Nuclear Ribosomal Cistron Research Articles

BACK-AND-FORTH HERMAPHRODITISM: PHYLOGENETIC CONTEXT OF REPRODUCTIVE SYSTEM EVOLUTION IN SUBDIOECIOUSDAPHNE LAUREOLA

Recent phylogenetic analyses of sexual reproductive systems supported the evolutionary pathway from hermaphroditism to dioecy via gynodioecy in different groups of angiosperms. In this study, we explore the evolution of sexual reproductive systems in Daphne laureola L. (Thymelaeaceae), a species with variation in reproductive system among population. Sequences from the ITS region of the nuclear ribosomal cistron and two plastid markers (psbA-trnH and ndhF) were analyzed and used to map the population reproductive system along the molecular phylogeny. Our results support D. laureola as a monophyletic lineage with three different clades within the Iberian Peninsula. The hermaphroditic populations belong to two different clades, whereas gynodioecy is ubiquitous but characteristic of the third clade, which grouped together all the North-Western Iberian populations sampled, including the apparently oldest haplotype sampled. Gynodioecy appears as the most likely basal condition of the 13 analyzed populations, but different evolutionary transitions in reproductive sexual system were traced within each D. laureola clade. Both ecological conditions and (meta)population dynamics may help explain plant reproductive system evolution at the microevolutionary scale. Phylogenetic studies in which the historical relationships between populations differing in reproductive system can be ascertained will help to clarify the process.

Suitability of chloroplast LSU rDNA and its diverse group I introns for species recognition and phylogenetic analyses of lichen-forming Trebouxia algae

To date, species identification of lichen photobionts has been performed principally on the basis of microscopic examinations and molecular data from nuclear-encoded genes. In plants, the chloroplast genome has been more readily exploited than the nuclear genome for systematic investigations. At the present time, very little information is available about the chloroplast genome of lichen-forming algae. For this reason, we have sequenced a portion of the gene encoding for the chloroplast large sub-unit rRNA (LSU rDNA) as a new molecular marker. Sequencing of the chloroplast LSU rDNAs revealed the existence of an unusual diversity of group I introns (a total of 31) within 15 analyzed Trebouxia species. The number, sequence and insertion site of these introns were very different among species, contributing to their recognition. A relatively large intron-free portion of the chloroplast LSU rDNA and part of the nuclear ribosomal cistron (18S–5.8S–26S) between the nuclear internal transcribed spacers (nrITS) were subjected to phylogenetic analyses. The obtained results indicate that data combination from both nuclear and chloroplast sequences can improve phylogenetic accuracy. Herein, we propose the suitability of both intronic and exonic sequences of the chloroplast LSU rDNA for species recognition, and an exonic sequence spanning from position 879 to 1837 in the Escherichia coli 23S rDNA for phylogenetic analyses of Trebouxia phycobionts.

Systematics of thePhialophora verrucosacomplex: new insights from analyses of β-tubulin, large subunit nuclear rDNA and ITS sequencesThis paper is one of a selection of papers published in the Special Issue on Systematics Research.

Phialophora Medlar, as defined currently, is a genus encompassing melanized, anamorphic Ascomycota that produce one-celled conidia from phialides with distinct, darkened collarettes. The type species, Phialophora verrucosa Medlar, is closely related to Phialophora americana (Nannf.) S. Hughes, the anamorph of Capronia semiimmersa (Candoussau & Sulmont) Untereiner & Naveau (Herpotrichiellaceae, Chaetothyriales). To confirm that P. americana and P. verrucosa are distinct taxa, and to examine their phylogenetic relationships to species of Capronia and other representatives of the Chaetothyriales, we sequenced portions of the β-tubulin gene and nuclear ribosomal RNA cistron (ITS and LSU rDNA). We also compared isolates of P. americana grown on a number of media. Isolates of C. semiimmersa, Capronia svrcekiana Réblová, and P. americana produced phialides bearing deep, vase-shaped collarettes and formed a strongly supported clade that did not include P. verrucosa in a phylogeny inferred from the combined β-tubulin–ITS–LSU dataset. Capronia svrcekiana was found to be conspecific with C. semiimmersa based on the comparison of cultural, micromorphological, and molecular characters. In the LSU phylogeny, three recently described species of Phialophora ( Phialophora europaea de Hoog et al., Phialophora reptans de Hoog, and Phialophora sessilis de Hoog) were grouped outside of the clade containing sampled members of the Herpotrichiellaceae. While the position of these species in the Chaetothyriales remained unresolved, it was evident that P. europaea, P. reptans, and P. sessilis are not members of the P. verrucosa complex.

