Biology Reference
In-Depth Information
2000
). The following discussion describes some of the attributes of these tar-
gets, which are relevant because specific assumptions may be essential for an
appropriate phylogenetic-analysis method. More recently, microRNAs have been
studied, especially for the analysis of deep evolutionary divergences.
12.5.1 Mitochondria
Mitochondria are the cell's respiratory power plant for the generation of
ATP. Mitochondria are thought to have developed more than a billion years
ago when a free-living eubacterium took up residence within another cell
(
Margulis 1970
). Sequence analysis of modern mitochondrial DNA suggests that
α
-Proteobacteria, such as
Rickettsia
,
Anaplasma
and
Ehrlichia
, are the closest
contemporary relatives of that eubacterium (
Gray et al. 1999, Lang et al. 1999
).
The relatively low gene content of mtDNA, compared with even the small-
est eubacterial genome, indicates that loss or transfer of genetic information
occurred at an early stage in the evolution of the “protomitochondrial genome”
(
Gray et al. 1999
). What is not clear is whether mitochondria originated from
a single endosymbiotic event or more than one event (
Lang et al. 1999
). Over
the past billion years, mitochondria in Eukaryotes have evolved to the point that
genome size and gene content vary among the major groups (
Lang et al. 1999
).
Mutation rates in mtDNA are variable, with mammalian mtDNA having a
mutation rate at least 50 times greater than in plants (
Moritz et al. 1987, Lang
et al. 1999
). The number and type of genes present in mitochondria differ, with
perhaps seven to 10 independent losses having occurred for each gene. The
evolution of mitochondrial genes over the past billion years has been complex;
some genes apparently were transferred independently several times into the
nuclear genome; some genes were lost without transfer to the nuclear genome
due to gene substitution; and genes were acquired by lateral gene transfer as
well (
Gray et al. 1999
). There is no evidence that, once mitochondrial genes
were transferred to the nucleus, they were regained by the mitochondria and
there is no evidence for widespread and substantial lateral transfer of genetic
information into or between mitochondria (
Lang et al. 1999
). The predomi-
nantly vertical inheritance of genes is a prerequisite for phylogenetic analyses.
Within mitochondrial genomes, there are regions that diverge rapidly, whereas
other regions are highly conserved, making the different regions suitable for
analysis of different taxonomic levels (
Simon et al. 1991, Liu and Beckenbach
1992, Tamura 1992, Caterino et al. 2000
).
Animal mitochondria are small, circular (16-20 kb in length), and lack introns,
with the genes compactly arranged on both DNA strands. With a few exceptions,
animal mtDNA contains the same 37 genes coding for small and large subunit
