Faithful vertical transmission but ineffective horizontal transmission of bacterial endosymbionts during sexual reproduction of the black bean aphid,Aphis fabae

1. Insects are commonly infected with bacterial endosymbionts. In addition to the costs and benefits associated with harbouring these symbionts, their rates of vertical and horizontal transmission are important determinants of symbiont prevalence. 2. Aphids are cyclical parthenogens and show virtually perfect maternal transmission of endosymbionts during asexual reproduction. Less clear is the role of the annual sexual generation, during which overwintering eggs are produced. Data from pea aphids (Acyrthosiphon pisum Harris) suggest that maternal transmission failures and horizontal transmission via males may occur under sexual reproduction at least occasionally. No such data exist for other aphid species. 3. In the present study, the rates of maternal and paternal transmission of facultative endosymbionts during sexual reproduction in the black bean aphid, Aphis fabae (Scopoli) were examined. Crosses were performed between clones infected with Hamiltonella defensa, clones infected with Regiella insecticola and clones without facultative endosymbionts, and eggs were overwintered under three different conditions. 4. Only one of 205 offspring from crosses testing for maternal transmission failed to inherit the symbiont present in the maternal clone, and in crosses testing for horizontal transmission, only one of 412 offspring acquired a facultative symbiont from the father. 5. These results show that in A. fabae, maternal transmission of H. defensa and R. insecticola is extremely reliable also during sexual reproduction, indicating that maternal transmission failures are unlikely to exert a significant influence on frequencies of infection in the field. Paternal transmission of endosymbionts was exceedingly rare, suggesting that this route of horizontal transmission may be less important than hitherto assumed.     

Rheotaxis In Uronychia Setigera (Ciliata, Hypotrichida)

ABSTRACT. The behavior of populations of Uronychia setigera (Ciliata, Hypotrichida) exposed to water currents flowing at increasing velocities (300, 400, 500, 900, 1,700 μm/s) was analyzed using two techniques: 1) the ethogram and 2) the numerical indices recently proposed to measure the development in space and time of tracks of ciliates. Beyond a certain threshold value of the water velocity (˜ 300 μm/s), this species shows a definite positive rheotaxis, only if it moves in a more or less direct contact with the substrate. No rheotactic swimming ever occurs. Rheotaxis is a gradual, adaptive behavior: the higher the velocity of the current, the stronger the degree of the rheotactic response, as demonstrated by the increasing significance of the polar distribution of the tracks. Beyond 500 μm/s the water flow is so strong that it affects the locomotion of U. setigera continuously and strongly inducing this species to perform a new behavioral pattern, the Fast Backward Bidimensional Swimming. Under stressing water currents it reacts at first by creeping along straighter trajectories and then with faster locomotion, in such a way that its reaction is to a certain extent proportional to the drag of the currents. the rheotaxis of U. setigera is discussed as an adaptive response.     

Phylogeography and morphological variability in land snails: the Sicilian Marmorana (Pulmonata, Helicidae)

Land snails have long been recognized as suitable organisms for studying phenotypic differentiation and phylogeny in relation to geographical distribution. Morphological data (shell and anatomy biometry on different geographical scales) and partial sequences from mitochondrial genes (cytochrome oxidase subunit I , 16S rDNA) were used to test whether morphological patterns match phylogeny in a diversified group of Sicilian rock-dwelling land snails belonging to the genus Marmorana . The taxonomic implications of the three character sets (shell and anatomical biometry and molecular data) were also considered. The inferred phylogenetic relationships do not match morphological (shell and genitalia) patterns. This result may significantly modify the current taxonomy. Mitochondrial based reconstructions define several supported clades well correlated with geographic distribution and populations were found to be distributed parapatrically. The progressive decline in mitochondrial DNA sequence similarity over a distance of 250 km is consistent with a model of isolation by distance, a pattern previously recognized for other groups of land snails. For one clade of Marmorana , colonization along Mediterranean trade routes appears to be a possibility.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 809–823.     

Bdelloid rotifers and nematodes in a small Italian stream

SUMMARY. The bdelloid rotifers and nematodes of a small clean watercourse in the area surrounding the Po river near Parma have been studied. The samples were collected at two sites 400 m apart. The species found in twelve collections of benthos between May 1977 and June 1978 were eighteen belonging to Bdelloidea and forty-seven to Nematoda (Tylenchida have not been considered). Nearly all the bdelloid species were oligosaprobic or beta-mesosaprobic; the Shannon index of diversity was 3.0 at the first site and 2.4 at the second. Among the Nematoda the dominant species were Paraplectonema pedunculatum and Paraphanolaimus anisitsi which were not previously recorded for Italy. Nearly all nematodes are microphagous; predators represent only 10% of collected specimens. Polluted water indicator species (Secernentia group) were only 3.5% of the total nematodes recorded. Most rotifers and nematodes did not show seasonal fluctuations in numbers. The two sampling sites do not seem to have substantial differences in the taxocoenosis of Rotifera and Nematoda; their percentage of similarity is 69% and 79% for the two classes respectively.