There is strong inferential evidence for recent horizontal gene transfer of the P (mobile) element to Drosophila melanogaster from a species of the Drosophila willistoni group. One potential vector of this transfer is a semiparasitic mite, Proctolaelaps regalis DeLeon, whose morphology, behavior, and co-occurrence with Drosophila are consistent with the properties necessary for such a vector. Southern blot hybridization, polymerase chain reaction (PCR) amplification, and DNA sequencing showed that samples of P. regalis associated with a P strain of D. melanogaster carried P element sequences. Similarly, Drosophila ribosomal DNA sequences were identified in P. regalis samples that had been associated with Drosophila cultures. These results have potentially important evolutionary implications, not only for understanding the mechanisms by which genes may be transferred between reproductively isolated species, but also for improved detection of some host-parasite and predator-prey relationships.

The diagnostic assessment of postural instability is more pronounced during unstable-support conditions requiring active head movements. In addition to supporting return-to-duty decisions by flight surgeons, the CDP provides a standardized sensorimotor measure that can be used to evaluate the effectiveness of countermeasures designed to either minimize deconditioning on orbit or promote reconditioning upon return to Earth.

The P element, originally described in Drosophila melanogaster, is one of the best-studied eukaryotic transposable elements. In an attempt to understand the evolutionary dynamics of the P element family, an extensive phylogenetic analysis of 239 partial P element sequences has been completed. These sequences were obtained from 40 species in the Drosophila subgenus Sophophora. The phylogeny of the P element family is examined in the context of a phylogeny of the species in which these elements are found. An interesting feature of many of the species examined is the coexistence in the same genome of P sequences belonging to two or more divergent subfamilies. In general, P elements in Drosophila have been transmitted vertically from generation to generation over evolutionary time. However, four unequivocal cases of horizontal transfer, in which the element was transferred between species, have been identified. In addition, the P element phylogeny is best explained in numerous instances by horizontal transfer at various times in the past. These observations suggest that, as with some other transposable elements, horizontal transfer may play an important role in the maintenance of P elements in natural populations.Transposable elements are universal features of eukaryotic genomes and can be broadly divided into two different classes (1). Class I elements are characterized by DNA sequences with homology to reverse transcriptase and are often referred to as retrovirus-like elements or retroelements. Their mobility is achieved through an RNA intermediate. Transposition of Class II elements, such as the Drosophila elements mariner and P, is catalyzed by a transposase and occurs directly from DNA to DNA, without an RNA intermediate. The P element was first described in D. melanogaster where its mobility in the germ-line of hybrid flies is responsible for a type of hybrid dysgenesis (2). The complete, or canonical, P element is 2,907 base pairs long and has four ORFs (ORF 0-3) that encode an 87-kDa DNA-binding transposase (3). Also required for transposition are the element termini, which include flanking 31-bp perfect inverted repeats, 11-bp subterminal repeats, and unique terminal sequences comprising approximately 150 bp (4). The genomic copy number of D. melanogaster P elements varies from 0 to about 60 per genome (3). A minority of these are autonomous (transposase-competent) elements; most are internally deleted, nonautonomous (transposase-incompetent) elements. Defective P elements are generally smaller, variable in size, and derived from complete elements by internal deletions. The induction of these deletions is associated with active transposition of P elements.When active, transposable elements can behave as natural or spontaneous mutagens, inducing a wide variety of mutations, chromosomal aberrations, or other genetic changes. Although positive effects are not precluded, both theoretical and empirical studies of transposable elements suggest that this property of transposition has a negative effect on h...

The eubacterial genus Rickettsia belongs to the cu subgroup of the phylum Proteobacteriu. This genus is usually divided into three biotypes on the basis of vector host and antigenic cross-reactivity characteristics. However, the species Rickettsia bellii does not fit into this classification scheme; this organism has characteristics common to both the spotted fever group and the typhus group biotypes and also exhibits some unique features. Sequences of the 16s rRNA and 23s rRNA genes from Rickettsia rickettsii (spotted fever group), Rickettsia prowazekii (typhus group), and R. bellii were studied to determine the position of R. bellii in the rickettsia1 classification scheme. The 23s rRNA gene sequences described in this paper are the first 23s rRNA sequences reported for any member of the Rickettsiuceue. The 23s rRNA gene contains substantially more phylogenetic information than is contained in the 16s rRNA sequences, and the 23s rRNA gene sequence has diverged about 1.9 times faster in the three Rickettsia species which we studied. Taken together, the molecular data obtained from the two genes indicate that R. bellii is not a member of either the spotted fever group or the typhus group; rather, this organism appears to be the product of a divergence which predates the separation of the genus into the spotted fever group and the typhus group. Consequently, different combinations of the ancestral characteristics retained by R. bellii have been retained in the more derived lineages of the genus. A comparison of the 16s rRNA and 23s rRNA gene sequences of Rickettsia strains with other proteobacterial sequences confirmed that the genus Rickettsia is a unique deeply branching member of the cu subgroup of the Proteobacteria and that the Rickettsia species form a monophyletic cluster. While divergence of the contemporary members of the genus Rickettsia occurred recently, the unique evolutionary line represented by this genus appears to be very old.Rickettsia strains are small, gram-negative bacteria that are intimately associated with eukaryotic cells and are members of a diverse family, the Rickettsiaceae, which also includes other intracellular organisms, including the genera Ehrlichia, Wolbachia, Anaplasma, and Cowdria (4,36). Rickettsiae have natural arthropod hosts (either ticks, mites, or insects) and can be pathogenic for humans and other vertebrates. The obligately intracellular lifestyle and fastidious nature of these organisms have made them difficult to study. A number of genotypic and phenotypic characteristics indicate that the Rickettsia species are closely related, and this genus is now recognized as the sole member of a branch of the cx subgroup Proteobacteriu (8) on the basis of 16s rRNA cataloging and partial 16s rRNA gene sequence data (34).The genus Rickettsia is divided into three biotypes on the basis of immunological cross-reactivity and ecological characteristics (36). Rickettsia species are placed into the spotted fever group (SFG), the typhus group (TG), or the scrub typhus group on the basis...

Nucleotide sequences from two nuclear loci, alcohol dehydrogenase and internal transcribed spacer-1 of the nuclear ribosomal DNA repeats, and two mitochondrial genes, cytochrome oxidase I and cytochrome oxidase II, were determined from nine species in the Drosophila saltans species group. The partition homogeneity test and partitioned Bremer support were used to measure incongruence between phylogenetic hypotheses generated from individual partitions. Individual loci were generally congruent with each other and consistent with the previously proposed morphological hypothesis, although they differed in level of resolution. Since extreme conflict between partitions did not exist, the data were combined and analyzed simultaneously. The total evidence method gave a more resolved and highly supported phylogeny, as indicated by bootstrap proportions and decay indices, than did any of the individual analyses. The cordata and elliptica subgroups, considered to have diverged early in the history of the D. saltans group, were sister taxa to the remainder of the saltans group. The sturtevanti subgroup, represented by D. milleri and D. sturtevanti, occupies an intermediate position in this phylogeny. The saltans and parasaltans subgroups are sister clades and occupy the most recently derived portion of the phylogeny. As with previous morphological studies, phylogenetic relationships within the saltans subgroup were not satisfactorily resolved by the molecular data.