The phylum Chordata includes three subphyla: Cephalochordata (amphioxus or lancelets), Tunicata (ascidians, appendicularians and thaliaceans) and Vertebrata (gnathostomes plus agnathans). Sequencing of the genome of the sea lamprey was started in 2005 and that of the amphioxus genome was started in 2003. Both are essentially completed (1, 2). The genome of the ascidian Ciona intestinalis was published in 2002 (3), and an improved version 2.0 has just been released (http://genome.jgi-psf.org/Cioin2/Cioin2.home.html (4). Sequencing of the genome of the appendicularian tunicate Oikopleura dioica is underway and initial results are available at (http://www.genoscope.cns.fr/externe/GenomeBrowser/Oikopleura/). As stated by Henry Gee (5): “The genome sequence of a species of amphioxus, an iconic organism in the history of evolutionary biology, opens up a fresh vista on the comparative investigation of invertebrate chordates and vertebrates.” In this paper, Gee nicely summarizes the state of research on amphioxus, essentially stating that during most of the twentieth century, the amphioxus was neglected as a subject of study,… now this “eldritch” organism is set to re-enter public life.

Because of these advances in genomics, our symposium will bring together the latest findings from invertebrate chordate and agnathan genomes in regard to the evolution of developmental mechanisms and the neuroendocrine systems that are already challenging several current hypotheses and providing directions for new comparative studies.

This is the first symposium to examine both the evolution of developmental mechanisms and neuroendocrine systems in these basal chordates. Recent symposia have focused on EITHER the evolution of developmental mechanisms OR neuroendocrine systems, and therefore, have included very different slates of speakers and audiences. Thus, this is an exciting opportunity for a multidisciplinary symposium.

Amphioxus and tunicates are examples of contrasts. Although they share the fundamental chordate body plan with a dorsal, hollow nerve cord, paraxial muscles and notochord, they exemplify the extremes of evolutionary rates and developmental modes. Amphioxus, which is basal in the chordates, is evolving relatively slowly and its 520 mb genome has retained considerable synteny with those vertebrates (1, 2). Thus, amphioxus is proving to be an excellent model for the chordate ancestor. On the other hand, tunicates are evolving rapidly (6). They have small genomes (160 mb in Ciona and about 60 mb in the appendicularian tunicate Oikopleura) that have discarded many genes (e.g. several Hox genes, Gbx) and duplicated others (2 Pax2/5/8 genes, 2 for GnRH encoding 6 peptides (7). Tunicates also have reduced larval body plans (e.g. only about 330 neurons in the central nervous system of Ciona and even fewer in that of Oikopleura vs. 20,000 in that of amphioxus and millions or more in vertebrates (8). Tunicates are an excellent example of how much (genes and structure) can be thrown away and still have a viable organism. Tunicates show what evolution can do and raise the question of why amphioxus, in particular, has conserved so much.

The genome projects for the three invertebrate chordates are for the first time, allowing a good look into the neuroendocrine systems. For example, amphioxus has a functional thyroid hormone receptor, and vertebrate triiodothyronine, (T3, a thyroid hormone) induces metamorphosis in amphioxus as in the frog (9). However, as yet no gene homologous to the vertebrate T3 precursor, thyroglobulin has been found, suggesting that the active compound may be 3,3′,5-triiodo-thyroacetic acid, which in vertebrates, is a breakdown product of T3 thyroid hormone (9). Amphioxus does have homologs of the primary vertebrate endocrine organs (pineal gland, ovary, testis, ensostyle/thyroid, brain, gut and pituitary), although they are structurally more simple than their vertebrate counterparts. For example, although the pituitary homolog is considered a homolog of the adenohypophysis, a counterpart of the vertebrate neurohypophysis is lacking. Moreover, genome analyses indicate that amphioxus probably lacks homologs of several key pituitary hormones. Thus, pituitary control of reproduction is probably a vertebrate innovation (1).

