1,721,291 research outputs found
Molecular phylogeny of three subfamilies of the Neanuridae (Insecta, Collembola) and the position of the Antarctic species Friesea grisea Schaeffer
No full textThe nuclear 28S rRNA and the mitochondrial COII gene were used to establish phylogenetic relationships among species of the family Neanuridae, with special emphasis on species of the subfamily Neanurinae. Phylogenetic analysis was conducted using genetic distances, parsimony and likelihood methods. The D3-D5 fragment of the rRNA gene was very conserved, both in sequence and in secondary structure features. This fragment supplied little information on relationships at this level. The phylogenetic reconstruction based on 1st and 2nd codon positions of the COII gene was partly in accordance with morphological data, but it was discordant for the placement of some species. Relationships among the subfamilies Frieseinae, represented by the Antarctic species Friesea grisea, Pseudachorutinae and Neanurinae were uncertain. The subfamily Neanurinae and its tribes Neanurini and Paleonurini were shown as monophyletic taxa. Relationships between three species of the genus Bilobella were in accordance with morphological and biochemical data. Relationships between genera within the Neanurini were more controversial. In accordance with morphological hypotheses, a basal position of Thaumanura was suggested, but the molecular data placed Neanura muscorum in a derived position, in sharp contrast with morphological evidence. A close relationship was suggested between Deutonura conjuncta, Cansilianura malatestai and Lathriopyga longiseta. The disagreement between molecular and morphological data suggests that one or both data sets might be affected by a certain degree of homoplasy and that these data should be interpreted with caution in phylogenetic reconstructions
An assessment of the value of nuclear and mitochondrial genes in elucidating the origin and evolution of Isotoma klovstadi Carpenter (Insecta, Collembola)
No full textIn order to infer the origin and the evolution of Antarctic Collembola, a correct phylogenetic analysis depicting relationships among Antarctic and non-Antarctic species is required. A preliminary assessment of the value of DNA sequences in reconstructing phylogenetic relationships among the Antarctic Isotoma klovstadi and other non-Antarctic species was carried out by sequencing one mitochondrial gene (Cytochrome c oxidase, subunit II) and two nuclear genes (a fragment of the 28S rDNA and the Elongation Factor-1 alpha). Estimates of base composition heterogeneity revealed that in the two protein-coding genes (COII and EF-1 alpha) 3rd codon position sites are compositionally very heterogeneous and the analysis of these two genes was therefore performed only on 1st and 2nd codon position sites. Phylogenetic analyses using Maximum Likelihood, Maximum Parsimony and Minimum Evolution revealed that the COII and the EF-1 alpha genes are more suitable than the D3 fragment for the reconstruction of phylogenetic relationships within the Family Isotomidae to which Isotoma and several other genera of Antarctic Collembola belon
Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan Collembola (Insecta)
No full textLight and electron microscopy evidence have been obtained to describe the peculiar spermatogenesis in the collembolan species Sminthurus viridis and Allacma fusca (Sminthuridae), In these two species, the two sexes differ for the lack of two chromosomes (the sex chromosomes) in males (males, 2n = in; females, 2n = 12), While oogenesis seems to proceed normally, spermatogenesis is peculiar because the two daughter cells of the first meiotic division have different chromosome numbers (six and four). The cell receiving four chromosomes degenerates, while the cell receiving six chromosomes completes meiosis and produces identical spermatozoa (n = 6), At fertilization, pronuclei with six chromosomes fuse together to form zygotes with 2n = 12, Male embryos must lose two sex chromosomes during the first zygotic mitosis, as all male cells have 2n = 10 chromosomes. The sex chromosome system of these species can be identified as X1X1X2X2:X(1)X(1)0. Electron microscopy observations show that the same peculiar spermatogenesis occurs also in two others species of the same family, Caprainea marginata and Lipothrix lubbocki, The peculiar sex determination system described is similar but not identical to what is observed in other insect orders, and it may represent an evolutionary step toward parthenogenesis, It is suggested that this peculiar spermatogenesis is common to all Symphypleona
A molecular approach for studying the biology of the olive fly (Bactrocera oleae): mitochondrial genomics and microsatelliti loci
No full textRepeated regions in mitochondrial genomes: distribution, origin and evolutionary significance
No full textAll complete or nearly complete mitochondrial genomes of Metazoa (2819) have been subject to bioinformatic analysis to investigate the distribution and features of repeated and palinclromic sequences. Repeats are ubiquitous, with 29.9% of genomes containing at least one and 1.95% of total genome length being repeated. Repeat boundaries were tested for the presence of secondary structure motifs, consensus sequences or small repeats, features generally reported as associated with duplications. No significant relationship was detected, suggesting the non ubiquitousness of such features. A mechanism related to gene conversion is proposed to explain the origin of small interspersed repeats
