Biology Reference
In-Depth Information
may be polyploid, including species in the Orthoptera (Blaberidae, Tettigoniidae),
Hemiptera (Coccidae, Delphacidae), Embioptera (Oligotomidae), Lepidoptera
(Psychidae), Diptera (Chamaemyiidae, Chironomidae, Psychodidae, Simuliidae),
Coleoptera (Ptinidae, Chrysomelidae, Curculionidae), and Hymenoptera
(Diprionidae, Apidae) (
Otto and Whitton 2000
). Polyploid insects usually are
3n or 4n, but exceptions include curculionid weevil species that are 5n and
6n (
Retnakaran and Percy 1985
). Parthenogenesis has not been found in the
Diplura, Protura, Odonata, Plecoptera, Dermaptera, Grylloblattodea, Zoraptera,
Megaloptera, Mecoptera, and Siphonaptera, although only a few species in these
groups have been examined carefully.
Parthenogenesis can be divided into three major types: arrhenotoky, thelytoky,
and deuterotoky.
Deuterotoky
is the least common and involves the develop-
ment of unfertilized eggs into either males or females, and at least one insect,
a mayfly, is reported to exhibit facultative deuterotoky (
White 1973
). In
arrhe-
notoky
, insects are haplodiploid, with males developing from unfertilized haploid
eggs and females developing from fertilized diploid eggs. Typically, arrhenotok-
ous females determine the sex ratio, which is often biased toward females. The
entire order Hymenoptera and many species in the Hemiptera, Thysanoptera, and
Coleoptera are arrhenotokous (
Hartl and Brown 1970, White 1973
).
Thelytokous insect species have females only.
Thelytoky
has arisen repeatedly
in evolution, consists of several types, and can be induced experimentally in sev-
eral ways (
White 1973
; for examples, see Chapter 10). In some cases of thelytoky,
eggs only develop after penetration by a sperm (pseudogamy or gynogenesis),
but the sperm nucleus degenerates without fusing with the egg nucleus so that
it makes no genetic contribution to the embryo. The sperm may be derived from
the testis or ovotestis of a hermaphrodite or from a male of a different, but
closely related, species. Thelytoky may be the sole mode of reproduction in a
species, or it may alternate with sexual reproduction in regular manner (cyclical
thelytoky), as happens in some aphids (
Hales et al. 1997
), gall wasps, and some
cecidomyiids. In species that reproduce by cyclical thelytoky, genetic recombina-
tion is possible, but in species with complete thelytoky (only females) there is no
way in which mutations that have occurred in two unrelated individuals can be
combined in a third individual.
Thelytokous reproduction can be induced in the eggs of many species by
pricking the egg, exposing it to chemical agents, or heat. In many normally
bisexual insects, a few eggs deposited by virgin females can hatch spontane-
ously, and the incidence of such egg hatch can be increased by artificial selec-
tion.
White (1973)
suggests that the capacity for artificial parthenogenesis,
