Biology Reference
In-Depth Information
induced thelytoky, or facultative thelytoky indicates that some capacity for par-
thenogenesis is probably present in all eggs. Thelytokous species or thelytokous
populations of bisexual species have been found in the Diptera, Hymenoptera,
Lepidoptera, Orthoptera, and Coleoptera.
In the Hemiptera, both arrhenotoky and thelytoky occur (
Retnakaran and
Percy 1985
), but even more complex genetic systems can be found (
White 1973,
Haig 1993
). For example, in some mealybugs (Pseudococcidae), both males and
females develop from fertilized eggs but, in the embryos that develop into
males, the paternally derived chromosomes become heterochromatic, geneti-
cally inactive, and are not transmitted to the male progeny. This genetic system
has been called
parahaploidy
(
Brown and Nur 1964, Nur 1990
).
Chromosome
imprinting
is probably involved to ensure that the paternally derived chromo-
somes are eliminated and not the maternally derived chromosomes. The mecha-
nisms involved in chromosomal imprinting could be associated with methylation
of DNA (
Sapienza et al. 1987, Solter 1988, Wagner et al. 1993
).
4.4 Endopolyploidy is Common in Somatic Tissues of Arthropods
The discussion of ploidy is confusing because, in most insects, some of the
somatic tissues exhibit high levels of endopolyploidy. For example, haploid male
honey bees have about the same amount of DNA as females in their somatic
tissues because nuclei of the male undergo compensatory endomitosis so that
equal amounts of DNA are present. In some cases, haploid males are known to
exhibit higher levels of endopolyploidy in some tissues than the diploid females
of the same species.
4.5 Genetics of Insects Other than
D. melanogaster
Much of what we know about the genetics of insects is derived from the study of
Drosophila
species (
Ashburner 1989, Brody 1999
). Extensive genetic information is
available for
D. melanogaster
, including the complete sequences of the genome
(
Adams et al. 2000, Hawley and Walker 2000, Jabbari and Bernardi 2000, Otto 2000
;
for additional details on genome projects, see Chapter 7). For updated informa-
tion on the
Drosophila
genome and other aspects of
Drosophila
biology, search the
web at The Interactive Fly (
http://www.sdbonline.org/ly/aimain/1aahome.htm
) and
FlyBase, A Database of the
Drosophila
Genome.
Relatively little genetic information has been available for the majority of the
883,475 known insect species. Genetic studies previously were made on insects
of economic importance, such as mosquitoes, honey bees, the Mediterranean
fruit fly, the parasitoid wasp
Nasonia
, the silk moth,
Tribolium
beetles, tsetse,
