Biology Reference
In-Depth Information
grow larger than female gonads because they contain more germ cells. The sex-
ual identify of germ cells is determined by both the X:A ratio of the germ cells
and the X:A ratio of the surrounding soma (
Cline and Meyer 1996
). The expres-
sion of
Sxl
+
in the soma is required in the female germ line. Three genes,
ovo
+
,
ovarian tumor
+
, and
sans fille+,
+
, are important for growth and differentiation of
female germ cells. Thus, activation and splicing of
Sex lethal
+
in the ovary is reg-
ulated by a different set of proteins from those in the soma (
Vincent et al. 2001
).
Drosophila
males have fewer abdominal segments than females, which is due
to the sex-specific and segment-specific regulation of the Wingless morphogen.
Wingless
+
expression is suppressed in the developing terminal male abdom-
inal segment by a combination of the Hox protein Abdominal-B and the sex-
determination regulator
doublesex
+
, which results in cell death and suppression
of cell proliferation of the terminal segment (
Wang et al. 2011
).
10.6 Are Sex-Determination Mechanisms Diverse?
Sex-determination mechanisms in insects appear to be diverse (
White 1973
,
Lauge 1985
,
Retnakaran and Percy 1985
,
Wrensch and Ebbert 1993, Werren
and Beukeboom 1998
). Many insects have a
genetic sex-determination
system,
with genetic differences determining maleness or femaleness. Others appear to
have an
environmental sex-determination
system, in which there are no genetic
differences between males and females but temperature or host conditions
determine the sex. For example, in a few insects the hemolymph of the mother
determines the sex of the offspring.
Ploidy levels sometimes are important in sex determination: both sexes of
many arthropods are diploid (2n, diplo-diploidy), whereas others have haploid
males and diploid females (n and 2n, haplo-diploidy or
arrhenotoky
). Haplo-
diploidy has apparently developed at least 10 times independently in insects
(
Normark 2004
). Other species consist primarily of diploid females (
thelytoky
)
and haploid males rarely are produced. In some species, haploid males are pro-
duced by the loss of paternally derived chromosomes after fertilization (known
as
parahaploidy
or
pseudoarrhenotoky
or
male gamete loss
).
Males in apterygote and many pterygote insects are
heterogametic
(males are
XO, XY, XXO, XXY, or XYY and females are XX), but in some higher pterygotes
(Trichoptera, Lepidoptera) females may be the
heterogametic
(ZW) sex.
Mank
(2009)
noted that silk moths, in which females are heterogametic (ZW), lack a
global dosage-compensating mechanism for the Z chromosome, equalizing tran-
scription for only some genes. It appears that other lepidopteran species also do
not have globally dosage-compensated sex chromosomes (
Mank 2009
).