Portrait of a Species

Chlamydomonas reinhardtii, the first alga subject to a genome project, has been the object of numerous morphological, physiological, and genetic studies. The organism has two genetically determined mating types (plus and minus) and all stages of the simple life cycle can be evoked in culture. In the nearly 60 years since the first standard laboratory strains were isolated, numerous crosses and exchanges among laboratories have led to some confusion concerning strain genealogy. Here we use analyses of the nuclear internal transcribed spacer regions and other genetic traits to resolve these issues, correctly identify strains currently available, and analyze phylogenetic relationships with all other available similar chlamydomonad types. The presence of a 10-bp indel in ITS2 in some but not all copies of the nuclear ribosomal cistrons of an individual organism, and the changing ratios of these in crosses, provide a tool to investigate mechanisms of concerted evolution. The standard C. reinhardtii strains, plus C. smithii +, plus the new eastern North American C. reinhardtii isolates, comprise one morphological species, one biological species of high sexual intercompatibility, and essentially identical ITS sequences (except the tip of helix I of ITS2). However, variant RFLP patterns characterize strains from each geographic site.

Paramecium aureliaRevisited

The species Paramecium aurelia sensu latu, containing 15 sexually isolated subspecies (syngens), is the classic example of a sibling species complex in the ciliates. Using DNA sequence comparison, it is now possible to see whether this example parallels other studied sibling species complexes. We sequenced the internal transcribed spacer (ITS) region of the nuclear ribosomal cistron for 13 of the syngens plus two other Paramecium species and several Tetrahymena spp. Using available spirotrich sequences of the internal transcribed spacer 2 (ITS2), we established the RNA transcript folding pattern for ciliates. Ciliates exhibit the two highly conserved helices in their RNA transcript folding pattern in common with other eukaryotes, despite their unusual nuclear behavior and their presumed low copy number of micronuclear ribosomal repeats. Consequently, the set of 111-116 ITS2 nucleotide positions that are relatively conserved in evolution can be derived and used for comparative analysis. Mating behavior (i.e. gamete agglutination and fusion) is the character showing greatest correlation with the degree of ITS2 evolution in the P. aurelia complex, as also found in other eukaryotes. The degree of change in the ITS2 relatively conserved sequences found among the sibling species of P. aurelia is the same degree as found among the sibling species of the Drosophila melanogaster-mauritania-sechellia-simulans-yakuba species complex. The relatively conserved subregion of ITS2, determined from transcript secondary structure, is a tool for identifying the level of the biological species in the absence of knowledge of sexual compatibility in both micro- and macro-eukaryote species complexes.

