Biology Reference
In-Depth Information
In the phorid fly
Megaselia scalaris
, the sex-determining linkage group is not
fixed. Different chromosomes serve as the sex-determining pair in different
populations (
Traut 1994
).
Traut and Willhoeft (1990)
estimate that the male-
determining factor moved to a different linkage group, thereby creating new
Y chromosomes with a frequency of at least 0.06%, which is consistent with the
hypothesis that the sex-determining factor is moving by transposition. An alter-
native explanation is that mutations at multiple sex loci in the genome result in
males; however, the high rates of change (0.06%) are higher than expected if
due to mutation. Analysis of the sex-determination cascade in
M. scalaris
indi-
cates that
doublesex
+
is highly conserved when compared with
dsx
+
in
D. mela-
nogaster
(
Kuhn et al. 2000
), but
Sex-lethal
+
is not functionally conserved in
M. scalaris
(
Sievert et al. 2000
). Analyses of other insects also suggest that the
base of the sex-determination cascade is more highly conserved in function than
the upper level of the cascade (
Figure 10.2
).
In the blowfly
Chrysomya rufifacies
(Calliphoridae), females produce
either
female progeny only (
thelygenic
females)
or
male progeny only (
arrhenogenic
females) (
Clausen and Ullerich 1990
). Thelygenic females are heterozygous for
a dominant female-determining maternal-effect gene (
F'
), whereas arrheno-
genic females and males are homozygous for the recessive allele (
f
). This spe-
cies lacks differentiated sex chromosomes. DNA sequence homology between
the
D. melanogaster da
+
gene and a polytene band in the sex chromosomes of
C. rufifacies
was observed by
in situ
hybridization, suggesting that
F
in
C. rufi-
facies
and
da
+
in
D. melanogaster
are equivalent (
Clausen and Ullerich 1990
).
Muller-Holtkamp (1995)
found that the
Sex-lethal
+
gene homologue in
C. rufifa-
cies
is highly conserved in sequence and exon-intron organization.
10.6.2 Environmental Effects
Environmental conditions can influence sex determination in some arthro-
pods. Many haplo-diploid insects adjust the sex ratio of their progeny based on
environmental factors. For example, females of species in the genus
Encarsia
(Hymenoptera: Aphelinidae) develop as
autoparasitoids
of whiteflies (which
are considered the primary hosts). Males of the same
Encarsia
species develop
as parasitoids of
Encarsia
female pupae, which are considered the secondary
hosts. Virgin females deposit unfertilized eggs to produce haploid sons on sec-
ondary hosts (females of their own species), but typically do not oviposit in pri-
mary hosts (whiteflies), even if they are the only hosts available. When a virgin
female does deposit haploid male eggs in a primary host (whiteflies), these eggs
usually do not develop, for unknown reasons. An unusual population of
E. per-
gandiella
was found in which males could develop on the primary whitefly host.
It appears that these haploid males started out as fertilized diploid eggs, but
