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Ono, H, Koop D, Holland LZ.  2018.  Nodal and Hedgehog synergize in gill slit formation during development of the cephalochordate Branchiostoma floridae. Development. 145   10.1242/dev.162586   AbstractWebsite

The larval pharynx of the cephalochordate Branchiostoma (amphioxus) is asymmetrical. The mouth is on the left, and endostyle and gill slits are on the right. At the neurula, Nodal and Hedgehog (Hh) expression becomes restricted to the left. To dissect their respective roles in gill slit formation, we inhibited each pathway separately for 20 min at intervals during the neurula stage, before gill slits penetrate, and monitored the effects on morphology and expression of pharyngeal markers. The results pinpoint the short interval spanning the gastrula/neurula transition as the critical period for specification and positioning of future gill slits. Thus, reduced Nodal signaling shifts the gill slits ventrally, skews the pharyngeal domains of Hh, Pax1/9, Pax2/5/8, Six1/2 and IrxC towards the left, and reduces Hh and Tbx1/10 expression in endoderm and mesoderm, respectively. Nodal auto-regulates. Decreased Hh signaling does not affect gill slit positions or Hh or Nodal expression, but it does reduce the domain of Gli, the Hh target, in the pharyngeal endoderm. Thus, during the neurula stage, Nodal and Hh cooperate in gill slit development - Hh mediates gill slit formation and Nodal establishes their left-right position.

Holland, ND, Holland LZ, Heimberg A.  2015.  Hybrids between the Florida amphioxus (Branchiostoma floridae) and the Bahamas lancelet (Asymmetron lucayanum): Developmental morphology and chromosome counts. Biological Bulletin. 228:13-24. AbstractWebsite

The cephalochordate genera Branchiostoma and Asymmetron diverged during the Mesozoic Era. In spite of the long separation of the parental clades, eggs of the Florida amphioxus, B. floridae, when fertilized with sperm of the Bahamas lancelet, A. lucayanum (and vice versa), develop through embryonic and larval stages. The larvae reach the chordate phylotypic stage (i.e., the pharyngula), characterized by a dorsal nerve cord, notochord, perforate pharynx, and segmented trunk musculature. After about 2 weeks of larval development, the hybrids die, as do the A. lucayanum purebreds, although all were eating the same algal diet that sustains B. floridae purebreds through adulthood in the laboratory; it is thus unclear whether death of the hybrids results from incompatible parental genomes or an inadequate diet. The diploid chromosome count in A. lucayanum and B. floridae purebreds is, respectively, 34 and 38, whereas it is 36 in hybrids in either direction. The hybrid larvae exhibit several morphological characters intermediate between those of the parents, including the size of the preoral ciliated pit and the angles of deflection of the gill slits and anus from the ventral midline. Based on the time since the two parent clades diverged (120 or 160 million years, respectively, by nuclear and mitochondrial gene analysis), the cross between Branchiostoma and Asymmetron is the most extreme example of hybridization that has ever been unequivocally demonstrated among multicellular animals.

Wu, HR, Chen YT, Su YH, Luo YJ, Holland LZ, Yu JK.  2011.  Asymmetric localization of germline markers Vasa and Nanos during early development in the amphioxus Branchiostoma floridae. Developmental Biology. 353:147-159.   10.1016/j.ydbio.2011.02.014   AbstractWebsite

The origin of germline cells was a crucial step in animal evolution. Therefore, in both developmental biology and evolutionary biology, the mechanisms of germline specification have been extensively studied over the past two centuries. However, in many animals, the process of germline specification remains unclear. Here, we show that in the cephalochordate amphioxus Branchiostoma floridae, the germ cell-specific molecular markers Vasa and Nanos become localized to the vegetal pole cytoplasm during oogenesis and are inherited asymmetrically by a single blastomere during cleavage. After gastrulation, this founder cell gives rise to a cluster of progeny that display typical characters of primordial germ cells (PGCs). Blastomeres separated at the two-cell stage grow into twin embryos, but one of the twins fails to develop this Vasa-positive cell population, suggesting that the vegetal pole cytoplasm is required for the formation of putative PGCs in amphioxus embryos. Contrary to the hypothesis that cephalochordates may form their PGCs by epigenesis, our data strongly support a preformation mode of germ cell specification in amphioxus. In addition to the early localization of their maternal transcripts in the putative PGCs, amphioxus Vasa and Nanos are also expressed zygotically in the tail bud, which is the posterior growth zone of amphioxus. Thus, in addition to PGC specification, amphioxus Vasa and Nanos may also function in highly proliferating somatic stem cells. (C) 2011 Elsevier Inc. All rights reserved.

