Biology Reference
In-Depth Information
5.3. Origin of protist globins and their relationship to higher
eukaryote globins
Although there is no broad consensus on eukaryote origin, due to substantial
disagreement between the interpretations based on the fossil record and the
calculations based on molecular clock estimates, a recent estimate based on
multigene phylogenies (
Parfrey, Lahr, Knoll, & Katz, 2011
) suggested that
last eukaryote common ancestor (LECA) existed between 1679 and
1866 Ma. Furthermore, the major eukaryote lineages, such as the Amoe-
bozoa, Excavata, SAR and Opisthokonta, are thought to have diverged dur-
ing the time period 1000-1600 Ma. The lengthy evolution of unicellular
eukaryotes is undoubtedly responsible for the diversity in their globin
sequences, resulting in a formidable obstacle to the untangling of
globin phylogeny.
There is general agreement that the two major events in microbial
eukaryote evolution were the endosymbioses of an alpha-proteobacterium
and a cyanobacterium (
Andersson, Karlberg, Canback, & Kurland, 2003;
Gray, 1999; Gray, Lang, & Burger, 2004
), estimated to have occurred at
2.3-1.8 billion years ago and 1.5-1.6 billion years ago, respectively
(
Hedges, Blair, Venturi, & Shoe, 2004
), resulting in the emergence of mito-
chondria and chloroplasts, respectively. In contrast to the evolution of the
mitochondria, that of plastids turned out to be extremely complex, leading
to extensive diversification involving secondary and tertiary endosymbiotic
events, and including plastid loss and replacement within the emerging
microbial eukaryote lineages. It is now known that the primary plastids sur-
vived in green plants, red and green algae and glaucophytes (
Rodriguez-
Ezpeleta et al., 2005
).
Figure 9.3
shows a diagrammatic representation of
the many steps in plastid diversification that have been elucidated
(
Keeling, 2010
). We have proposed that the two foregoing endosymbiotic
events are also responsible for the transfer of bacterial globin genes to the
early eukaryote ancestor (
Vinogradov et al., 2007
). The extant cyanobacteria
have only SDgbs and TrHb1s, while the alpha-proteobacteria contain
SDgbs and FHbs, members of the three T subfamilies, and several GCSs
(
Vinogradov et al., 2013
). It is likely that all the TrHb1s in microbial eukary-
otes and some of the SDgbs are derived from the ancient endosymbiosis with
a cyanobacterium. The complexity of plastid evolution hints at the likely
complexity of microbial eukaryote globin phylogeny whose unravelling will
probably require the elucidation of the origins of the numerous microbial
eukaryote lineages. However, whilst
the elucidation of
the relations
