Biology Reference
In-Depth Information
an enlarged A
+
T-enriched region (9-13 kb) and a series of 0.8-2.0 kb tandemly
repeated sequences adjacent to the A
+
T region. Every weevil sampled in all
three species had two to five distinct size classes of mtDNA (exhibited hetero-
plasmy). The magnitude of the size differences, the number of size classes found
within individual weevils, and the abundant mtDNA heteroplasmy is unusual
(
Boyce et al. 1989
).
The dogma that mtDNA is exclusively inherited in a maternal manner has
been questioned in
Drosophila
and marine mussels. Incomplete maternal inheri-
tance of mtDNA occurs in
Drosophila simulans
(
Satta et al. 1988, Matsuura et al.
1991
), and the high level of heteroplasmy found in the three
Pissodes
species
could be due to paternal transmission of mtDNA.
Mitochondrial chromosomes are circular, supercoiled, double-stranded DNA
molecules. The mitochondrial chromosome of
D. yakuba
contains
≈
18.5 kb of
DNA, and each mitochondrion contains multiple copies of the chromosome
(
Figure 3.6
). Mitochondrial genes in insects lack introns and intergenic regions
usually are small or absent. The ribosomes found in the mitochondria are smaller
than the ribosomes in the cytoplasm.
Most eggs and somatic cells contain hundreds or thousands of mtDNA mol-
ecules, so a new mutation can result in a situation in which two or more mtDNA
genotypes coexist within an individual (heteroplasmy). Heteroplasmy, however,
is apparently a transitory state in germ cells. Thus, the majority of individuals are
effectively haploid with regard to the number of types of mtDNA transmitted to
the next generation.
Mitochondrial DNA evolves faster than single-copy nuclear DNA because
mitochondria are relatively inefficient in repairing errors during DNA replica-
tion or after DNA damage. For example, in Hawaiian
Drosophila
, mtDNA seems
to evolve
≈
3 times faster than the genes of nuclear DNA (
Moritz et al. 1987
).
Because mtDNA does not code for proteins involved directly in its own replica-
tion, transcription, or translation, mtDNA has a large number of length muta-
tions and transitions.
Mitochondrial DNA can be amplified easily from mitochondria by the poly-
merase chain reaction (PCR) (see Chapter 8 for a description of the PCR) because
there are multiple copies in each cell. Mitochondrial DNA is easier to purify from
cells than a specific segment of nuclear DNA. This is due to its buoyant den-
sity, high copy number within cells, and its location within an organelle, mak-
ing mtDNA a useful subject for systematics or population genetics studies, as is
described in Chapters 12 and 13.