CLOSTERIUM MONILIFERUM‐EHRENBERGII(CHAROPHYCEAE, CHLOROPHYTA) SPECIES COMPLEX VIEWED FROM THE 1506 GROUP I INTRON AND ITS2 OF NUCLEAR rDNA

To assess phylogenetic relationships and speciation modes inClosterium, we sequenced two noncoding regions of the nuclear ribosomal cistron, the 1506 group I intron in small subunit and the internal transcribed spacer 2, for a total of 58 strains of theClosterium moniliferum‐ehrenbergiispecies complex. These include both homothallic and heterothallicC. moniliferumErenberg ex Ralfs v.moniliferum, heterothallicC. moniliferumv.submoniliferum(Woronichin) Krieger, and heterothallicC. ehrenbergiiMeneghini ex Ralfs that can be divided into several mating groups. We found no or very little sequence divergence within single mating groups ofC. ehrenbergiiand among all heterothallic strains ofC. moniliferumv.moniliferumorC. moniliferumv.submoniliferum.Nevertheless, sequence divergence was much greater between those mating groups ofC. ehrenbergiiand also among the three traditional taxa.Maximum parsimony and maximum likelihood analyses showed that the taxonC. ehrenbergiiwas not monophyletic. The two varieties ofC. moniliferumappeared as a sister clade to certain mating groups ofC. ehrenbergii. Among the clades that were recovered in different trees by maximum parsimony and maximum likelihood analyses, we consistently found two large conspicuous clades: clade I consisted of mating groups A, B, C, H, K, and L ofC. ehrenbergiiwhose zygospores have smooth‐walls, and clade II contained the mating groups D, E, I, J, and S whose zygospores are scrobiculate. Phylogenetic incongruences observed are discussed from the viewpoints of the different molecular nature of the group I intron and internal transcribed spacer 2, as well as putative rapid diversification of the mating groups and probable ancient ancestral hybridization.

Ribosomal ITS sequences and plant phylogenetic inference

One of the most popular sequences for phylogenetic inference at the generic and infrageneric levels in plants is the internal transcribed spacer (ITS) region of the 18S–5.8S–26S nuclear ribosomal cistron. The prominence of this source of nuclear DNA sequence data is underscored by a survey of phylogenetic publications involving comparisons at the genus level or below, which reveals that of 244 papers published over the last five years, 66% included ITS sequence data. Perhaps even more striking is the fact that 34% of all published phylogenetic hypothesis have been based exclusively on ITS sequences. Notwithstanding the many important contributions of ITS sequence data to phylogenetic understanding and knowledge of genome relationships, a number of molecular genetic processes impact ITS sequences in ways that may mislead phylogenetic inference. These molecular genetic processes are reviewed here, drawing attention to both underlying mechanism and phylogenetic implications. Among the most prevalent complications for phylogenetic inference is the existence in many plant genomes of extensive sequence variation, arising from ancient or recent array duplication events, genomic harboring of pseudogenes in various states of decay, and/or incomplete intra- or inter-array homogenization. These phenomena separately and collectively create a network of paralogous sequence relationships potentially confounding accurate phylogenetic reconstruction. Homoplasy is shown to be higher in ITS than in other DNA sequence data sets, most likely because of orthology/paralogy conflation, compensatory base changes, problems in alignment due to indel accumulation, sequencing errors, or some combination of these phenomena. Despite the near-universal usage of ITS sequence data in plant phylogenetic studies, its complex and unpredictable evolutionary behavior reduce its utility for phylogenetic analysis. It is suggested that more robust insights are likely to emerge from the use of single-copy or low-copy nuclear genes.

Phylogenetic Relationships in Phoradendreae (Viscaceae) Inferred from Three Regions of the Nuclear Ribosomal Cistron. I. Major Lineages and Paraphyly of Phoradendron

Abstract Phoradendron is a genus of morphologically diverse mistletoes that currently lacks a satisfactory supraspecific classification. Its subtle distinction from Dendrophthora moreover suggests that Phoradendron may not be monophyletic. This study uses sequences from three regions of the nuclear ribosomal DNA cistron to infer phylogenetic relationships in the two genera. These DNA regions differ in their rates of nucleotide substitution. Sequences of the ITS spacers are sufficiently variable to resolve relationships among closely related species but are barely alignable with those of more distantly related species and outgroups. Alignment is possible, however, among sequences of the 5.8S gene and among two regions of the 26S nuclear rDNA. Maximum parsimony analyses suggested that neither Phoradendron nor Dendrophthora is monophyletic. Three major clades were identified whose topologies suggest that many characters traditionally used for primary subdivision are homoplastic. Communicating Editor: Alan Wh...

