Molecular analysis of the genomic organization of the Hymenoptera Diadromus pulchellus and Eupelmus vuilleti

The genomic organization of two parasitic wasps was analyzed by DNA reassociation. Cot curves revealed a pattern with three types of components. A highly repetitive DNA, accounting for 15 to 25% of the genome, was identified as satellite DNA. The moderately repetitive DNA corresponds to 26 to 42% of the genome in both species, and shows large variations in complexity, repetitive frequency and a number of sub-components between males and females. These variations are seen as resulting from DNA amplification during somatic and sexual differentiation. Dot blot analyses show that such DNA amplifications concern several types of structural and regulatory genes. The presence of repeated mobile elements was studied by the Roninson method to compare the repeated sequence patterns of Diadromus pulchellus and Eupelmus vuilleti with those of Drosophila melanogaster. The occurrence and organization of mobile elements in these Hymenoptera differ from those of the neighboring order of Diptera. The repetitive and unique components define very large genomes (1 to 3 × 10⁹ base pairs). The genomic organization in Parasitica appears to be an extreme drosophilan type. We propose that the germinal genome of these parasitic wasps is primarily composed of satellite DNA blocks and very long stretches of unique sequences, separated by a few repeated and/or variously deleted, interspersed elements of each mobile element family.     

Mitochondrial rps14 is a transcribed and edited pseudogene in Arabidopsis thaliana

We have isolated and analysed a 2 kb region of the mitochondrial genome of Arabidopsis thaliana (Columbia) showing a high level of nucleotide identity with the mitochondrial (mt) rps14 small-subunit ribosomal protein gene from Oenothera berteriana and Vicia faba, as well as with an open reading frame (ORF) located upstream of the nad3 locus in O. berteriana. The rps14 locus is present as a single copy in the A. thaliana mt genome and has a translational stop codon located near the initiation codon, as well as a deletion of one nucleotide that disturbs the coding sequence. The cloning and sequencing of nine amplified mt rps14 cDNAs clearly demonstrated that this gene is transcribed and that the mRNA precursors are edited at three positions, all involving C-to-U conversions. No editing events changing the stop codon and restoring the correct coding sequence were witnessed within the 9 individual cDNA clones. Therefore, we conclude that the single rps14 sequence of the mitochondrial genome from A. thealiana is in fact a pseudogene that is transcribed and edited but not translated.     

A rapid procedure for the construction of PCR cDNA libraries from small amounts of plant tissue

We describe a general method for the preparation of λZAP II cDNA libraries from very small amounts (<50 mg) of plant tissue. We have achieved this by combining an efficient method for RNA extraction with a modified PCR protocol for the synthesis and amplification of cDNA. Using this protocol we have found it possible to generate cDNA libraries containing more than 10⁶ clones from as little as 1 μg of total RNA.     

Inheritance of the group I rDNA intron in Tetrahymena pigmentosa

We have previously argued from phylogenetic sequence data that the group I intron in the rRNA genes of Tetrahymena was acquired by different Tetrahymena species at different times during evolution. We have now approached the question of intron mobility experimentally by crossing intron⁺ and intron^? strains looking for a strong polarity in the inheritance of the intron (intron homing). Based on the genetic analysis we find that the intron in T. pigmentosa is inherited as a neutral character and that intron⁺ and intron^? alleles segregate in a Mendelian fashion with no sign of intron homing. In an analysis of vegetatively growing cells containing intron⁺ and intron^? rDNA, initially in the same macronucleus, we similarly find no evidence of intron homing. During the course of this work, we observed to our surprise that progeny clones from some crosses contained three types of rDNA. One possible explanation is that T. pigmentosa has two rdn loci in contrast to the single locus found in T. thermophila. Some of the progeny clones from the genetic analysis were expanded for several hundred generations, and allelic assortment of the rDNA was demonstrated by subcloning analysis. © 1992 Wiley-Liss, Inc.     

The possible role of soluble salts in chemical evolution

Summary A model is proposed for a prebiotic environment in which concentration, condensation, and chemical evolution of biomolecules could have taken place. The main reactions expected of proteins, nucleic acids, lipids, and some of their precursors in this environment are examined.The model is based on our previously developed concept of a fluctuating system in which hydration and dehydration processes take place in a cyclic manner. In the present model, however, high concentrations of soluble salts, such as chlorides and sulfates, are taken into account, whereas previously a more or less salt-free system had been assumed. Thus the preponderance of surfaces of soluble salts is implied, even though sparingly soluble minerals, such as clay minerals or quartz, are also present.During the dehydration stage biomolecules tend to leave the solution and concentrate at certain microenvironments, such as in micelles and aggregates, at the liquid-gas surface and, possibly, at the emerging solid surfaces. Moreover, in these brines, and especially during the last stages of dehydration, high temperatures are attainable, which may enhance certain reactions between the organic molecules, and result in a net increase of condensation over degradation.In the dehydrated state, solid-state condensation and synthesis reactions are possible in which the surface of soluble salts may serve as a catalyst. Several reports in the literature support this hypothesis. Hydration brings about dissolution of the minerals and redistribution of the biomolecules. In such a system, evolutionary processes like those postulated by White (1980) and by Lahav and White (1980) are possible. Moreover, since several soluble salts of known geological occurrence are optically active in their crystalline state, the involvement of the model system in the selection and evolution of chiral organic compounds should also be considered. In addition, organic molecules in the above microenvironments are also expected to undergo selective interactions based on factors such as molecular pattern and chiral recognition and hydrophobicity. The proposed system emphasizes the need to develop the theoretical background and experimental methods for the study of interactions among biomolecules in the presence of high salt concentrations and solid surfaces of soluble salts, as well as interactions between the biomolecules and these surfaces.     

MOLECULAR DELINEATION OF SPECIES AND SYNGENS IN VOLVOCACEAN GREEN ALGAE (CHLOROPHYTA)1

Two species of the colonial green flagellate family Volvocaceae are worldwide in distribution yet exhibit contrasting species structure. Geographically disparate isolates of Gonium pectorale Mueller can interbreed while isolates of Pandorina morum Bory behave quite differently. More than 20 sexually isolated subpopulations occur within this species; these have been termed “syngens” (sensu Sonneborn). Because prezygotic barriers to mating cause intersyngen pairings to fail, breeding analyses cannot be used to estimate genetic relatedness among the syngens of P. morum. DNA comparisons provide an alternative method of assessing genetic relatedness. We compared the nucleotide sequence of the internal transcribed spacer (ITS) region of the nuclear ribosomal repeat among clones of P. morum and of G. pectorale. Members of syngens of P. morum with distribution restricted to one small geographical area show great similarity. Likewise, members of any syngen of worldwide distribution show near uniformity, even those from different continents. However, the ITS sequence of each syngen differs from that of other syngens. In contrast, G. pectorale, which has an ITS region that is remarkably uniform throughout the world, appears to consist of a single syngen within North America and Europe by mating tests. The molecular data are in complete conformity with previous syngen assignment. Because the latter is based on mating affinity, with two complementary mating types per syngen, the evolution of new mating type pairs appears to be the basis of microevolution in these algae. We infer that either P. morum is a more ancient species than G. pectorale or that P. morum has a less stable genome. In either case, the biogeographic distribution of certain syngens may reflect climatological changes of the past.