Mitochondrial DNA polymorphism and phylogenetic relationships inHevea brasiliensis

Using fourteen random mitochondrial DNA probes, we have examined restriction fragment length polymorphism (RFLP) in wild and cultivatedHevea brasiliensis. A total of 395 accessions, including 345 from various prospectings collected in Brazil, Colombia and Peru and 50 cultivated clones, were analyzed. Two other species (H. benthamiana andH. pauciflora) were also included in the study for comparison. The high level of mitochondrial polymorphism allowed us to divide all the accessions analyzed into 212 distinct genotypes. The genetic variability of cultivated clones was limited to four genotypes forming two clusters. In contrast, considerable genetic variation was found in the wild collections. In almost all cases, accessions displaying the same RFLP profile were restricted to the same geographical area (same or neighbor administrative districts). In addition, accessions whose genetic closeness was predicted by RFLP profiles were also clustered according to geographical origin. In a few cases, however, similar RFLP profiles were found for accessions originating from geographically distant districts. This discrepancy can be explained either by seed dispersion (by river) or possibly by similar genetic events occurring independently in different geographical locations. Chloroplast DNA RFLP was also analyzed in 217 accessions, representative of 126 distinct mitochondrial genotypes. Very few differences were found, indicating that the chloroplast genome is more highly conserved than the mitochondrial genome.     

‘Lymphocyte-like’cells in ascidians: Precursors for vertebrate lymphocytes?

There has been much speculation that the evolutionary precursors of vertebrate lymphocytes may exist in the ascidians (proto chordates), but conclusive evidence has remained elusive especially as membrane bound immunoglobulin has not been detected in any invertebrate. This paper reviews new evidence which indicates that the ‘lymphocyte-like’ cells in ascidians have functional, as well as morphological, similarities with vertebrate lymphocytes. Firstly, these cells have been linked with in vivo non-self recognition events such as allograft rejection in solitary ascidians and non-fusion reactions between colonial ascidians. Secondly, they have been shown to be cytotoxic towards xenogenic targets in vitro and to use cytolytic mechanisms similar to those of cytotoxic T-cells. Thirdly, they are able to proliferate in vitro in response to mitogens or allogeneic cells. It is therefore suggested that, apart from immunoglobulin production and clonal selection, there is persuasive evidence that the ‘lymphocyte-like’ cells of ascidians constitute a primordial form of vertebrate lymphocyte.     

Biosynthesis of δ-aminolevulinic acid from glutamate by Sulfolobus solfataricus

The extremely thermophilic, obligately aerobic bacterium Sulfolobus solfataricus forms the tetrapyrrole precursor, -aminolevulinic acid (ALA), from glutamate by the tRNA-dependent five-carbon pathway. This pathway has been previously shown to occur in plants, algae, and most prokaryotes with the exception of the -group of proteobacteria (purple bacteria). An alternative mode of ALA formation by condensation of glycine and succinyl-CoA occurs in animals, yeasts, fungi, and the -proteobacteria. Sulfolobus and several other thermophilic, sulfur-dependent bacteria, have been variously placed within a subgroup of archaea (archaebacteria) named crenarchaeotes, or have been proposed to comprise a distinct prokaryotic group designated eocytes. On the basis of ribosomal structure and certain other criteria, eocytes have been proposed as predecessors of the nuclear-cytoplasmic descent line of eukaryotes. Because aplastidic eukaryotes differ from most prokaryotes in their mode of ALA formation, and in view of the proposed affiliation of eocytes to eukaryotes, it was of interest to determine how eocytes form ALA. Sulfolobus extracts were able to incorporate label from [1-¹⁴C]glutamate, but not from [2-¹⁴C]glycine, into ALA. Glutamate incorporation was abolished by preincubation of the extract with RNase. Sulfolobus extracts contained glutamate-1-semialdehyde aminotransferase activity, which is indicative of the five-carbon pathway. Growth of Sulfolobus was inhibited by gabaculine, a mechanism-based inhibitor of glutamate-1-semialdehyde aminotransferase, an enzyme of the five-carbon ALA biosynthetic pathway. These results indicate that Sulfolobus uses the five-carbon pathway for ALA formation.Abbreviations AHA 4-amino-5-hexynoic acid - ALA -aminolevulinic acid, Gabaculine, 3-amino-2,3-dihydrobenzoic acid - GSA glutamate 1-semialdehyde     

CHROMOSOMAL VERSUS MITOCHONDRIAL DNA EVOLUTION: TRACKING THE EVOLUTIONARY HISTORY OF THE SOUTHWESTERN EUROPEAN POPULATIONS OF THE SOREX ARANEUS GROUP (MAMMALIA,INSECTIVORA)

