Comparative study of the evolution of nuclear ribosomal spacers incorporating secondary structure analyzes within Dodonaeoideae, Hippocastanoideae and Xanthoceroideae (Sapindaceae)

Floral nectaries in Sapindaceae s.s.: morphological and structural diversity, and their systematic implications

We investigated the morphology and structure of the floral nectary in 11 Neotropical genera belonging to the subfamilies Dodonaeoideae and Paullinioideae (Sapindaceae) from southern South America representing three tribes (Dodonaeaeae, Paullinieae, and Melicocceae), in relation to other floral traits in species with contrasting morphological flower characteristics. Nectary organization was analyzed under light, stereoscopic, and scanning electron microscopes; Diplokeleba floribunda N.E. Br. was also observed using transmission electron microscopy. Our comparative data may contribute to the understanding of floral nectary evolution and systematic value in this family. The nectaries were studied in both staminate and pistillate flowers. All the floral nectaries are typical of Sapindaceae: extrastaminal, receptacular, structured, and persistent. The anatomical analysis revealed a differentiated secretory parenchyma and an inner non-secretory parenchyma; the nectary is supplied by phloem traces and, less frequently, by phloem and xylem traces. Nectar is secreted through nectarostomata of anomocytic type. The anatomical analysis showed the absence of nectary in the three morphs of Dodonaea viscosa flowers. Nectary ultrastructure is described in D. floribunda. In this species, the change in nectary color is related to progressive accumulation of anthocyanins during the functional phase. We found relatively small variation in the nectary structural characteristics compared with large variation in nectary morphology. The latter aspect agreed with the main infrafamilial groupings revealed by recent phylogenetic studies, so it is of current valuable systematic importance for Sapindaceae. In representatives of Paullinieae, the reduction of the floral nectary to 4-2 posterior lobes should be interpreted as a derived character state.

Molecular characterization ofOpisthorchis noverca(Digenea: Opisthorchiidae) based on nuclear ribosomal ITS2 and mitochondrial COI genes

Opisthorchiasis is a public health problem in South-East Asian countries and Eastern Europe. The infection implicates mainly two species ofOpisthorchis, namelyO. viverriniandO. felineus,that occur mostly in fish-eating mammals and humans, although there are rare reports of human cases involving two other species,O. novercaandO. guayaquilensis.Opisthorchis novercahas been reported frequently in dogs and pigs from the Indian subcontinent, with rare reports from cattle and human subjects. With a view to supplementing morphology-based identification of this species, the present study aimed to provide molecular characterization ofO. noverca, using rDNA internal transcribed spacer 2 (ITS2) and mitochondrial cytochrome oxidase I (mt COI) markers so as to determine its genetic correlation with other species of Opisthorchiidae, and also to generate a taxon-specific molecular marker based on the ITS2 region. The phylogenetic relationship betweenO. novercaand other species of the genus was determined using molecular sequence data. To strengthen the result, secondary structure sequence analyses of ITS2 with hemi-compensatory base changes (hCBCs), and amino acid sequence analyses, were also evaluated. Our results confirm thatO. novercais a distinct and valid species.

A close-up view on ITS2 evolution and speciation - a case study in the Ulvophyceae (Chlorophyta, Viridiplantae)

BACKGROUND

The second Internal Transcriber Spacer (ITS2) is a fast evolving part of the nuclear-encoded rRNA operon located between the 5.8S and 28S rRNA genes. Based on crossing experiments it has been proposed that even a single Compensatory Base Change (CBC) in helices 2 and 3 of the ITS2 indicates sexual incompatibility and thus separates biological species. Taxa without any CBC in these ITS2 regions were designated as a 'CBC clade'. However, in depth comparative analyses of ITS2 secondary structures, ITS2 phylogeny, the origin of CBCs, and their relationship to biological species have rarely been performed. To gain 'close-up' insights into ITS2 evolution, (1) 86 sequences of ITS2 including secondary structures have been investigated in the green algal order Ulvales (Chlorophyta, Viridiplantae), (2) after recording all existing substitutions, CBCs and hemi-CBCs (hCBCs) were mapped upon the ITS2 phylogeny, rather than merely comparing ITS2 characters among pairs of taxa, and (3) the relation between CBCs, hCBCs, CBC clades, and the taxonomic level of organisms was investigated in detail.

RESULTS

High sequence and length conservation allowed the generation of an ITS2 consensus secondary structure, and introduction of a novel numbering system of ITS2 nucleotides and base pairs. Alignments and analyses were based on this structural information, leading to the following results: (1) in the Ulvales, the presence of a CBC is not linked to any particular taxonomic level, (2) most CBC 'clades' sensu Coleman are paraphyletic, and should rather be termed CBC grades. (3) the phenetic approach of pairwise comparison of sequences can be misleading, and thus, CBCs/hCBCs must be investigated in their evolutionary context, including homoplasy events (4) CBCs and hCBCs in ITS2 helices evolved independently, and we found no evidence for a CBC that originated via a two-fold hCBC substitution.

CONCLUSIONS

Our case study revealed several discrepancies between ITS2 evolution in the Ulvales and generally accepted assumptions underlying ITS2 evolution as e.g. the CBC clade concept. Therefore, we developed a suite of methods providing a critical 'close-up' view into ITS2 evolution by directly tracing the evolutionary history of individual positions, and we caution against a non-critical use of the ITS2 CBC clade concept for species delimitation.