factors for long-term culture of lancelets in the laboratory.