Biology Reference
In-Depth Information
10.14 Sex and the Sorted Insects
451
10.14.1
Genetic Control
451
10.14.2
Genetic Improvement of Parasitoids
453
10.15 Conclusion 454
References Cited
454
10.1 Overview
Resolving the molecular genetic basis of sex determination in arthropods has
applied applications that potentially could result in improved genetic-control
programs for pest insects or in useful genetic modifications of beneficial spe-
cies. Sex determination has been studied extensively in
Drosophila melanogas-
ter
. Sex determination in
D. melanogaster
has three components: 1) dosage
compensation, 2) somatic-sexual development, and 3) germ-line sexual develop-
ment. The primary cue for determining sex in
D. melanogaster
is the number of
X chromosomes relative to autosomes (A) in a cell (X:A ratio). This ratio deter-
mines somatic sex, germ-line sex, and dosage compensation by regulating the
functions of sets of regulatory genes. One model suggested that sex determina-
tion in all insects is based on modifications of the
Drosophila
scheme in which
a hierarchy of key regulatory genes control sexual development. At the top of
the regulatory cascade is
Sex lethal
+
(
Sxl
+
), which must be ON to determine the
female pathway. If
Sxl
+
is OFF, the male pathway is the “default” developmental
process. Sex subsequently is determined by the differential splicing of messenger
RNAs. At the end of the pathway, a DNA-binding regulatory protein coded for
by the
doublesex
+
gene influences sex determination. Although sex determina-
tion in arthropods is clearly determined by chromosomal and genetic processes,
environment also plays a role in some. Furthermore microorganisms, including
Wolbachia
,
Rickettsia
,
Cardinium
,
Spiroplasma
, and viruses, are able to modify
sex determination or sex ratio in many arthropods.
10.2 Introduction
Sexual reproduction results in genetic variability through crossing over, as well
as variation due to Mendel's laws of Independent Assortment and Independent
Segregation. As a result of meiosis, genes obtained from different parents
can be combined in a single descendant (review the discussion of meiosis in
Chapter 3). New genotypes thus are constructed from preexisting variability by
the mechanisms of
segregation
,
assortment
, and
recombination
during meio-
sis. Homologous chromosomes separate (segregate) randomly to yield haploid
gametes containing chromosomes derived from both the individual's maternal
and paternal genomes. Recombination occurs during crossing over between