Analyses of the Ciona genome have revealed 4 gonadotropin-releasing hormone (GnRH) receptors, a corticotropin-releasing hormone (CRH) receptor and genes for insulin and insulin growth factor (IGF) and their receptors as well as genes for several somatostatin receptors, calcitonin receptor, and the receptors for CRH, calcitonin and somatostatin (10, 11). The emerging picture from amphioxus and tunicates reveals a core neuroendocrine system present at the base of the chordates to which early vertebrates added additional complexity.

Lampreys: As an agnathan, the oldest extant lineage of vertebrates, the sea lamprey has become a model system for analysis of the evolution of many genes and systems including the evolution of the neuroendocrine regulation of reproduction (12-15) and the evolution of development (EvoDev) (16). Moreover, lampreys have been key to the question of when the two rounds of whole genome duplications occurred in the vertebrate lineage. Comparisons of vertebrate genomes with those of amphioxus and tunicates were in general agreement in that one whole genome duplication occurred at the base of the vertebrates. However, these data have failed to resolve whether the second duplication occurred before or after the agnathan/gnathostome split (2). Recently, Karaku et al. (2009) have concluded from an analysis including 55 gene families in the sea lamprey that the second whole genome duplication occurred before the agnathan/ gnathostome split. Lampreys as basal vertebrates were identified in a key position such that the mapping of the lamprey genome started in Jan 2005 (http://www.genome.gov/12511858). Nine non-mammalian organisms were chosen by NIH for mapping the genome, each of which represents a position on the evolutionary timeline marked by important changes in animal anatomy, physiology, development or behavior (NIH News Release August 4, 2004). It is estimated that the current coverage of the genome is about 5.9X, which infers that much of the genome has been sequenced and available for analysis using the trace archives and the partially assembled genome by Ensembl (http://pre.ensembl.org/Petromyzon_marinus/index.html). The phylogenetic position of lampreys as a basal vertebrate allows lampreys to be a basis for understanding the molecular evolution of the genes encoding receptors and hormones that arose in the vertebrates.

The acquisition of a hypothalamic-pituitary axis was a seminal event in vertebrate evolution leading to the neuroendocrine control of many complex functions including growth, reproduction, osmoregulation, stress and metabolism (18). Findings from the amphioxus, tunicate and lamprey genomes open a new understanding of the gonadotropin-releasing hormone (master molecule in the control of reproduction) and can help to delineate the evolution of the complex neuro/endocrine axis of reproduction. To date, biochemical, molecular, immunocytochemical and functional studies on the structure and function of the GnRHs in lampreys have established that similar to all other vertebrates, the lamprey has a hypothalamic-pituitary-gonadal axis and that there is a high degree of conservation of the mechanisms of GnRH action (15, 19). Generally, gnathostomes have one or two GnRHs that act as hypothalamic hormones, two pituitary gonadotropins (luteinizing hormone, LH, and follicle stimulating hormone, FSH), and one gonadal FSH receptor and one LH receptor compared to the lamprey that have two, possibly three, hypothalamic GnRHs, only one pituitary gonadotropin and one gonadal glycoprotein receptor (18). The identification of the primary amino acid and cDNA sequences of two forms of GnRH, lamprey GnRH-I and -III, a novel GnRH (lamprey GnRH-II), the cDNA of one GnRH receptor as well as the identification of one pituitary gonadotropin-beta have been completed (13, 15, 19, 20). The lamprey GnRH receptor identified shares several characteristics of both type-I and type-II vertebrate GnRH receptors (14, 20). The high conservation of the GnRH and its receptor throughout vertebrate species makes the lamprey model highly appropriate for examining the GnRH system in terms of its ligands and novel receptors. Similarly, the recent identification of a novel GnRH, lamprey gonadotropin (GTH) beta (21) and two glycoprotein receptors-a GTH-like receptor and a TSH-like receptor – (12, 22) provides an opportunity for comparative and evolutionary analysis of the neuroendocrine system in vertebrates in comparison to basal chordates. Sower et al. (18) hypothesized that the glycoprotein hormone/glycoprotein hormone receptor systems emerged as a link between the neuro-hormonal and peripheral control levels during the early stages of gnathostome divergence. The significance of the results obtained by analysis of the hypothalamic-pituitary-gonadal and hypothalamic pituitary-thyroid axes in sea lamprey may transcend the limited scope of the corresponding physiological compartments by providing important clues in respect to the interplay between genome wide events (duplications), coding sequence (mutation) and expression control level evolutionary mechanisms in definition of the chemical control pathways in vertebrates.