Comparison of ITS RFLP patterns of Gracilaria (Rhodophyceae, Gracilariales) populations from Chile and New Zealand and an examination of interfertility of Chilean morphotypes

Restriction fragment length polymorphism (RFLP) patterns of the internal transcribed spacer (ITS) of the nuclear ribosomal cistron and crossability trials were used to characterize four morphotypes of Gracilaria from Lenga, Isla Santa Maria and Maullin, Chile, and two morphotypes from sites in New Zealand. PCR products from all Chilean morphotypes resulted in a major single band of ca. 1198 bp. ITS-RFLP profiles generated with the restriction enzymes Cla I, Hae III, Pst I, Hha I, Rsa I and Taq I, were identical in all cases. All crosses within, as well as between, morphotypes resulted in cystocarp differentiation, with the production of viable carpospores. Based upon these data, it is concluded that the four morphotypes from Chile correspond to a single species, G. chilensis, and that the ITS-RFLP pattern is a useful marker to predict genetic relatedness at the specific level in Gracilaria. A comparison of the ITS-RFLP patterns of the Chilean morphotypes with the patterns of two samples of G. chilensis from New Zealand revealed that the sample from Scorching Bay, Wellington, fits the Chilean ITS-RFLP patterns. The population from Blockhouse Bay, Auckland, appears to correspond to another species.

Asterocladon lobatum gen. et sp. nov., a new brown alga with stellate chloroplast arrangement, and its systematic position judged from nuclear rDNA sequences

We describe a small brown alga, which appeared in laboratory cultures of material collected on the coast of Brazil. It forms oligostichous microthalli with apical growth, phaeophycean hairs, and plurilocular sporangia. Ultrastructural studies revealed a stellate chloroplast–pyrenoid arrangement. Because this combination of characters does not fit with any known brown algal genus, we introduce this isolate as a new taxon: Asterocladon lobatum D.G. Müller, E.R. Parodi et A.F Peters gen. et sp. nov. To infer the phylogenetic relationship between A. lobatum and other brown algae with stellate chloroplasts, partial DNA sequences of the small and large subunits of the nuclear ribosomal cistron of A. lobatum were compared with homologous sequences of the filamentous algae Asteronema ferruginea (Harvey) Delépine et Asensi, A. rhodochortonoides (Børgesen) D.G. Müller et E.R. Parodi, Bachelotia antillarum (Grunov) Gerloff, all four members of the Scytothamnales with compact thalli, and five representatives of the Ectocarpales sensu lato. Parsimony and distance analyses indicated that the genus Asteronema is polyphyletic. They further suggested that A. ferruginea and Bachelotia antillarum are members of the Scytothamnales. Asrerocladon lobatum and Asteronema rhodochortonoides formed a separate clade that does not belong to the Scytothamnales. According to the molecular phylogeny, none of the filamentous brown algae with stellate chloroplast arrangement belongs to the Ectocarpales, where they are presently classified.