Holland, LZ, Short S.  2010.  Alternative Splicing in Development and Function of Chordate Endocrine Systems: A Focus on Pax Genes. Integrative and Comparative Biology. 50:22-34.   10.1093/icb/icq048   AbstractWebsite

Genome sequencing has facilitated an understanding of gene networks but has also shown that they are only a small part of the answer to the question of how genes translate into a functional organism. Much of the answer lies in epigenetics-heritable traits not directly encoded by the genome. One such phenomenon is alternative splicing, which affects over 75% of protein coding genes and greatly amplifies the number of proteins. Although it was postulated that alternative splicing and gene duplication are inversely proportional and, therefore, have similar effects on the size of the proteome, for ancient duplications such as occurred in the Pax family of transcription factors, that is not necessarily so. The importance of alternative splicing in development and physiology is only just coming to light. However, several techniques for studying isoform functions both in vitro and in vivo have been recently developed. As examples of what is known and what is yet to be discovered, this review focuses on the evolution and roles of the Pax family of transcription factors in development and on alternative splicing of endocrine genes and the factors that regulate them.

Holland, LZ, Albalat R, Azumi K, Benito-Gutierrez E, Blow MJ, Bronner-Fraser M, Brunet F, Butts T, Candiani S, Dishaw LJ, Ferrier DEK, Garcia-Fernandez J, Gibson-Brown JJ, Gissi C, Godzik A, Hallbook F, Hirose D, Hosomichi K, Ikuta T, Inoko H, Kasahara M, Kasamatsu J, Kawashima T, Kimura A, Kobayashi M, Kozmik Z, Kubokawa K, Laudet V, Litman GW, McHardy AC, Meulemans D, Nonaka M, Olinski RP, Pancer Z, Pennacchio LA, Pestarino M, Rast JP, Rigoutsos I, Robinson-Rechavi M, Roch G, Saiga H, Sasakura Y, Satake M, Satou Y, Schubert M, Sherwood N, Shiina T, Takatori N, Tello J, Vopalensky P, Wada S, Xu AL, Ye YZ, Yoshida K, Yoshizaki F, Yu JK, Zhang Q, Zmasek CM, de Jong PJ, Osoegawa K, Putnam NH, Rokhsar DS, Satoh N, Holland PWH.  2008.  The amphioxus genome illuminates vertebrate origins and cephalochordate biology. Genome Research. 18:1100-1111.   10.1101/gr.073676.107   AbstractWebsite

Cephalochordates, urochordates, and vertebrates evolved from a common ancestor over 520 million years ago. To improve our understanding of chordate evolution and the origin of vertebrates, we intensively searched for particular genes, gene families, and conserved noncoding elements in the sequenced genome of the cephalochordate Branchiostoma floridae, commonly called amphioxus or lancelets. Special attention was given to homeobox genes, opsin genes, genes involved in neural crest development, nuclear receptor genes, genes encoding components of the endocrine and immune systems, and conserved cis-regulatory enhancers. The amphioxus genome contains a basic set of chordate genes involved in development and cell signaling, including a fifteenth Hox gene. This set includes many genes that were co-opted in vertebrates for new roles in neural crest development and adaptive immunity. However, where amphioxus has a single gene, vertebrates often have two, three, or four paralogs derived from two whole-genome duplication events. In addition, several transcriptional enhancers are conserved between amphioxus and vertebrates-a very wide phylogenetic distance. In contrast, urochordate genomes have lost many genes, including a diversity of homeobox families and genes involved in steroid hormone function. The amphioxus genome also exhibits derived features, including duplications of opsins and genes proposed to function in innate immunity and endocrine systems. Our results indicate that the amphioxus genome is elemental to an understanding of the biology and evolution of nonchordate deuterostomes, invertebrate chordates, and vertebrates.