 

References Cited or Pertinent

1. Holland LZ, Albalat R, Azumi K, Benito-Gutierrez E, Blow MJ, Bronner-Fraser M, Brunet F, Butts T, Candiani S, Dishaw LJ, Ferrier DE, 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 A, Ye Y, Yoshida K, Yoshizaki F, Yu JK, Zhang Q, Zmasek CM, de Jong PJ, Osoegawa K, Putnam NH, Rokhsar DS, Satoh N, Holland PW 2008 The amphioxus genome illuminates vertebrate origins and cephalochordate biology. Genome Res 18:1100-1111

2. Putnam NH, Butts T, Ferrier DE, Furlong RF, Hellsten U, Kawashima T, Robinson-Rechavi M, Shoguchi E, Terry A, Yu JK, Benito-Gutierrez EL, Dubchak I, Garcia-Fernandez J, Gibson-Brown JJ, Grigoriev IV, Horton AC, de Jong PJ, Jurka J, Kapitonov VV, Kohara Y, Kuroki Y, Lindquist E, Lucas S, Osoegawa K, Pennacchio LA, Salamov AA, Satou Y, Sauka-Spengler T, Schmutz J, Shin IT, Toyoda A, Bronner-Fraser M, Fujiyama A, Holland LZ, Holland PW, Satoh N, Rokhsar DS 2008 The amphioxus genome and the evolution of the chordate karyotype. Nature 453:1064-1071

3. Dehal P, Satou Y, Campbell RK, Chapman J, Degnan B, De Tomaso A, Davidson B, Di Gregorio A, Gelpke M, Goodstein DM, Harafuji N, Hastings KE, Ho I, Hotta K, Huang W, Kawashima T, Lemaire P, Martinez D, Meinertzhagen IA, Necula S, Nonaka M, Putnam N, Rash S, Saiga H, Satake M, Terry A, Yamada L, Wang HG, Awazu S, Azumi K, Boore J, Branno M, Chin-Bow S, DeSantis R, Doyle S, Francino P, Keys DN, Haga S, Hayashi H, Hino K, Imai KS, Inaba K, Kano S, Kobayashi K, Kobayashi M, Lee BI, Makabe KW, Manohar C, Matassi G, Medina M, Mochizuki Y, Mount S, Morishita T, Miura S, Nakayama A, Nishizaka S, Nomoto H, Ohta F, Oishi K, Rigoutsos I, Sano M, Sasaki A, Sasakura Y, Shoguchi E, Shin-i T, Spagnuolo A, Stainier D, Suzuki MM, Tassy O, Takatori N, Tokuoka M, Yagi K, Yoshizaki F, Wada S, Zhang C, Hyatt PD, Larimer F, Detter C, Doggett N, Glavina T, Hawkins T, Richardson P, Lucas S, Kohara Y, Levine M, Satoh N, Rokhsar DS 2002 The draft genome of Ciona intestinalis: insights into chordate and vertebrate origins. Science 298:2157-2167.

4. Satou Y, Mineta K, Ogasawara M, Sasakura Y, Shoguchi E, Ueno K, Yamada L, Matsumoto J, Wasserscheid J, Dewar K, Wiley G, Macmil S, Roe B, Zeller R, Hastings K, Lemaire P, Lindquist E, Endo T, Hotta K, Inaba K 2008 Improved genome assembly and evidence-based global gene model set for the chordate Ciona intestinalis: new insight into intron and operon populations. Genome Biol 9:R152

5. Gee H 2008 Evolutionary biology: The amphioxus unleashed. Nature 453:999-1000

6. Holland L, Gibson-Brown J 2003 The Ciona intestinalis genome: when the constraints are off. Bioessays 25:529-532

7. Sherwood NM, Tello JA, Roch GJ 2006 Neuroendocrinology of protochordates: insights from Ciona genomics. Comp Biochem Physiol A Mol Integr Physiol 144:254-271