MEDITERRANEAN CAULERPA TAXIFOLIA AND C. MEXICANA (CHLOROPHYTA) ARE NOT CONSPECIFIC

In 1984, Caulerpa taxifolia (Vahl) C. Agardh was reported along the coast of Monaco. Over the past decade it has spread along 60 km of the Mediterranean coastline and presently represents a potential risk to biodiversity. Several explanations have been advanced regarding the presence of C. taxifolia in the Mediterranean. One hypothesis maintains that the alga was introduced accidentally into the sea at Monaco, where it has been used as a decorative alga in aquaria. Caulerpa taxifolia has not been reported in earlier marine floras of the Mediterranean, and its sudden appearance has suggested that it may be a recent introduction. Another hypothesis proposes that C. taxifolia and Caulerpa mexicana Sonder ex Kützing are morphological variants of one another and hence conspecific taxa. Caulerpa mexicana has been found in the eastern Mediterranean since at least 1941. In order to establish the taxonomic identities of these taxa, individuals from five populations of C. taxifolia and four populations of C. mexicana were collected from within and outside of the Mediterranean. Comparative DNA sequence analysis of the nuclear ribosomal cistron, including the 3′‐end of the 18S, ITS1, 5.8S, and ITS2 regions, show clear phylogenetic separation of the two taxa using parsimony and maximum likelihood analyses. Separation is maintained whether the analyses are based on just the more conserved 18S data or just the fast‐ evolving spacers. The two species are thus not conspecific. For specimens of uncertain identity (i.e. taxifolia–mexicana intermediates), a PCR diagnostic amplification can easily be performed because the ITS1 in C. taxifolia is 36 nucleotides shorter than the ITS1 in C. mexicana. Whether or not C. taxifolia has been present for a longer period of time in the marine flora, either as a cryptic endemic species or as the result of one or more introductions, represents an additional hypothesis that will require identification of biogeographic populations from throughout the world, as well as a population‐level study of the Mediterranean region.

Molecular and morphological investigations of three brown algal genera with stellate plastids: evidence for Scytothamnales ord. nov. (Phaeophyceae)

Recent molecular phylogenies of the Phaeophyceae have demonstrated that the morphologically simple ephemeral species referred to as Ectocarpales sensu lato are not primitive in a phylogenetic sense. They form a monophyletic group and share parietal plastids with pyrenoids. The clade is distant from the orders of morphologically advanced brown algae with discoid plastids and no or reduced pyrenoids. Sequences of parts of the nuclear ribosomal cistron of species of the genera Scytothamnus, Stereocladon, and Splachnidium, which have stellate chloroplasts with a single pyrenoid in the center of the cells, provide strong evidence that none of these algae belong to the Ectocarpales sensu lato. Neither do they group with the Fucales, with which Splachnidium shares receptacles and conceptacles. Although an analysis of small subunit sequences fails to clearly separate them from the Laminariales, with which they do not share morphological, cytological, or reproductive characters, a combined analysis of partial small subunit sequences and more conserved parts of the otherwise highly variable internal transcribed spacers (ITS1 and ITS2) places them between Laminariales and Ectocarpales sensu lato. We propose Scytothamnales ord. nov. with the two closely related genera Scytothamnus and Stereocladon in the family Scytothamnaceae and the more distant Splachnidium in its own monotypic family. Adenocystis utricularis (Bory) Skottsberg, which previously has been included in Scytothamnaceae despite its different, discoid plastid type, remains a member of Ectocarpales sensu lato.

Ribosomal DNA sequences support taxonomic separation of the two species of Chorda: reinstatement of Halosiphon tomentosus (Lyngbye) Jaasund (Phaeophyceae, Laminariales)

The brown algal family Chordaceae contains the single genus Chorda with two terete unbranched species, C. filum and C. tomentosa. They share a similar superficial morphology, but differences in anatomy and reproductive characters have made assignment to a single genus controversial. Comparison of DNA sequences of variable parts of the small subunit of the nuclear ribosomal cistron, and of relatively conserved areas of both internal transcribed spacers, of several taxa of the Sporochnales–Desmarestiales–Laminariales complex revealed that the two species of Chorda are not closely related and do not form a clade. This is taken as support for recognition of Halosiphon tomentosus (Lyngbye) Jaasund, although it was originally based on the erroneous observation of a Streblonema-like gametophyte in the life history of H. tomentosus.

