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become haploid males after the loss of the paternal set of chromosomes shortly
after fertilization ( Hunter et al. 1993 ).
The haplo-diploid twospotted spider mite Tetranychus urticae (Acari:
Tetranychidae) appears to modify its sex ratio based on the size of eggs in
females before fertilization ( Macke et al. 2011 ). Eggs that are larger before fer-
tilization (which implies higher quality provisioning of eggs) produce a more
female-biased sex ratio after fertilization. Unfertilized eggs produce males,
which are smaller than females. Male eggs produced by mated females are
smaller than male eggs produced by virgin females (who can only produce
haploid male progeny). This suggests that virgin females have a male-fitness
advantage over mated females, perhaps because larger males are more likely to
outcompete smaller males to mate with females.
10.6.3 Postzygotic Sex Determination
In several collembolans, including Sminthurus viridis and Allacma fusca (sub-
order Symphpleona) the two sexes differ by having 10 chromosomes in males
and 12 in females ( Dallai et  al. 1999, 2000 ). Sex determination occurs after the
zygote forms (rather than at syngamy). Two chromosomes are eliminated in male
embryos in both the somatic and germ-line cells ( Dallai et al. 2000 ). Oogenesis is
normal but spermatogenesis is unusual; daughter cells of the first meiotic division
have either six or four chromosomes. The cell receiving four chromosomes degen-
erates, but the cell with six completes meiosis and produces identical sperm. At
fertilization, the pronuclei with six chromosomes fuse to form a zygote with
12 chromosomes. Male embryos then lose two sex chromosomes during the first
mitosis, resulting in 10 chromosomes. The mechanism of chromosome elimination
during early embryogenesis must be regulated by the genetic constitution of the
mother, which means that females could regulate the sex ratio of their progeny.
In fact, these species appear to have a female-biased sex ratio. Dallai et al. (2000)
suggested that this aberrant meiosis and the large number of females in these
species could be considered a step toward the evolution of parthenogenesis.
10.7 A Single Model?
Given the above-mentioned examples of the diverse sex-determining systems,
is it likely that a single model can describe sex determination in all insects?
Nothiger and Steinmann-Zwicky (1985) proposed that all the sex-determination
mechanisms in insects are variations upon a theme ( Figure 10.5 ). In their model,
there is a gene equivalent to Sxl + , a repressor (R) that inactivates Sxl + , a gene
that activates Sxl + , and a gene that is equivalent to dsx + that is expressed in two
alternative forms to interact with one or the other of the two sets of male- and
female-differentiation genes lower in the hierarchy.
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