Biology Reference
In-Depth Information
abnormal growth. Diploidy requires passing through a single cell each genera-
tion so the soma can begin anew without these mutations. Diploidy also permits
the efficient repair of double-stranded breaks in DNA. However, although
>
20
hypotheses have been proposed as to why sexual reproduction is maintained,
we still do not know why it is maintained by evolution (
Schurko et al. 2008
).
10.3.3 Origin of Sex
The reasons for maintaining sexual reproduction in current populations are
“likely to be quite different from the mechanisms by which sex got started in
the first place” (
Crow 1994
). It is generally accepted that sex was determined
initially by an allelic difference at a gene located on a homologous pair of auto-
somes (
Rice 1994, Lucchesi 1999
). The two sexes thus consisted of individuals
heterozygous or homozygous at this sex-determining locus. The transformation
of autosomes bearing the sex-determining gene into heteromorphic sex chro-
mosomes (such as X and Y) is thought to have occurred by the accumulation of
mutations in the neighborhood of the sex-limited allele. The retention of such
mutations could be facilitated by a reduction in the rate of recombination in the
chromosome of the individuals bearing the sex-limited allele.
Understanding the mechanisms of sex determination in insects provides
insights into the regulation of development of a significant character in eukary-
otes. Such knowledge could provide useful tools for the genetic improvement
of arthropod natural enemies of pest arthropods and weeds, genetic modifica-
tion of pests, or improve the methods by which genetic-control programs are
achieved (
LaChance 1979, Shirk et al. 1988, Stouthamer et al. 1992, Grenier et al.
1998, Heinrich and Scott 2000, Robinson and Franz 2000
). Also see Chapter 14
for a discussion of genetic manipulation of pest and beneficial arthropods.
Schurko et al. (2008)
noted that it is not always easy to determine whether a
species reproduces sexually. An example is the fungus-growing ant
Mycocepurus
smithii
that long has been thought to be asexual (thelytokous) (
Rabeling et al.
2011
). However, when 234 populations of
M. smithii
, from 39 localities rang-
ing in distribution from Mexico to Argentina and some Caribbean islands, were
evaluated using 12 microsatellite markers both thelytokous (89.7%) and sexual
(10.3%) populations were found. Males appeared to be absent from thelytok-
ous populations, but sperm was found in the reproductive tracts of queens in
the sexual populations. Thelytoky was assumed to be occurring if all individu-
als in the population tested shared genotypes, whereas sexuality was assumed if
there was an increase in unique multilocus genotypes, indicating genetic recom-
bination had occurred by sexual reproduction. The genetic uniformity across all
