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Holland, LZ, Holland ND.  2007.  A revised fate map for amphioxus and the evolution of axial patterning in chordates. Integrative and Comparative Biology. 47:360-372.   10.1093/icb/icm064   AbstractWebsite

The chordates include vertebrates plus two groups of invertebrates (the cephalochordates and tunicates). Previous embryonic fate maps of the cephalochordate amphioxus (Branchiostoma) were influenced by preconceptions that early development in amphioxus and ascidian tunicates should be fundamentally the same and that the early amphioxus embryo, like that of amphibians, should have ventral mesoderm. Although detailed cell lineage tracing in amphioxus has not been done because of limited availability of the embryos and because cleavage is radial and holoblastic with the blastomeres nearly equal in size and not tightly adherent until the mid-blastula stage, a compilation of data from gene expression and function, blastomere isolation and dye labeling allows a more realistic fate map to be drawn. The revised fate map is substantially different from that of ascidians. It shows (1) that the anterior pole of the amphioxus embryo is offset dorsally from the animal pole only by about 20 degrees, (2) that the ectoderm/mesendoderm boundary (the future rim of the blastopore) is at the equator of the blastula, which approximately coincides with the 3rd cleavage plane, and (3) that there is no ventral mesoderm during the gastrula stage. Involution or ingression of cells over the blastopore lip is negligible, and the blastopore, which is posterior, closes centripetally as if by a purse string. During the gastrula stage, the animal pole shifts ventrally, coming to lie about 20 degrees ventral to the anterior tip of the late gastrula/early neurula. Comparisons of the embryos of amphioxus and vertebrates indicate that in spite of large differences in the mechanics of cleavage and gastrulation, anterior/posterior and dorsal/ventral patterning occur by homologous genetic mechanisms. Therefore, the small, nonyolky embryo of amphioxus is probably a reasonable approximation of the basal chordate embryo before the evolution of determinate cleavage in the tunicates and the evolution large amounts of yolk in basal vertebrates.

Yu, JK, Holland LZ, Holland ND.  2002.  An amphioxus nodal gene (AmphiNodal) with early symmetrical expression in the organizer and mesoderm and later asymmetrical expression associated with left-right axis formation. Evolution & Development. 4:418-425.   10.1046/j.1525-142X.2002.02030.x   AbstractWebsite

The full- length sequence and zygotic expression of an amphioxus nodal gene are described. Expression is first detected in the early gastrula just within the dorsal lip of the blastopore in a region of hypoblast that is probably comparable with the vertebrate Spemann's organizer. In the late gastrula and early neurula, expression remains bilaterally symmetrical, limited to paraxial mesoderm and immediately overlying regions of the neural plate. Later in the neurula stage, all neural expression disappears, and mesodermal expression disappears from the right side. All along the left side of the neurula, mesodermal expression spreads into the left side of the gut endoderm. Soon thereafter, all expression is down- regulated except near the anterior and posterior ends of the animal, where transcripts are still found in the mesoderm and endoderm on the left side. At this time, expression also begins in the ectoderm on the left side of the head, in the region where the mouth later forms. These results suggest that amphioxus and vertebrate nodal genes play evolutionarily conserved roles in establishing Spemann's organizer, patterning the mesoderm rostrocaudally and setting up the asymmetrical left - right axis of the body.

Yu, JK, Holland LZ, Jamrich M, Blitz IL, Holland ND.  2002.  AmphiFoxE4, an amphioxus winged helix/forkhead gene encoding a protein closely related to vertebrate thyroid transcription factor-2: expression during pharyngeal development. Evolution & Development. 4:9-15.   10.1046/j.1525-142x.2002.01057.x   AbstractWebsite

The full-length sequence and developmental expression of amphioxus AmphiFoxE4 are described. Transcripts of the gene are first detected in the pharyngeal endoderm, where the club-shaped gland is forming and subsequently in the definitive gland itself. AmphiFoxE4 is closely related to vertebrate genes encoding the thyroid-specific transcription factor-2 (TTF2), which plays an early developmental role in the morphogenesis of the thyroid gland and a later role in hormone-mediated control of thyroid function. In amphioxus, AmphiFoxE4 expression is not thyroid specific because the club-shaped gland, the only structure expressing the gene, is not homologous to the vertebrate thyroid; in-stead, the thyroid homologue of amphioxus is a specialized region of the pharyngeal endoderm called the endostyle. We propose that (a) the pharynx of an amphioxus-like ancestor of the vertebrates included a club-shaped gland that expressed FoxE4 as well as an endostyle that did not, and (b) the club-shaped gland soon disappeared in the vertebrate line of descent but (c) not before there was a homeogenetic transfer of FoxE4 expression from the club-shaped gland to the nearby endostyle. Such a transfer could have provided part of the genetic program enabling the endostyle to separate from the pharyngeal endoderm and migrate away as the rudiment of the thyroid gland.

Schubert, M, Holland LZ, Holland ND.  2000.  Characterization of two amphioxus Wnt genes (AmphiWnt4 and AmphiWnt7b) with early expression in the developing central nervous system. Developmental Dynamics. 217:205-215.   10.1002/(sici)1097-0177(200002)217:2<205::aid-dvdy7>;2-f   AbstractWebsite

Full-length sequences and developmental expression patterns of two amphioxus Wnt genes (AmphiWnt4 and AmphiWnt7b) are described for the first time. The dynamic expression pattern of AmphiWnt4 suggests roles in the development of the posterior mesoderm, central nervous system, muscular somites, heart, and endostyle (a homolog of the vertebrate thyroid). The less diverse expression domains of AmphiWnt7b indicate that this gene may be involved only in the development of the central nervous system and the endostyle, In contrast to amphioxus, vertebrate embryos do not express Wnt4 homologues in the posterior mesoderm, somites, or heart; instead, Wnt genes of other subfamilies are expressed in these developing vertebrate organs, Because the developmental genetic programs of amphioxus may approximate those in the invertebrate chordate ancestor of the vertebrates, it is possible that some developmental functions of an ancestral Wnt4 gene may have been assumed by genes of other Wnt subfamilies during vertebrate evolution, possibly as a result of functional redundancy among Wnt subfamilies. (C) 2000 Wiley-Liss, Inc.

Panopoulou, GD, Clark MD, Holland LZ, Lehrach H, Holland ND.  1998.  AmphiBMP2/4, an amphioxus bone morphogenetic protein closely related to Drosophila decapentaplegic and vertebrate BMP2 and BMP4: Insights into evolution of dorsoventral axis specification. Developmental Dynamics. 213:130-139.   10.1002/(sici)1097-0177(199809)213:1<130::aid-aja13>;2-z   AbstractWebsite

Amphioxus AmphiBMP2/4 appears to be a single gene closely related to vertebrate BMP2 and BMP4. In amphioxus embryos, the expression patterns of AmphiBMP2/4 suggest patterning roles in the ectodermal dorsoventral axis (comparable to dorsoventral axis establishment in the ectoderm by Drosophila decapentaplegic and vertebrate BMP4). In addition AmphiBMP2/4 may be involved in somite evagination, tail bud growth, pharyngeal differentiation (resulting in club-shaped gland morphogenesis), hindgut regionalization, differentiation of olfactory epithelium, patterning of the anterior central nervous system, and establishment of the heart primordium, One difference between the developmental role of amphioxus AmphiBMP2/4 and vertebrate BMP4 is that the former does not appear to be involved in the initial establishment of the dorsoventral polarity of the mesoderm, Dev. Dyn. 1998;213:130-139. (C) 1998 Wiley-Liss, Inc.