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to chimerism) among early organisms may have occurred when primitive eukary-
otes picked up genes from their food, and he suggested that “You are what you
eat.” According to this scenario, the ancestor of the Eukaryota was archaeal, but
many eukaryotic nuclear genes today are of eubacterial origin because horizon-
tal transfer occurred. Some eubacterial genes could have moved horizontally
from mitochondria, but this is unlikely because the nuclear genes of mitochon-
drial origin are few and limited to proteins that are reimported into the mito-
chondria.
Doolittle (1998)
speculates that eubacterial genes with other functions
could have moved into the eukaryotic genome when phagocytic unicellular
eukaryotes fed on an
α
-proteobacterium. DNA from these food bacteria would
have moved repeatedly into the nuclear genome.
Doolittle (1998)
argued, “all
genes that can be replaced by food-derived [eubacterial] genes will be, in the
fullness of time. We should not think of such gene replacement as idiosyncratic
or exceptional, but as the normal course. It is, instead, the persistence of some
genes of archaeal ancestry that requires special explanation.”
Forterre (2011)
suggests another origin of the Eukaryota, in which an
archaean was engulfed by a eubacterium followed by invasions of viruses,
which provided many proteins.
Poole and Neumann (2011)
suggest an archaean
engulfed a mitochondrial ancestor.
Cotton and McInerney (2010)
report that
archaebacterial-derived genes are more essential to yeast viability, more highly
expressed and more connected and central to the yeast protein-interaction net-
work, indicating that genes of archaebacterial origin are more important to
yeast metabolism than genes of eubacterial origin and speculated that archae-
bacterial genes originated in the ancestral nuclear component of the eukaryotic
genome. Clearly, the debate continues as to the origin of the Eukaryota due to
the difficulty of analyzing such deep evolutionary events.
A consequence of lateral (horizontal) gene transfer is that phylogenetic analyses
of different genes can result in conflicting phylogenies, causing confusion (
Katz
1998, Bushman 2002
). Evidence from sequences of 66 protein-coding genes from
members of the three domains suggests that some eukaryotic genes are more sim-
ilar to archaeal genes, whereas others appear to share ancestry with eubacterial
genes (
Brown and Doolittle 1997, Katz 1998
). Archaebacteria and the Eukaryota
share genes involved in the genetic machinery of the cell, whereas Eubacteria and
the Eukaryota share genes that regulate metabolic processes. These analyses “chal-
lenge the traditional view that vertical transmission of genetic material from one
generation to the next is the predominant force in evolution” (
Katz 1998
).
The origin of the Eukaryota continues to be studied and debated (
Kelly et al.
2011
). Key characters involved in the emergence of eukaryotes include the