Chromosome elimination and sex determination in springtails (Insecta, Collembola)
No full textA post-zygotic mechanism of sex determination is described in the two symphypleonans Dicyrtomina ornata (Nicolet) and Ptenothrix italica Dallai. The process consists of the loss of two sex chromosomes from the male embryo. At the end of the first meiotic division of spermatogenesis, a second chromosome elimination occurs, allowing half the secondary spermatocytes, later transformed into spermatids, to receive a complete haploid set of chromosomes. The secondary spermatocytes, which receive an incomplete set of chromosomes, degenerate. Males of the two collembolan species, therefore, produce a reduced number (50%) of spermatozoa. Females of D. ornata have 2n = 12 and males 2n = 10 chromosomes; females of P. italica have 2n = 14 and males 2n = 12 chromosomes. In both species, oogenesis proceeds normally and chromosomes pair and form chiasmata in meiotic prophase. The adaptive significance of this post-zygotic mechanism of sex determination is discussed. The mechanism seems to be a characteristic feature of the suborder Symphypleona. The neanurid Arthropleona Anurida maritima (Guérin), which was studied for comparative analysis, has 2n = 8 chromosomes and normal spermatogenesis producing haploid nuclei with four chromosomes
An Algorithm to Construct Greedy Drawings of Triangulations
No full textWe show an algorithm to construct a greedy drawing of every given triangulation. The algorithm relies on two main results. First, we show how to construct greedy drawings of a fairly simple class of graphs, called triangulated binary cactuses. Second, we show that every triangulation can be spanned by a triangulated binary cactus. Further, we discuss how to extend our techniques in order to prove that every triconnected planar graph admits a greedy drawing. Such a result, which proves a conjecture by Papadimitriou and Ratajczak, was independently shown by Leighton and Moitra
Genetic diversity and taxonomy in soil dwelling insects: the genus Orchesella
No full textGenetic diversity and inter- and intraspecific differentiation are evaluated in 10 populations of four different species of the soil-dwelling genus Orchesella (Collembola, Entomobryidae). The degree of genetic variability is consistent with that observed in other insects and soil organisms. Values of genetic distance are very high and suggest that the species differentiated a long time ago. Two different procedures are used to estimate the rate of gene flow among six populations of O. villosa, but they give contradictory results: this species seems to be subdivided into two groups of populations (inland and coastal), which might represent two subspecies
On Planar Greedy Drawings of 3-Connected Planar Graphs
No full textA graph drawing is greedy if, for every ordered pair of vertices (x, y), there is a path from x to y such that the Euclidean distance to y decreases monotonically at every vertex of the path. Greedy drawings support a simple geometric routing scheme, in which any node that has to send a packet to a destination “greedily” forwards the packet to any neighbor that is closer to the destination than itself, according to the Euclidean distance in the drawing. In a greedy drawing such a neighbor always exists and hence this routing scheme is guaranteed to succeed. In 2004 Papadimitriou and Ratajczak stated two conjectures related to greedy drawings. The greedy embedding conjecture states that every 3-connected planar graph admits a greedy drawing. The convex greedy embedding conjecture asserts that every 3-connected planar graph admits a planar greedy drawing in which the faces are delimited by convex polygons. In 2008 the greedy embedding conjecture was settled in the positive by Leighton and Moitra. In this paper we prove that every 3-connected planar graph admits a planar greedy drawing. Apart from being a strengthening of Leighton and Moitra’s result, this theorem constitutes a natural intermediate step towards a proof of the convex greedy embedding conjecture
Allozyme variation in reference and metal-exposed natural populations of Orchesella cincta (L.) (Insecta, Collembola)
No full textEnvironmental pollution may affect genetic variation in populations inhabiting polluted sites. In this study, allozyme variation was studied in natural populations of the soil-dwelling insect Orchesella cincta. The populations originated from eight sites with different histories of metal contamination and natural enrichment in The Netherlands, Belgium and F.R.G. Slight but significant divergence was observed for four polymorphic loci; 18 loci were monomorphic at all sites. The highest mean values for heterozygosity (H), degree of polymorphism (P) and mean number of alleles per locus (A) were 0.090, 0.182 and 1.36, respectively. Geographic distance was not correlated to genetic divergence. Comparison of allozyme data from investigated populations and from a reference population in Italy showed a remarkable genetic homogeneity in this species, the highest value for Nei's genetic distance (D) being 0.035. Among NW European populations, a correlation between metal tolerance characteristics and allozyme frequencies was observed for glutamate-oxaloacetate transaminase (EC. 2.6.1.1)
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