Biology Reference
In-Depth Information
and some have only females (thelytoky). In some insect species, diploid males
may undergo chromosome heterochromatinization and loss during develop-
ment to become haploid (parahaploidy). This diversity of genome organization
in insects reflects their long evolutionary history.
An understanding of the stages of embryonic development and many of the
major genes that influence these stages in
Drosophila melanogaster
is emerg-
ing. A field of study called “evo-devo” compares development among organisms
and is based on molecular analyses of development. Embryonic development
in
D. melanogaster
is determined first by maternal genes and then by zygotic
genes. Maternal-effect genes determine the polarity of the embryo. Next, seg-
mentation genes influence the development of major bands or parasegments.
Another determination is accomplished by the interaction of homeotic genes
that provide a finer definition of segmental structures.
4.2 Introduction
Insects are both extremely numerous and diverse. Insects comprise approximately
half of all described eukaryotic species and approximately ¾ of all described ani-
mals. There are
≈
883,475 described insect species in
>
762 families organized in
32 orders (
Daly et al. 1998
,
Whitfield and Purcell 2013
). Many insect species may
remain undescribed because some estimates suggest there are as many as 2 mil-
lion insect species. Insects have a long evolutionary history, live in a great vari-
ety of habitats, exhibit diverse types of life styles, have an extraordinary range of
structural variations, eat an astonishing variety of food, and are among the most
abundant animals on earth. The long evolutionary history of insects has provided
sufficient time for them to develop a diversity of genetic systems (for an overview
of insect evolution, see Chapter 12).
This chapter provides only a small sample of the diversity of insect genetic
systems. More extensive reviews of insect cytogenetics can be found in
White
(1973) and Wagner et al. (1993)
. This chapter also provides an overview of the
diversity of microbial symbionts associated with insects, the diversity of their
transposable elements, gene regulation in some insect-specific genes, as well as
a brief introduction to the molecular genetics of insect embryonic development,
all topics that are important in understanding the mechanism of transforming
insects by injection of cloned genes into embryos (see Chapter 9).
4.3 Genetic Systems in Insects
Most insects are diploid (2n) in their somatic cells and haploid (n) in their gam-
etes. Other systems can be found; some insect groups are parthenogenetic and
