Symbiosis and the Origin of Species - Insect Symbiosis - page 227

is usually gamete duplication (Stouthamer, 1997) in which haploid eggs do not complete the Ýrst
mitotic division and diploidy is restored. Because of this mechanism, it is typically thought that
PI Wolbachia would be restricted to the Hymenoptera and other haplodiploids (e.g., thrips and
mites), where infected virgin females would lay all female (diploid) offspring. The Ýnding that PI
Wolbachia can also do their business through apomictic parthenogenesis (Weeks and Breeuwer,
2001) suggests that diplodiploid systems are also susceptible to these phenomena. Apomictic
asexuality is actually the most common form of parthenogenesis in diplodiploid arthropods (Suoma-
lainen et al., 1987).
What are the consequences of PI bacteria on species formation? The onset of asexuality does
not directly prohibit gene Þow between the asexual and sexual population because asexuals can
still exchange genes with sexual mates, as shown in Trichogramma wasps (Stouthamer and Kazmer,
1994). For example, asexual females still retain the ability to mate with a sexual male and may do
so unless no mates can be found. Additional isolating barriers must therefore accompany the shift
in reproductive strategy to complete the speciation event.
Premating isolation may be easily achieved since the onset of asexuality can indirectly lead
to the degradation of Ýtness characters involved in sexual reproduction, including male and female
mating behavior, male fertility, secondary sexual characteristics, fertilization processes, oviposition
behaviors, and developmental requirements. For example, an asexual female may lose her ability
to accept a sexual male through mutational degradation of genes required for mating behavior.
Both genetic drift and selection can drive the evolution of mutations involved in sexual degrada-
tion, and these forces may show sex-speciÝc patterns (Pijls et al., 1996). For example, deleterious
mutations in male Ýtness are expected to accumulate neutrally because males are neither produced
nor needed in asexual races, assuming that maleÏfemale sexually antagonistic alleles are rare. In
contrast, mutations in genes encoding female sexual traits may be strongly selected for due to
antagonistic pleiotropy. For example, mutations that erode sexual traits in females (e.g., mating
behavior or sperm usage) could pleiotropically cause a Ýtness increase in asexual females. Female
sexual degradation may therefore proceed rapidly via natural selection. This potential difference in
decay rate for male and female sexual traits leads to the prediction that female traits may degrade
before male traits, at least in large populations where the efÝcacy of selection is strong and drift is low.
This asymmetry actually strengthens the possibility for asexual speciation because decay of female
function may fortify reproductive isolation more so than decay of male function. If asexuality is
complete and males are never (or rarely) produced, then male sexual decay is not necessary for the
completion of asexual speciation. In contrast, female sexual decay is necessary because females make
up the bulk of an asexual population and have the potential to mate with sexual males. Thus, the
simplest mode of asexual speciation is the acquisition of a completely penetrant PI Wolbachia , followed
by the Ýxation of a single mutation that inhibits mating between sexual males (uninfected) and asexual
females (infected). Table 17.4 shows a list of studies that have examined the kinds of sexual degradation

TABLE 17.4
Asexuality in the Hymenoptera Frequently Leads to Sexual Degradation in Female Traits,
but Not in Male Traits

System Female Trait Male Trait Ref.

Trichogramma Ð Ð Stouthamer et al., 1990

Apoanagyrus diversicornis Mating behavior Ð Pijls et al., 1996

Encarsia hispida Oviposition behavior Ð Hunter, 1999

Galeopsomyia fausta Mating behavior Ð Argov et al., 1999

Telenomus nawai Fertilization Ð Arakaki et al., 2000

Encarsia pergandiella Oviposition behavior Ð Zchori-Fein et al., 2001

Muscidifurax uniraptor Mating behavior and anatomy Sperm production Gottlieb and Zchori-Fein, 2001