Biology Reference
In-Depth Information
change, and (usually) maternal inheritance of mtDNA make it suitable for exam-
ining population history and evolution among closely related taxa (
Lansman
et al. 1981, Gray 1989, Simon et al. 1991, Caterino et al. 2000
), as well as deeper
evolutionary relationships (
Lang et al. 1999, Caterino et al. 2000
). Molecular
studies of mtDNA can be done using RFLPs or sequencing of specific regions of
the mtDNA after cloning or amplification by the PCR (
Satta and Takahata 1990,
Pashley and Ke 1992, White and Densmore 1992
). Gene order can be used as a
phylogenetic tool (
Boore and Brown 1998
).
The lack of recombination in mitochondrial DNA means that fixation of an
advantageous mutation by selection will cause fixation of all other polymor-
phisms by “genetic hitchhiking.” Even the quickly evolving noncoding origin of
replication region may not have neutral-allele frequencies because this region
is linked to the rest of the genome (
Ballard and Kreitman 1995
).
Ballard and
Kreitman (1995)
point out that violation of the neutral-evolution assumption
can have important phylogenetic implications. It violates a major assumption
of one phylogenetic analysis method (UPGMA clustering, described in Section
12.6.7). Also, selection or parasite-induced sweeps can mimic the effects of inter-
population migration or population bottlenecks (
Mooers and Holmes 2000,
Yang and Bielawski 2000
).
Mitochondria have been used as “molecular clocks” to time the divergences
of organisms from each other. However, dating of evolutionary events can be
problematic when nonneutral evolution within species is combined with altered
rates of evolution in the sister taxon (
Ballard and Kreitman 1995
). Furthermore,
in many cases, it appears that mitochondrial molecular clocks tick at different
rates in different lineages and at different times within a lineage.
There are difficulties in working with mtDNA. These include the lack
of recombination, which makes mtDNA essentially a single heritable unit.
Although recombination in mtDNA has not been observed in insects, it has
been found in fungi when
heteroplasmy
(presence of two types of mitochon-
dria) occurs (
Saville et al. 1998
). Lack of recombination potentially produces
gene diversity estimates that have larger standard errors than those deter-
mined using nuclear loci that can recombine. Biparental inheritance of mito-
chondria occurs occasionally in insects, which can complicate population studies
(
Lansman et al. 1983, Kondo et al. 1990, Matsuura et al. 1991, Fontaine et al.
2007
). Introgression of mitochondria between
Drosophila
species has been sug-
gested as an explanation for the presence of mitochondria from a related spe-
cies (
Aubert and Solignac 1990
).
Fontaine et al. (2007)
reviewed cases in which
mitochondria were shown to be paternally inherited. Such “paternal leakage”
appears to occur most often (or be detected most often) when closely related
