DIPLOMO: the tool for a new type of evolutionary analysis

Assessment of codivergence of mastreviruses with their plant hosts

BACKGROUND

Viruses that have spent most of their evolutionary time associated with a single host lineage should have sequences that reflect codivergence of virus and host. Several examples for RNA viruses of host-virus tree congruence are being challenged. DNA viruses, such as mastreviruses, are more likely than RNA viruses to have maintained a record of host lineage association.

RESULTS

The full genomes of 28 isolates of Wheat dwarf virus (WDV), a member of the Mastrevirus genus, from different regions of China were sequenced. The analysis of these 28 entire genomes and 18 entire genome sequences of cereal mastreviruses from other countries support the designation of wheat, barley and oat mastrevirus isolates as separate species. They revealed that relative divergence times for the viruses WDV, Barley dwarf virus (BDV), Oat dwarf virus (ODV) and Maize streak virus (MSV) are proportional to divergence times of their hosts, suggesting codivergence. Considerable diversity among Chinese isolates was found and was concentrated in hot spots in the Rep A, SIR, LIR, and intron regions in WDV genomes. Two probable recombination events were detected in Chinese WDV isolates. Analysis including further Mastrevirus genomes concentrated on coding regions to avoid difficulties due to recombination and hyperdiversity. The analysis demonstrated congruence of trees in two branches of the genus, but not in the third. Assuming codivergence, an evolutionary rate of 10-8 substitutions per site per year was calculated. The low rate implies stronger constraints against change than are obtained by other methods of estimating the rate.

CONCLUSION

We report tests of the hypothesis that mastreviruses have codiverged with their monocotyledonous hosts over 50 million years of evolution. The tests support the hypothesis for WDV, BDV and ODV, but not for MSV and other African streak viruses.

PATRISTIC: a program for calculating patristic distances and graphically comparing the components of genetic change

BACKGROUND

Phylogenies are commonly used to analyse the differences between genes, genomes and species. Patristic distances calculated from tree branch lengths describe the amount of genetic change represented by a tree and are commonly compared with other measures of mutation to investigate the substitutional processes or the goodness of fit of a tree to the raw data. Up until now no universal tool has been available for calculating patristic distances and correlating them with other genetic distance measures.

RESULTS

PATRISTICv1.0 is a java program that calculates patristic distances from large trees in a range of file formats and allows graphical and statistical interpretation of distance matrices calculated by other programs.

CONCLUSION

The software overcomes some logistic barriers to analysing signals in sequences. In additional to calculating patristic distances, it provides plots for any combination of matrices, calculates commonly used statistics, allows data such as isolation dates to be entered and reorders matrices with matching species or gene labels. It will be used to analyse rates of mutation and substitutional saturation and the evolution of viruses. It is available at http://biojanus.anu.edu.au/programs/ and requires the Java runtime environment.

Genetic differentiation in the winter pine processionary moth (Thaumetopoea pityocampa--wilkinsoni complex), inferred by AFLP and mitochondrial DNA markers

The winter pine processionary moth has become an important pine pest in the last century, as a consequence of the spread of pine cultivation in the Mediterranean region. The pattern of genetic differentiation of this group, that includes two sibling species (Thaumetopoea pityocampa and Th. wilkinsoni), has been studied in nine populations using amplified fragment length polymorphism (AFLP) and single strand conformation polymorphism-sequence analysis (SSCP) of the mitochondrial cytochrome oxidase 1 (COI) and cytochrome oxydase 2 (COII). Results indicate the existence of strong genetic differentiation between the two species that became separated before the Quaternary ice ages. Moreover data indicate that Th. pityocampa has a strong geographical structure, particularly evident at the nuclear level, where all pairwise phiST resulted to be highly significant and individuals from the same population resulted to be strongly clustered when an individual tree was reconstructed. The estimates of the absolute number of migrants between populations (Nm), obtained from mitochondrial and nuclear DNA markers, suggest that gene flow is low and that a gender-related dispersal could occur in this species. The males appear to disperse more than females, contributing to the genetic diversity of populations on a relatively wide range, reducing the risks of inbreeding and the genetic loss associated with bottlenecks occurring in isolated populations.

The haemagglutinin gene, but not the neuraminidase gene, of 'Spanish flu' was a recombinant

Published analyses of the sequences of three genes from the 1918 Spanish influenza virus have cast doubt on the theory that it came from birds immediately before the pandemic. They showed that the virus was of the H1N1 subtype lineage but more closely related to mammal-infecting strains than any known bird-infecting strain. They provided no evidence that the virus originated by gene reassortment nor that the virus was the direct ancestor of the two lineages of H1N1 viruses currently found in mammals; one that mostly infects human beings, the other pigs. The unusual virulence of the virus and why it produced a pandemic have remained unsolved. We have reanalysed the sequences of the three 1918 genes and found conflicting patterns of relatedness in all three. Various tests showed that the patterns in its haemagglutinin (HA) gene were produced by true recombination between two different parental HA H1 subtype genes, but that the conflicting patterns in its neuraminidase and non-structural-nuclear export proteins genes resulted from selection. The recombination event that produced the 1918 HA gene probably coincided with the start of the pandemic, and may have triggered it.

Evolution and origins of tobamoviruses

More than a dozen tobamoviruses are known. In nature, each species probably survives by moving between several closely related host species. Each infected plant contains a population of variants, but in most host populations the tobamovirus population is stable. The phylogenetic relationships of tobamovirus species broadly correlate with those of their angiosperm hosts. The simplest explanation for this correlation is that they have coevolved with the angiosperms, and hence, like them, are about 120-140 million years old. Gene sequence differences between species also indicate that the tobamoviruses are an ancient genus. Their gene sequences, and the protein motifs they encode, link them to tobraviruses, hordeiviruses and soil-borne wheat mosaic virus, more distantly to the tricornaviruses, and even to hepatitis virus E and other furoviruses, rubiviruses and alphaviruses. Their progenitors may have been associated with charophycean algae, and perhaps also plasmodiophoromycete fungi.