From: Iyer, Lakshminarayan (NIH/NLM/NCBI) [E]
Sent: Friday, December 16, 2005 10:08 AM
To: NLM/NCBI List ncbi-seminar
Subject: Seminar Tuesday Dec 20, 11:00 am

Tuesday, December 20, 2005,
Bdlg. 38a, B2 Conference Room
11 AM

Lessons on virus evolution from comparative genomics of
Nucleo-cytoplasmic large DNA viruses 

Lakshminarayan M. Iyer
NCBI, NLM, NIH

The origin of large double stranded DNA viruses is a subject of intense
speculation. In general, there are two broad theories: 1) Large double
stranded DNA viruses evolved in the earliest phases of life's evolution
and were the primitive precursors of cellular systems.  2) Large double
stranded DNA viruses were secondary derivatives of cellular systems that
underwent drastic degeneration as a consequence of extreme parasitism. I
will describe our recent studies on the Nucleo-cytoplasmic large DNA
viruses (NCLDV) that throws light on these issues. The NCLDV are a
monophyletic lineage that include diverse large DNA viruses such as the
poxviruses, African swine fever virus, Iridoviruses, Phycodnaviruses and
the giant Mimivirus.  Previously, using comparative sequence analyses,
we identified a common core of genes in the NCLDV that predicted a
fairly complex ancestral virus encoding components for replication,
transcription, capping and polyadenylation, transcription regulation and
virus packaging. I will present several new advancements that we have
made since then. In particular, analyses of the primases and the
segregating ATPases throw light on the origins of the replication and
packaging components of the NCLDV. Additionally, the availability of
diverse new genomes clarifies the phylogenetic inter-relationships
within the NCLDV and suggests that the divergence of the major NCLDV
families occurred at an early stage of evolution, prior to the
divergence of the main eukaryotic lineages. I will present data that
provides evidence for the subsequent evolution of the NCLDV genomes by
expansion of paralogous gene families and acquisition of numerous genes
via horizontal gene transfer from the eukaryotic hosts, other viruses,
and bacteria. Interestingly a significant number of genes are also
shared between the NCLDV and various bacteriophages, although a vertical
relationship between the NCLDV and a particular bacteriophage cannot be
established. On the basis of these observations, I will present two
alternative scenarios for the origin of the NCLDV and other groups of
large DNA viruses. One of these scenarios posits an early assembly of a
large DNA virus precursor from which various large DNA viruses diverged
through an ongoing process of xenologous and non-orthologous
displacements and occasional acquisition of novel genes from various
sources, whereas the second scenario posits convergent emergence on
multiple occasions of large DNA viruses from a small precursor through
repeated independent accretion of similar sets of genes. 


Lakshminarayan Iyer
NCBI, NLM, NIH
Bethesda, MD-20894
Work tel no. 301-594-7090

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