Holland, ND, Holland LZ.  2006.  Stage- and tissue-specific patterns of cell division in embryonic and larval tissues of amphioxus during normal development. Evolution & Development. 8:142-149.   10.1111/j.1525-142X.2006.00085.x   AbstractWebsite

The distribution of dividing cells is described for embryos and larvae of amphioxus (Branchiostoma floridae) pulse labeled with bromodeoxyuridine. Because cell division is assessed for all of the developing tissues, this is the first comprehensive study of developmental cell proliferation for an animal lacking a stereotyped cell lineage. In amphioxus, cell divisions are virtually synchronous during cleavage, but become asynchronous at the blastula stage. Starting at the neurula stage, after the origin of the mesoderm, the proportion of dividing cells progressively declines in the somitic mesoderm and notochord. Other tissues, however, deviate from this pattern. For example, in the mid-neurula, there is a brief, intense burst of mitosis at the anterior end of the neural plate. Also, from the neurula through the early larval stage, all of the ectoderm cells cease dividing and develop cilia that propel the animal through the water; subsequently, in the epidermis of later larvae, mitosis resumes and the proportion of ciliated cells declines as muscular undulation gradually replaces ciliation for swimming. Finally, in the early larvae, there is a terminal arrest of cell division in three cell types that differentiate early to participate in feeding as soon as the mouth opens-namely the ciliated pharyngeal cells that produce the feeding current and the secretory cells of the club-shaped gland and endostyle that export food-trapping mucus into the pharynx. In sum, these stage- and tissue-specific changes in cell proliferation intensity illustrate how the requirements of embryonic and larval natural history can shape developmental programs.

Kreslova, J, Holland LZ, Schubert M, Burgtorf C, Benes V, Kozmik Z.  2002.  Functional equivalency of amphioxus and vertebrate Pax258 transcription factors suggests that the activation of mid-hindbrain specific genes in vertebrates occurs via the recruitment of Pax regulatory elements. Gene. 282:143-150.   10.1016/s0378-1119(01)00840-x   AbstractWebsite

Pax genes encode transcription factors that control key developmental decisions in various animal phyla. The Pax2/5/8 subfamily plays a key role in specification and/or maintenance of vertebrate mid-hindbrain boundary (MHB) region by directly regulating expression of other genes, most notably En2. In the invertebrate chordate amphioxus, expression of AmphiPax2/5/8 is found in many sites that are homologous to the regions of the vertebrate embryo expressing orthologous genes Pax2, Pax5 or Pax8. However, no co-expression of AmphiPax2/5/8 and AmphiEn is detected in the region of the neural tube that might correspond to the vertebrate MHB. Based on this observation and the absence of AmphiWnt expression in this region it appears that amphioxus does not have a MHB. Here we investigated the possibility that the AmphiPax2/5/8, as a key component of MHB development, has lost some of the properties of its vertebrate counterparts. We have analyzed both the DNA-binding and transactivation properties of AmphiPax2/5/8 as well as its ability to interact with the groucho co-repressor. In all these assays AmphiPax2/5/8 is indistinguishable from the human Pax5. In addition, we found two alternatively spliced AmphiPax2/5/8 isoforms that function similarly to the alternatively spliced isoforms of human Pax8. Analysis of the AmphiEn regulatory region provided no evidence for AmphiPax2/5/8 binding and transactivation. Therefore, in amphioxus, AmphiPax2/5/8, although capable of performing all the necessary functions has not been recruited for a developmental mechanism which usually sets up MHB development in vertebrates. (C) 2002 Elsevier Science B.V. All rights reserved.