8. Nicol D, Meinertzhagen IA 1991 Cell counts and maps in the larval central nervous system of the ascidian Ciona intestinalis (L.). . J Comp Neurol 309:415-429

9. Paris M, Escriva H, Schubert M, Brunet F, Brtko J, Ciesielski F, Roecklin D, Vivat-Hannah V, Jamin EL, Cravedi JP, Scanlan TS, Renaud JP, Holland ND, Laudet V 2008 Amphioxus postembryonic development reveals the homology of chordate metamorphosis. Curr Biol 18:825-830

10. Campbell RK, Satoh N, Degnan BM 2004 Piecing together evolution of the vertebrate endocrine system. Trends Genet 20:359-366

11. Sekiguchi T, Kawashima T, Satou Y, Satoh N 2007 Further EST analysis of endocrine genes that are preferentially expressed in the neural complex of Ciona intestinalis: Receptor and enzyme genes associated with endocrine system in the neural complex. General and Comparative Endocrinology 150:233-245

12. Freamat M, Kawauchi H, Nozaki M, Sower SA 2006 Identification and cloning of a glycoprotein hormone receptor from sea lamprey, Petromyzon marinus. J Mol Endocrinol 37:135-146

13. Kawauchi H, Sower SA 2006 The dawn and evolution of hormones in the adenohypophysis. Gen Comp Endocrinol 148:3-14

14. Silver MR, Sower SA 2006 Functional characterization and kinetic studies of an ancestral lamprey GnRH-III selective type II GnRH receptor from the sea lamprey, Petromyzon marinus. J Mol Endocrinol 36:601-610

15. Sower SA 2003 The Endocrinology of Reproduction in Lampreys and Applications for Male Lamprey Sterilization. J Great Lakes Research 29:50-65

16. Kuratani S, Kuraku S, Murakami Y 2002 Lamprey as an evo-devo model: Lessons from comparative embryology and molecular phylogenetics. Genesis 34:175-183

17. Kuraku S, Meyer A, Kuratani S 2009 Timing of Genome Duplications Relative to the Origin of the Vertebrates: Did Cyclostomes Diverge before, or after? Mol Biol Evol 2009 Jan;26:47-59.

18. Sower SA, Freamat M, Kavanaugh SI 2009 The Origins of the Vertebrate Hypothalamic-Pituitary-Gonadal (HPG) & Hypothalamic-Pituitary Thyroid (HPT) Endocrine Systems: New Insights From Lampreys. General Comparative Endocrinology 161:16120-16129

19. Kavanaugh SI, Nozaki M, Sower SA 2008 Origins of gonadotropin-releasing hormone (GnRH) in vertebrates: identification of a novel GnRH in a basal vertebrate, the sea lamprey. Endocrinology 149:3860-3869

20. Silver MR, Nucci NV, Root AR, Reed KL, Sower SA 2005 Cloning and characterization of a functional type II gonadotropin-releasing hormone receptor with a lengthy carboxy-terminal tail from an ancestral vertebrate, the sea lamprey. Endocrinology 146:3351-3361

21. Sower SA, Moriyama S, Kasahara M, Takahashi A, Nozaki M, Uchida K, Dahlstrom JM, Kawauchi H 2006 Identification of sea lamprey GTHbeta-like cDNA and its evolutionary implications. Gen Comp Endocrinol 148:22-32

22. Freamat M, Sower S 2008 A sea lamprey glycoprotein hormone receptor similar with Gnathostome thyrotropin hormone receptor. Journal of Molecular Endocrinology 41:219-228

Sponsors: National Science Foundation and DCE (SICB)

Objectives
This symposium focuses on the evolution of chordate genomes, in particular, those events that occurred before the appearance of jawed vertebrates. The aim is to relate the evolution of chordate genomes to the evolution of body forms and functions. Endocrine systems and embryonic development emphasize that genome sequences are only important in the context of the organism.

Organized by: Stacia Sower (University of New Hampshire) and Linda Z. Holland (University of Calfornia San Diego)

Schedule