Inter- and intraspecific genetic variation in Caulerpa (Chlorophyta) based on nuclear rDNA ITS sequences

Caulerpa (Chlorophyta, Ulvophyceae) is a common marine tropical-subtropical genus of about 70 species, inhabiting the eulittoral zone on rocks and corals as well as mangroves. The genus is particularly diverse and abundant along southern Australian coasts, where it is thought to have originated. Here, we compare sequences from the nuclear ribosomal cistron among five species of Caulerpa, including nine populations of Caulerpa filiformis from two biogeographic regions : five from Australia and four from South Africa. Species relationships were well resolved by internal transcribed spacer (ITS) sequences and supported by high bootstrap values as follows: (C. geminata (C. simpliciuscula (C. trifaria (C. scalpelliformis (C. filiformis – Australia, C. filiformis – South Africa ))))). Nucleotide divergence within C. filiformis was low with four and five nucleotide differences present in ITS1 and ITS2 respectively. Unexpectedly, the 5.8S rRNA gene showed eight nucleotide differences between the Australian and South African populations and may indicate cryptic species. The Australian C. filiformis is probably not a South African introduction. These sequences provide an independent means for assessment of species relationships and appear, in so far as limited taxon sampling permits, to follow chloroplast ultrastructural groupings proposed 20 years ago.

DNA SEQUENCE VARIATION IN THE RIBOSOMAL INTERNAL TRANSCRIBED SPACER REGION OF FRESHWATER CLADOPHORA SPECIES (CHLOROPHYTA)1

ABSTRACTFreshwater species of Cladophora (Chlorophyta) are globally distributed and occupy an unusually wide range of ecological habitats. Delineating species is difficult because most easily observed morphological traits are highly variable and because sexual reproduction has not been clearly documented. Synthesizing ecological data on freshwater Cladophora species is problematic because it is unclear whether freshwater Cladophora species comprise many genetically distinct species or a few ecologically and morphologically variable and/or plastic species. We determined nucleotide sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal cistron of freshwater Cladophora species from a wide range of habitats and geographic locations. We compared these sequences to those derived from culture collections of C. fracta and C. glomerata, the two most commonly reported freshwater Cladophora species. Cladophora fracta and C. glomerata had very similar ITS sequences (95.3%). All other sequences were identical to those from the C. fracta or C. glomerata culture collections with the exception of one California sample that was similar to both C. fracta (95.6%) and C. glomerata (92.4%). ITS genotypes did not correlate with morphology or geography. This analysis shows that common freshwater Cladophora species comprise very few (possibly one) ecologically and morphologically variable species.

A REVISION OF THE SYSTEMATICS OF THE NIZYMENIACEAE (GIGARTINALES, RHODOPHYTA) BASED ON POLYSACCHARIDES, ANATOMY, AND NUCLEOTIDE SEQUENCES1

ABSTRACT The Australian endemic family Nizymeniaceae, based on Nizymenia australis Sonder, consists of three species in the two genera Nizymenia (1 sp.) and Stenocladia (2 spp.). We have reassessed the generic composition of the family based on evidence from nonfibrillar wall polysaccharides, vegetative anatomy, and the nucleotide sequences of an internal transcribed spacer, ITS 2, of the nuclear ribosomal cistron. Investigation of the polysaccharides by constituent sugar analysis, sulfate content determination, and methylation analysis, combined with gas chromatography‐mass spectrometry and infrared analysis, showed that the polysaccharides elaborated by the three species were branched, highly sulfated xylogalactans. These polysaccharides also contained significant amounts of mono‐O‐methyl galactose (5–8 mol% of total sugars), mainly 4‐O‐methyl galactose. Although no discrete chemical structures could be assigned to the polysaccharides, the analyses showed that those from Nizymenia australis and Stenocladia australis (Sonder) Silva were more alike than either was to that from S. furcata (Harvey) J. Agardh. This polysaccharide affinity was echoed by a suite of vegetative anatomical features. However, the only likely synapomorphy was the presence of refractive, thick‐walled medullary rhizines in both N. australis and S. australis. The ITS 2 sequences were inferred from direct sequencing of the products of polymerase chain reaction amplification. Comparison of the ITS 2 sequences of its three species with those of two outgroups indicated that the family Nizymeniaceae is monophyletic but that interspecific relationships within the family could not be resolved. We conclude that there is insufficient evidence to separate any of the species from the rest at the genus level. Therefore, all three species are consolidated into the genus Nizymenia. This necessitates nomenclatural changes of Stenocladia australis to Nizymenia conferta (Harvey) Chiovitti, Saunders, et Kraft comb. nov.

