Evolution of "early" molluscs
- 出版日期：2003.
- 页数：163 p. :
- 分类号：a555
MARC全文
02h0020637 20101216115203.0 cr un||||||||| 100928s2003 xx ||||f|||d||||||||eng | CNY371.35 (UnM)AAI3091651 UnM UnM NGL a555 Okusu, Akiko. Evolution of "early" molluscs [electronic resource] : integrating phylogenetic, developmental, and morphological approaches. 2003. 163 p. : digital, PDF file. Source: Dissertation Abstracts International, Volume: 64-05, Section: B, page: 2076. ; Adviser: Gonzalo Giribet. Thesis (Ph.D.) -- Harvard University, 2003. The main goal of this dissertation is to achieve a better understanding of molluscan evolution, especially of the basal nodes, by integrating studies of comparative morphology, comparative embryology, and phylogenetic analyses of molecular sequence data. Molluscs are a group of metazoans that show an unusual amount of body plan disparity the origin of which is still uncertain. Relatively little is known about the “basal” molluscs Neomeniomorpha, Chaetodermomorpha and Polyplacophora. This thesis attempts to deepen our understanding of morphology, embryogenesis, and phylogenetic relationships of these molluscan groups. New species of western pacific Epimenia species (Mollusca, Neomeniomorpha) are described using radular, spicule, and body-coloration characteristics. It is shown here that radular morphology and body coloration are more useful in distinguishing species within the genus Epimenia than internal morphology alone. The embryogenesis and development of neomenioid aplacophorans is described for the first time using scanning electron and epifluorescent microscopy. Hoechst nuclear staining and programmed cell death staining of the neomenioid larvae show that the larval test of “pericalymma” is merely an enlarged pre-oral sphere of a trochophore larva. Phylogenetic relationships of chitons (Mollusca, Polyplacophora) are investigated using nuclear markers 18S rRNA, 28S rRNA, and histone H3, and mitochondrial 16S rRNA and cytochrome c oxidase subunit I. From the resulting topologies, it is inferred that the egg hulls in chitons evolved from smooth forms to ornamented with simple or with strongly hexagonal cupules with flaps, and to a spiny form; the sperm evolved from typical ectaquasperm with a spherical nucleus head towards a filamentous nucleus with complex mitochondria; and the adanal gills restricted to the posterior region extended towards a more anteriorly position becoming abanal. The classical taxonomy of chitons, based mostly on characters such as shell valves and spicules, is refuted by my results. Phylogenetic relationships among representatives of all molluscan classes are analyzed using DNA sequence data from three markers, 18S rRNA, 28S rRNA, and histone H3. The topologies support monophyly of the molluscan classes Chaetodermomorpha (=Caudofoveata), Neomeniomorpha (=Solenogastres), Polyplacophora, Scaphopoda, and Cephalopoda (Nautiloida and Coleoida), as well as that of other protostome phyla such as Nemertea, Sipuncula, and Platyhelminthes. However, no analysis supports monophyly of Mollusca, Gastropoda or Bivalvia. The possible implications of molluscan polyphyly are discussed. Lastly, the constant issues of dealing with contamination and amplification of exogenous nucleic acids from the DNA sequences of neomenioid aplacophorans are presented and a solution to contamination problems is presented. Mollusks, Fossil. aCN bNGL http://proquest.calis.edu.cn/umi/detail_usmark.jsp?searchword=pub_number%3DAAI3091651&singlesearch=no&channelid=%CF%B8%C0%C0&record=1 NGL Bs628 rCNY371.35 ; h1 xhbs1003