Biology Reference
In-Depth Information
the egg.Ò Attempts to culture the infection were unsuccessful (Hertig, 1936). Unfortunately, fol-
lowing the initial description,
W. pipientis
essentially disappeared from scientiÝc literature for more
than three decades.
Although the name
W. pipientis
failed to attract scientiÝc attention in the years following its
description,
Wolbachia
-induced embryo lethality became the research focus of multiple laboratories
and Ýeld trials sponsored by the World Health Organization (WHO).
C. pipiens
was again the
experimental subject when crosses intended to clarify
Culex
systematics identiÝed intriguing pat-
terns of incompatibility resulting from interstrain hybridizations (reviewed in Laven, 1967c). Cross-
ing experiments revealed a complex pattern of both bidirectional and unidirectional incompatibility.
Additional backcrossing experiments with the hybrid offspring demonstrated that compatibility
types were determined by a maternally inherited factor, prompting Laven to coin the term Ñcyto-
plasmic incompatibilityÒ to describe the crossing phenomena (Laven, 1967c). A total of 17
Culex
crossing types were deÝned by Laven (Laven, 1967c). Contemporary with the
Culex
-crossing
experiments, similar patterns of CI were also described in additional mosquitoes (Rozeboom and
Kitzmiller, 1958),
Nasonia
(Mormoniella)
vitripennis
(Saul, 1961; Ryan and Saul, 1968), and
Tribolium confusum
(Stanley, 1961). However,
W. pipientis
failed to be incriminated as the etio-
logical agent responsible for CI.
Not knowing the agent responsible for CI did not prevent researchers from proposing and
testing applied strategies that employed CI and targeted the control of medically important
Culex
populations.
C. quinquefasciatus
Say (
C. pipiens fatigans
Wiedemann) is the primary vector for
human Ýlariasis in Southeast Asia. The proposed strategy was similar to conventional sterile insect
technique (SIT) (Knipling, 1955) but employed releases of cytoplasmically incompatible males
instead of irradiated or chemosterilized males (Barr, 1966; Laven, 1967a). The strategy attracted
support from the WHO, resulting in Ýeld trials of the strategy in conjunction with its Filariasis
Research Unit in Rangoon, Burma (Anonymous, 1964). Cytoplasmically incompatible males were
repeatedly released into a relatively isolated population, with the goal of reducing the vector
population and interrupting the transmission of Ýlariasis. In one study, Ýeld releases of incompatible
males were continued over a 12-week period in the village of Okpo, Burma. To assess strategy
efÝcacy,
C. quinquefasciatus
egg rafts were Ýeld-collected and monitored for hatching. During the
course of the experiment, raft egg hatching dropped from 95.7 to 0%. By the end of the study, all
Ýeld-collected egg rafts failed to hatch (Laven, 1967a). The success of this strategy for the sup-
pression of Ýeld populations led to the proposed adaptation of this strategy for additional targets
including economically important pest species (Davidson, 1974; Pal and LaChance, 1974; Brower,
1979). However, the success of the CI and related strategy Ýeld tests were tainted by a public-
relations scandal, in which the Indian press accused the WHO and involved governments of
conducting biological warfare research (Anonymous, 1975, 1976; Hanlon, 1975; Tomiche, 1975;
Wood, 1975; Curtis and Curtis, 1976; Curtis and Von Borstol, 1978; Walgate, 1978). Due in part
to the allegations and negative attention, research with CI for the suppression of pest populations
was not continued.
Of equal concern, scientiÝc critics argued that the CI-suppression strategy was impractical due
to the requirement that only males be released (Pal and Whitten, 1974). With traditional SIT
strategies, releases are designed to consist primarily of males for maximum efÝciency, but the
absolute elimination of females from releases is not imperative to the success of the strategy
(Knipling, 1998; Krafsur, 1998). In contrast, accidental female release in the CI-based SIT strategy
could permit the establishment of the new cytotype in the Ýeld, resulting in a Ýeld population that
was compatible with additional releases of males. Thus, the end result would be the replacement
of the Ýeld population with the released vector population that harbored the new cytotype. Additional
male releases would no longer be incompatible, permitting recovery of the host population.
Although all-male releases were possible for small pilot experiments, the complete elimination of
females was not considered practical on a scale required for large-area control or eradication
programs (Pal and Whitten, 1974). Subsequently, interest in the applied use of CI shifted from