The shrews of the Sorex araneus group have undergone a spectacular chromosome evolution. The karyotype of Sorex granarius is generally considered ancestral to those of Sorex coronatus and S. araneus. However, a sequence of 777 base pairs of the cytochrome b gene of the mitochondrial DNA (mtDNA) produces a quite different picture: S. granarius is closely related to the populations of S. araneus from the Pyrenees and from the northwestern Alps, whereas S. coronatus and S. araneus from Italy and the southern Alps represent two well-separated lineages. It is suggested that mtDNA and chromosomal evolution are in this case largely independant processes. Whereas mtDNA haplotypes are closely linked to the geographical history of the populations, chromosomal mutations were probably transmitted from one population to another. Available data suggest that the impressive chromosome polymorphism of this group is quite a recent phenomenon.     

The origins and evolutionary expansion of the Strongylida (Nematoda)

The Strongylida are thought to have arisen from free-living rhabditoid nematodes, but the relationships between the major groupings within the Strongylida, the Strongylina, the Metastrongylina, Trichostrongylina and the Ancylostomatina are far from clear in spite of the abundance of morphological data now available for analysis. Evolutionary mechanisms including co-evolution, host switching, host dispersal, use of intermediate hosts, various sites of localisation within the definitive host and modifications of life-cycle strategies appear to have been utilised in the expansion of the Strongylida, with different mechanisms predominating in different families or superfamilies. Co-evolution appears to have been a major mode of evolution in the Strongylina, in contrast to the Trichostrongylina, which have used host dispersal and host-switching to great advantage. The phylogeny of the Ancylostomatina shows little association with host evolution, but does match the feeding preferences of the hosts. The Metastrongylina have utilised intermediate hosts and life cycle modifications including a shift to extra-intestinal sites as major means of diversification, in contrast to the other sub-orders. The review, while indicating much progress in our understanding of the phylogeny of the Strongylida, also reveals that enormous gaps still exist, and emphasises the tentative nature of many of the phylogenetic hypotheses tendered to date.     

Phylogeny of Drosophila and related genera inferred from the nucleotide sequence of the Cu,Zn Sod gene

The phylogeny and taxonomy of the drosophilids have been the subject of extensive investigations. Recently, Grimaldi (1990) has challenged some common conceptions, and several sets of molecular data have provided information not always compatible with other taxonomic knowledge or consistent with each other. We present the coding nucleotide sequence of the Cu,Zn superoxide dismutase gene (Sod) for 15 species, which include the medfly Ceratitis capitata (family Tephritidae), the genera Chymomyza and Zaprionus, and representatives of the subgenera Dorsilopha, Drosophila, Hirtodrosophila, Scaptodrosophila, and Sophophora. Phylogenetic analysis of the Sod sequences indicates that Scaptodrosophila and Chymomyza branched off the main lineage before the major Drosophila radiations. The presence of a second intron in Chymomyza and Scaptodrosophila (as well as in the medfly) confirms the early divergence of these two taxa. This second intron became deleted from the main lineage before the major Drosophila radiations. According to the Sod sequences, Sophophora (including the melanogaster, obscura, saltans, and willistoni species groups) is older than the subgenus Drosophila; a deep branch splits the willistoni and saltans groups from the melanogaster and obscura groups. The genus Zaprionus and the subgenera Dorsilopha and Hirtodrosophila appear as branches of a prolific “bush” that also embraces the numerous species of the subgenus Drosophila. The Sod results corroborate in many, but not all, respects Throckmorton's (King, R.C. (ed) Handbook of Genetics. Plenum Press, New York, pp. 421–469, 1975) phylogeny; are inconsistent in some important ways with Grimaldi's (Bull. Am. Museum Nat. Hist. 197:1–139, 1990) cladistic analysis; and also are inconsistent with some inferences based on mitochondrial DNA data. The Sod results manifest how, in addition to the information derived from nucleotide sequences, structural features (i.e., the deletion of an intron) can help resolve phylogenetic issues. Correspondence requests to: F. J. Ayala     

