title: Reproductive isolation among sympatric cryptic species in marine diatoms.
abstract: Pseudo-nitzschia is a marine cosmopolitan genus of chain-forming planktonic diatoms. As for the vast majority of phytoplankton organisms, species identification within this genus mostly relies upon morphological features. Taxa were initially identified based on cell shape and gross morphology of their composite silica cell wall, called the frustule. Yet, observations of the frustule in electron microscopy showed many additional characters for species identification and results of molecular studies have demonstrated that genetically distinct groups might exist within morpho-species. However, these studies have not addressed the biological meaning of these genetic differences. Here, we bridge that gap by comparing ultrastructural features and sequence data (three ribosomal and one plastid marker) of 95 strains with results of mating experiments among these strains. Experiments were performed on two morphologically distinct entities: P. delicatissima and P. pseudodelicatissima. Each of the two entities consisted of multiple genetically distinct and reproductively isolated taxa, all occurring in sympatry: P. delicatissima was composed of three phylogenetic and reproductively distinct groups, whereas P. pseudodelicatissima consisted of up to five. Once these taxa had been defined both genetically and biologically, subtle ultrastructural differences could be detected as well. Our findings not only show that cryptic genetic variants abound in sympatry, but also that they are reproductively isolated and, therefore, biologically distinct units.
authors.affiliation: Stazione Zoologica A. Dohrn, Villa Comunale, 80121 Napoli, Italy. amato@szn.it
authors.forename: Alberto
authors.initials: A
authors.lastname: Amato
authors.forename: Wiebe H C F
authors.initials: WH
authors.lastname: Kooistra
authors.forename: Jung Hee Levialdi
authors.initials: JH
authors.lastname: Ghiron
authors.forename: David G
authors.initials: DG
authors.lastname: Mann
authors.forename: Thomas
authors.initials: T
authors.lastname: Pröschold
authors.forename: Marina
authors.initials: M
authors.lastname: Montresor
mesh_terms: D021903:DNA, Ribosomal Spacer / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification; D017377:Diatoms / Q000145:classification* / Q000235:genetics; D014644:Genetic Variation; D012339:RNA, Ribosomal, 28S / Q000032:analysis* / Q000235:genetics
chemical_list: D021903:DNA, Ribosomal Spacer; D012339:RNA, Ribosomal, 28S