In other words, it is not simply a matter of weak purifying selection allowing active spread of the elements, but an effect of the element mutational dynamics introducing directional bias in the evolutionary process.
Given that the signs of respiratory disease reported in this study were those associated with the occurrence of respiratory disease in the previous two years, cows and pregnant heifers might have experienced disease caused by BRSV some time previously, following the active spread of the virus among youngstock detected in this study at the time of testing.
A certain amount of invagination does take place, assisted by an active spreading of the animal hemisphere of the embryo; as a result, the ectoderm covers the endodermal and mesodermal areas.
However, an approximate balance between decreased fitness of the host and active spreading of the parasitic element, combined with a complex population structure could lead to long persistence times without the homing cycle operating, especially when the selective disadvantage to the carrier and the mobility of the element were not the same in different subpopulations.
The active spread of EAV-HP in modern chickens is believed to involve a helper virus due to the numerous point mutations, deletions and insertions in EAV-HP sequences inactivating viral gene products [ 3], and that none of the proviruses found to date have been observed to produce infectious virions [ 2].
The identification of isolates belonging to cluster SMI-049 during several years and the large accumulation of new cases in 1999 indicated an active spread of TB in Sweden.
Tuberculosis (TB) remains a serious public health threat and economic burden in the Russian Federation with escalating rates of multi- and extensive drug resistance (MDR and XDR) against a background of poor infection control and active spread of resistance strains [ 1, 2].
At its northern terminus, the Sumatran Fault Zone links with submarine strike-slip faults that transform into the active spreading centers of the Andaman Sea (Curray et al. 1979), and at its southern terminus, it ends within the large pull-apart basin of the Sunda Strait (Baroux et al. 1998; Bellier and Sebrier 1995; McCaffrey 1991; McCarthy and Elders 1997; Sieh and Natawidjaja 2000).
Alternatively, the low levels of variability could be exclusively a consequence of their fissiparous reproduction, as described in Schmidtea mediterranea[ 14] and other organisms [ 15], and its wide distribution may be a consequence of its active spread over the years.
The very active systemic spreading of the silencing signal through the phloem was first observed in the solanaceous plants, tobacco and tomato [ 78, 79] and later demonstrated also for Arabidopsis [ 80].
Once the mixture is active, spread it evenly on one side of each hilt piece and glue them firmly to the center piece, one on each side.
