Experimental evidence indicating that mastreviruses probably did not co-diverge with their hosts.
BACKGROUND
Despite the demonstration that geminiviruses, like many other single stranded DNA viruses, are evolving at rates similar to those of RNA viruses, a recent study has suggested that grass-infecting species in the genus Mastrevirus may have co-diverged with their hosts over millions of years. This "co-divergence hypothesis" requires that long-term mastrevirus substitution rates be at least 100,000-fold lower than their basal mutation rates and 10,000-fold lower than their observable short-term substitution rates. The credibility of this hypothesis, therefore, hinges on the testable claim that negative selection during mastrevirus evolution is so potent that it effectively purges 99.999% of all mutations that occur.


RESULTS
We have conducted long-term evolution experiments lasting between 6 and 32 years, where we have determined substitution rates of between 2 and 3 x 10(-4) substitutions/site/year for the mastreviruses Maize streak virus (MSV) and Sugarcane streak Réunion virus (SSRV). We further show that mutation biases are similar for different geminivirus genera, suggesting that mutational processes that drive high basal mutation rates are conserved across the family. Rather than displaying signs of extremely severe negative selection as implied by the co-divergence hypothesis, our evolution experiments indicate that MSV and SSRV are predominantly evolving under neutral genetic drift.


CONCLUSIONS
The absence of strong negative selection signals within our evolution experiments and the uniformly high geminivirus substitution rates that we and others have reported suggest that mastreviruses cannot have co-diverged with their hosts.
Harkins
Gordon W
GW
South African National Bioinformatics Institute, University of the Western Cape, Cape Town, South Africa. gordon@sanbi.ac.za
Delport
Wayne
W
Duffy
Siobain
S
Wood
Natasha
N
Monjane
Adérito L
AL
Owor
Betty E
BE
Donaldson
Lara
L
Saumtally
Salem
S
Triton
Guy
G
Briddon
Rob W
RW
Shepherd
Dionne N
DN
Rybicki
Edward P
EP
Martin
Darren P
DP
Varsani
Arvind
A
D004279:DNA, Viral / Q000235:genetics*; D019143:Evolution, Molecular; D017791:Geminiviridae / Q000235:genetics* / Q000302:isolation & purification; D010802:Phylogeny; D017354:Point Mutation; D031786:Saccharum / Q000821:virology; D012641:Selection, Genetic; D012689:Sequence Homology, Nucleic Acid; D003313:Zea mays / Q000821:virology
D004279:DNA, Viral