Biology Reference
In-Depth Information
Sex-determining chromosomes (Y or W) differ from autosomes in that there
is no genetic recombination, they are male- or female-specific, and usually con-
tain a large amount of heterochromatic DNA and few functional genes (
Kaiser
and Bachtrog 2010
). X chromosomes are female-biased and Z chromosomes are
male-biased in their mode of inheritance and are hemizygous in the heteroga-
metic sex.
Several different models have been proposed to explain sex determination
in the haplo-diploid Hymenoptera (
Cook 1993, Beukeboom 1995, Dobson and
Tanouye 1998a, Heimpel and de Boer 2008
). In the honey bee and the para-
sitoid
Bracon
(
=
Habrobracon
)
hebetor
, sex is determined by a series of alleles
at a single locus (
single-locus, multiple-allele model
) (
Whiting 1943
). In honey
bees, the single locus (probably located on chromosome 8) has several alleles (19
so far). Individuals that are heterozygous for this locus are normal fertile (dip-
loid) honey bee females, hemizygotes (unfertilized haploid eggs) became fertile
drones (males), and homozygotes are sterile diploid males with degenerated
testes containing reduced quantities of diploid sperm (
Beye et al. 1996, 1999
).
Usually, worker bees eat any diploid honey bee males.
Under the single-locus, multiple-allele model, inbreeding should produce
homozygous (diploid) males in the parasitoid
Bracon hebetor
. However,
Heimpel
et al. (1999)
evaluated the diversity of sex-determining alleles in five U.S. popu-
lations and estimated that there were at least 20 alleles. This high allelic diver-
sity suggests that the production of diploid males should be rare unless extreme
inbreeding occurs.
In the parasitoid
Nasonia vitripennis
, which is arrhenotokous, no comple-
mentary sex-determination locus was found (
Beukeboom et al. 2007
). The
doublesex
+
gene is homologous with
dsx
+
from other insects, including
Apis
,
and differential splicing occurs in males and females, indicating it is involved
in somatic sex differentiation (
Oliveira et al. 2009
). However, maternal mRNA,
in combination with zygotic transcription is essential for female development.
Thus, males develop as a result of
maternal imprinting
that
prevents
zygotic
transcription of the
transformer
+
gene in unfertilized eggs. However, if fertil-
ized, transcription of
tra
+
is initiated, which autoregulates the female-specific
doublesex
+
transcript, leading to female development (
Verhulst et al. 2010a
)
(
Table 10.2
).
The haplo-diploid turnip sawfly,
Athalia rosae ruficornis,
, when inbred, pro-
duces both diploid and triploid males, suggesting that sex in this hymenopteran
is determined by the single-locus, multiple-allele system (
Figure 10.3
,
Naito
and Suzuki 1991
). This sex-determination system also has been found in the