Nuclear DNA markers of the Australian tetraploid Microseris scapigera and its North American diploid relatives

The allotetraploid lactucean Microseris scapigera of Australia and New Zealand has presumably arisen in western North America by hybridization between an annual and a perennial diploid species followed by polyploidization and long-distance dispersal. A phylogenetic tree of various North American diploids, based on RFLPs in the nuclear DNA, confirmed the division of the genus into a clade containing the diploid annuals and a clade containing the diploid perennials. Four RFLP markers were shared among all accessions of M. scapigera and all the diploid accessions. Twelve markers found in the outgroup (Uropappus lindleyi) were absent in all Microseris. A cladogram of plants from six populations of M. scapigera based on eight RFLP markers shows a progressive specialization of three clades of two populations each. Two populations without any markers differentiating them from the North American diploids form the basic clade. These consist of plants with an apparently derived morphology that are self-compatible (or agamospermic) and thereby differ from most M. scapigera. Few markers in M. scapigera could be attributed to one or the other parental genome. As yet, we have found only one ITS 1 sequence of the nuclear ribosomal cistrons in M. scapigera. This sequence has features of both parental sequences.

Restriction enzyme mapping of the nuclear ribosomal cistron in selected Laminariales (Phaeophyta): a phylogenetic assessment

Restriction enzyme mapping of the nuclear ribosomal cistron was completed for a variety of Laminariales. Taxa investigated included Alaria marginata Postels and Ruprecht, Egregia menziesii (Turner) Areschoug, Eisenia arborea Areschoug, Lessoniopsis littoralis (Tilden) Reinke, Macrocystis integrifolia Bory, Nereocystis leutkeana (Mertens) Postels and Ruprecht, Postelsia palmaeformis Ruprecht, and Pterygophora californica Ruprecht, with Sargassum muticum (Yendo) Fensholt (Fucales) as an outgroup. The restriction maps establish a foundation for future phylogenetic, as well as other molecular, investigations in the kelp. We also assess restriction enzyme mapping of the nuclear ribosomal cistron for suitability in resolving intrafamilial and interfamilial taxonomic relationships in the Laminariales. The intergenic spacer proved too variable to be of use for phylogenetic comparisons at this level. Conversely, the gene regions were highly conserved, with only three restriction-site differences observed among all the laminarialean taxa investigated. Key words: Laminariales, rRNA, restriction enzyme mapping.

Sequence homology of nuclear and mitochondrial DNAs of different yeasts

1. Both nuclear and mtDNA of four different yeasts show approximately 10% homology as measured by DNA · DNA filter hybridization. These homologous sequences are mainly attributable to the ribosomal cistrons. 2. Melting curve analysis shows that the heterologous mitochondrial DNA · DNA hybrids contain several times more mismatching than the nuclear DNA · DNA hybrids. 3. DNA · rRNA hybridization shows that the sequences of the ribosomal cistrons in both the nuclear and the mitochondrial genome have been conserved during evolution. 4. However, melting curve analysis of the DNA · rRNA hybrids shows that the sequence of the nuclear ribosomal cistrons have undergone considerably fewer nucleotide substitutions than their mitochondrial counterparts. 5. The results suggest that the mitochondrial ribosomal cistrons have evolved more rapidly than the nuclear cistrons. This is discussed in the light of theories on the rate of molecular evolution.