Search results for "genus"
showing 10 items of 755 documents
On the slope of hyperelliptic fibrations with positive relative irregularity
2016
Let $f:\, S \to B$ be a locally non-trivial relatively minimal fibration of hyperelliptic curves of genus $g\geq 2$ with relative irregularity $q_f$. We show a sharp lower bound on the slope $\lambda_f$ of $f$. As a consequence, we prove a conjecture of Barja and Stoppino on the lower bound of $\lambda_f$ as an increasing function of $q_f$ in this case, and we also prove a conjecture of Xiao on the ampleness of the direct image of the relative canonical sheaf if $\lambda_f<4$.
Theta-characteristics on singular curves
2007
On a smooth curve a theta–characteristic is a line bundle L with square that is the canonical line bundle ω. The equivalent conditionHom(L, ω) ∼= L generalizes well to singular curves, as applications show. More precisely, a theta–characteristic is a torsion–free sheaf F of rank 1 with Hom(F , ω) ∼= F . If the curve has non ADE–singularities then there are infinitely many theta–characteristics. Therefore, theta–characteristics are distinguished by their local type. The main purpose of this article is to compute the number of even and odd theta–characteristics (i.e. F with h(C,F) ≡ 0 resp. h(C,F) ≡ 1 modulo 2) in terms of the geometric genus of the curve and certain discrete invariants of a …
A note on the unirationality of a moduli space of double covers
2010
In this note we look at the moduli space $\cR_{3,2}$ of double covers of genus three curves, branched along 4 distinct points. This space was studied by Bardelli, Ciliberto and Verra. It admits a dominating morphism $\cR_{3,2} \to {\mathcal A}_4$ to Siegel space. We show that there is a birational model of $\cR_{3,2}$ as a group quotient of a product of two Grassmannian varieties. This gives a proof of the unirationality of $\cR_{3,2}$ and hence a new proof for the unirationality of ${\mathcal A}_4$.
Irreducibility of Hurwitz spaces of coverings with one special fiber and monodromy group a Weyl group of type D d
2007
Let Y be a smooth, connected, projective complex curve. In this paper, we study the Hurwitz spaces which parameterize branched coverings of Y whose monodromy group is a Weyl group of type D d and whose local monodromies are all reflections except one. We prove the irreducibility of these spaces when $$Y \simeq \mathbb {P}^{1}$$ and successively we extend the result to curves of genus g ≥ 1.
First Data on the Molecular Phylogeography of Scincid Lizards of the Genus Mabuya
2000
A 487-bp fragment of the mitochondrial 16S rRNA gene was sequenced in 26 species of the circumtropical lizard genus Mabuya and used to analyze phylogenetic relationships within the genus. The species from Africa and Madagascar formed a monophyletic group relative to the included Asian and South American taxa. The Malagasy species included (M. elegans, M. cf. dumasi, and M. comorensis) did not appear as a monophylum. Combined and separate analysis of the 16S data and additional sequences of the mitochondrial 12S rRNA, ND4, and cytochrome b genes (a total of 2255 bp) in one Asian, two Malagasy, and two African species also did not result consistently in a monophyletic grouping of the Malagasy…
In situ localization of the Antennapedia gene on the chromosomes of nine Drosophila species of the obscura group.
2008
The homeotic Antennapedia gene, cloned from the genomic DNA of D. subobscura, was localized on the polytene chromosomes of nine species of the Drosophila obscura group. In all of them, the probe used hybridized on chromosomes equivalent to the E element of Muller's terminology. These results are consistent with the idea that single copy genes do not move around the genome and that chromosomal elements have conserved their genetic identity during evolution.
Gypsy endogenous retrovirus maintains potential infectivity in several species of Drosophilids.
2008
Abstract Background Sequences homologous to the gypsy retroelement from Drosophila melanogaster are widely distributed among drosophilids. The structure of gypsy includes an open reading frame resembling the retroviral gene env, which is responsible for the infectious properties of retroviruses. Results In this study we report molecular and phylogeny analysis of the complete env gene from ten species of the obscura group of the genus Drosophila and one species from the genus Scaptomyza. Conclusion The results indicate that in most cases env sequences could produce a functional Env protein and therefore maintain the infectious capability of gypsy in these species.
Distribution of the bilbo Non-LTR Retrotransposon in Drosophilidae and its Evolution in the Drosophila obscura Species Group
2001
The bilbo element is a non-LTR retrotransposon isolated from Drosophila subobscura. We conducted a distribution survey by Southern blot for 52 species of the family Drosophilidae, mainly from the obscura and melanogaster groups. Most of the analyzed species bear sequences homologous to bilbo from D. subobscura. In the obscura group, species from the same species subgroup also share similar Southern blot patterns. To investigate the phylogenetic relationship among these elements, we analyzed eight copies of a short sequence of the element from several species of the obscura group. The obtained phylogram agrees with the phylogeny of the species, which suggests vertical transmission of the ele…
Leaf domatia in the section Alaternus (Miller) DC. of the genus Rhamnus (Rhamnaceae)
1993
Anatomical and morphological study of leaves from three species of Rhamnus (R. alaternus, R. myrtifolius and R. ludovici-salvatoris) which comprise the section Alaternus (Miller) DC. from the Iberian Peninsula and Balearic Islands, have revealed the presence of domatia, which are macroscopic infundibuliform structures. Although not all the leaves of one single plant showed domatia, all the individuals of each species examined presented these structures.
Gobio Genus Species Integrated Management System – Târnava Rivers Study Case (Transylvania, Romania)
2019
Abstract The lotic habitats quality indicative congeners species Gobio gobio, Gobio kessleri and Gobio albipinnatus populations’ dynamic in time (2004-2019) and space revealed a decreasing trend in these rivers ecological status. The ADONIS:CE tool has been used to build a backing management system model, based on these indicative fish species habitat needs, indicators for favourable conservation status, pressures and threats. This management system implementation in the field will favour the amelioration of lotic habitats and the ecological status recovering of two of the local fish species of conservative interest (G. kessleri and G. albipinnatus).