Biology Reference
In-Depth Information
composed primarily of heterochromatin. The X chromosome is usually more
like an autosome in function and appearance. However, because it exists in one
copy in the heterogametic sex (XY or XO), some form of dosage compensation is
required to equalize the amount of gene product in the two sexes (for a discus-
sion of dosage compensation, see Chapter 10).
The female is usually the
homogametic sex
, with two X chromosomes, and
thus produces only eggs containing an X chromosome. In some insects, such as
the Lepidoptera, females are the heterogametic sex. In this case, the sex chro-
mosomes often are designated as W and Z, with the W chromosome analogous
to the Y chromosome of the male (
White 1973, Wagner et al. 1993
).
3.18 Extranuclear Inheritance in Mitochondrial Genes
Genes located in the nucleus show Mendelian inheritance because they segre-
gate in a regular manner during meiosis. However, not all genes in eukaryotic
organisms are located in the nucleus.
Mitochondria
are inherited cytoplasmi-
cally, and they primarily are transmitted through the maternal gametes.
Mitochondria are self-replicating organelles that occur in the cytoplasm of all
eukaryotes. Mitochondria are considered to be the descendants of an aerobic
eubacterium that became an endosymbiont within an early anaerobic cell that
may or may not have contained a nucleus (
Kobayashi 1998
). Mitochondria are
thought to have originated once (
Gray et al. 1999
). Based on studies of DNA
sequences, members of the rickettsial subdivision of the
α
-Proteobacteria, a
group of obligate intracellular parasites that include the
Rickettsia
,
Anaplasma
,
and
Ehrlichia
, are the closest known relatives of mitochondria (
Gray et al. 1999
).
Mitochondria have evolved and the mitochondrial genomes of insects have
departed radically from the ancestral pattern (
Gray et al. 1999
). Mitochondrial
genomes range in size from 6kb to
>
2 Mb (
Sogin 1997
). Genome size has
been reduced after endosymbiosis because some mitochondrial genes became
expendable in the internal environment of the host cell (
Blanchard and Lynch
2000
). Some nuclear genes have replaced the function of the mitochondrial
(mt) genes, but much of the reduction in mitochondrial genome size occurred
through the
transfer
of mitochondrial protein-coding genes into the nuclear
genome. As a result, the mitochondrion has an incomplete set of genes for its
own function (
Blanchard and Lynch 2000
).
The movement of mitochondrial genes into the nuclear genome is an exam-
ple of
horizontal gene transfer
. The result of such horizontal transfer means
that genes originally located in the mitochondria, but now in the nucleus,