The evolutionary expansion of the trypanosomatid flagellates

The trypanosomatids combine a relatively uniform morphology with ability to parasitise a very diverse range of hosts including animals, plants and other protists. Along with their sister family, the biflagellate bodonids, they are set apart from other eukaryotes by distinctive organisational features, such as the kinetoplast-mitochondrion and RNA editing, isolation of glycolysis enzymes in the glycosome, use of the flagellar pocket for molecular traffic into and out of the cell, a unique method of generating cortical microtubules, and bizarre nuclear organisation. These features testify to the antiquity and isolation of the kinetoplast-bearing flagellates (Kinetoplastida). Molecular sequencing techniques (especially small subunit ribosomal RNA gene sequencing) are now radically reshaping previous ideas on the phylogeny of these organisms. The idea that the monogenetic (MG) trypanosomatids gave rise to the digenetic (DG) genera is losing ground to a view that, after the bodonids, the African trypanosomes (DG) represent the most ancient lineage, followed by Trypanosoma cruzi (DG), then Blastocrithidia (MG), Herpetomonas (MG) and Phytomonas (DG), with Leptomonas (MG), Crithidia (MG), Leishmania (DG) and Endotrypanum (DG) forming the crown of the evolutionary tree. Vast genetic distances (12% divergence) separate T. brucei and T. cruzi, while the Leishmania species are separated by very short distances (less than 1% divergence). These phylogenetic conclusions are supported by studies on RNA editing and on the nature of the parasite surface. The trypanosomatids seem to be able to adapt with ease their energy metabolism to the availability of substrates and oxygen, and this may give them the ability to institute new life cycles if host behaviour patterns allow. Sexual processes, though present in at least some trypanosomatids, may have played only a minor part in generating diversity during trypanosomatid evolution. On the other hand, the development of altruistic behaviour on the part of some life cycle stages may be a hitherto unconsidered way of maximising fitness in this group. It is concluded that, owing to organisational constraints, the trypanosomatids can undergo substantial molecular variation while registering very little in the way of morphological change.     

Evidence for phylogenetic congruence among sulfur-oxidizing chemoautotrophic bacterial endosymbionts and their bivalve hosts

Sulfur-oxidizing chemoautotrophic (thioautotrophic) bacteria are now known to occur as endosymbionts in phylogenetically diverse bivalve hosts found in a wide variety of marine environments. The evolutionary origins of these symbioses, however, have remained obscure. Comparative 16S rRNA sequence analysis was used to investigate whether thioautotrophic endosymbionts are monophyletic or polyphyletic in origin and to assess whether phylogenetic relationships inferred among these symbionts reflect those inferred among their hosts. 16S rRNA gene sequences determined for endosymbionts from nine newly examined bivalve species from three families (Vesicomyidae, Lucinidae, and Solemyidae) were compared with previously published 16S rRNA sequences of thioautotrophic symbionts and free-living bacteria. Distance and parsimony methods were used to infer phylogenetic relationships among these bacteria. All newly examined symbionts fall within the gamma subdivision of the Proteobacteria, in clusters containing previously examined symbiotic thioautotrophs. The closest free-living relatives of these symbionts are bacteria of the genus Thiomicrospira. Symbionts of the bivalve superfamily Lucinacea and the family Vesicomyidae each form distinct monophyletic lineages which are strongly supported by bootstrap analysis, demonstrating that host phylogenies inferred from morphological and fossil evidence are congruent with phylogenies inferred for their respective symbionts by molecular sequence analysis. The observed congruence between host and symbiont phylogenies indicates shared evolutionary history of hosts and symbiont lineages and suggests an ancient origin for these symbioses. Correspondence to: D.L. Distel     

PHYLOGENETIC RELATIONSHIPS WITHIN THE COLONIAL VOLVOCALES (CHLOROPHYTA) INFERRED FROM CLADISTIC ANALYSIS BASED ON MORPHOLOGICAL DATA1

A cladistic analysis was used to deduce the phylogenetic relationships within the colonial Volvocales. Forty-one pairs of characters related to gross morphology and ultrastructure of vegetative colonies as well as asexual and sexual reproduction were analyzed based on parsimony, using the PAUP 3.0 computer program, for 25 species belonging to nine volvocacean and goniacean genera of the colonial Volvocales. Chlamydomonas reinhardtii Dangeard was the outgroup. The strict consensus tree indicated the presence of two monophyletic groups, one composed of all the volvocacean species analyzed in this study and the other containing the goniacean species except for the four-celled species Gonium sociale (Dujardin) Warming. In addition, these two groups constitute a large monophyletic group, to which G. sociale is a sister group. A new combination Tetrabaena socialis (Dujardin) Nozaki et Itoh and a new family Tetrabaenaceae Nozaki et Itoh are thus proposed for G. sociale. In addition, the analysis suggests that the volvocacean genera Eudorina and Pleodorina are paraphyletic groups, respectively, and that the monotypic genus Yamagishiella has no autapomorphic characters and represents primitive features of the anisogamous and oogamous genera of the Volvocaceae. Phylogenetic relationships within the Volvocaceae and the Goniaceae, as well as the various modes of sexual reproduction exhibited by these organisms, are discussed on the basis of the analysis.