Biology Reference
In-Depth Information
loci within colonies suggests mitotic parthenogenesis (
apomixis
or reproduc-
tion without meiosis) is occurring. Four populations from the Amazonian area
of South America appear to reproduce both sexually and asexually because the
progeny were a mixture of recombinant and clonal genotypes. Spermathecae
of four queens were dissected and haploid sperm were present, confirming sex-
ual reproduction was occurring, although no males were found. Thus, this spe-
cies appears to be a mosaic of asexual and sexually reproducing populations,
with sex being lost repeatedly in different lineages. The sexual populations are
common in the center of this species' geographic distribution where there are
high local population densities. The asexual lineages have “rapidly dispersed
throughout much of Latin America, leading to the current widespread geo-
graphic distribution.” This ant has the most extensive geographic distribution
of any fungus-growing species, indicating that both sexual and asexual popula-
tions have sufficient fitness to persist. The authors note that it is possible that
the loss of a single recessive allele can cause thelytoky and suggest that the high
propensity for switching from sexual to asexual reproduction in
M. smithii
may
be due to a small number of genes. Alternatively, it could be due to partheno-
genesis-inducing symbionts.
10.4 Sex Determination Involves Soma and Germ-Line Tissues
Sex determination involves both the soma and germ-line tissues (ovaries and
testes). Sexual dimorphism in adult insects is often extreme, with differences in
setal patterns, pigmentation, external genitalia, internal reproductive systems
and behavioral patterns (
Greenspan and Ferveur 2000, Kopp et al. 2000
).
How do sexually determined differences in the soma and germ line arise? The
details are pretty clear for
D. melanogaster
, and some information is available
for other economically important insects such as fruit flies, moths, honey bees,
and mosquitoes.
First, let's review the basic sex-determination system in
D. melanogaster
.
Then, sex determination in other insects is described. Finally, examples are pro-
vided that illustrate the importance of extrachromosomal and microbial genes
in modifying sex in many arthropods.
10.5 Sex Determination in
Drosophila melanogaster
Developing an understanding of sex determination in
D. melanogaster
has relied
on identifying a relatively few spontaneous mutants (
Table 10.1
), which indi-
cated that the number of genes involved is relatively low (
Belote et al. 1985, Slee
and Bownes 1990, Cline and Meyer 1996
). Sex determination in
D. melanogaster
