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whereas Brachypteres lay diapausing embryos that are heavier and darker than
direct developing eggs ( Kimura & Masaki, 1977 ). Although there is a cor-
relation between the polyphenic trait and the type of egg laid, they are
not inextricably linked. Manipulation of DH signaling during pupal life,
for example, can revert the developmental pathway without switching
the morph type. Macropterus pupae develop into moths laying diapausing
embryos when injected with DH, whereas Brachypterus pupae develop into
moths laying direct developing embryos when injected with anti-DH (DH
antagonist) ( Fig. 8.5 B) ( Uehara et al., 2011 ).
Interestingly, Bombyx embryonic diphenism is also induced by ECD
since injections of 20-ECD into “nondiapause” moths induce them to lay
developing, but giant, eggs that are larger and heavier than normal, and are
similar in appearance to the diapausing type ( Kawaguchi, Banno, Doira, &
Fujii, 1989 ). Hence, diapausing signals might orchestrate the timing, or the
levels, of developmental hormones for the induction of seasonal polyphenism
although the precise interactions between DH and ECD remain ill defined.
7.1. Adaptive advantages of seasonal polyphenisms
As with diapause, the seasonal polyphenisms are thought to provide adaptive
advantages. The swallowtail Papilio polyxenes diapauses as freeze-tolerant
pupa specified under autumnal short days. Larvae set for pupal diapause
(short days) develop into autumnal dark larvae upon exposure to short pho-
toperiods during the first three instars. In this case, the dark form has adaptive
advantages since it helps to elevate larval body temperatures to grow rapidly
during the autumn so that the diapausing pupal stage can be reached prior to
the first freeze ( Hazel, 2002 ). Another example is the noctuid moth,
Mamestra brassicae , in which the fourth and fifth instar larvae exhibit seasonal
color morphs ranging from a pale green to a dark morph. As above, dark-
morph larvae are thought to optimize the capture of solar radiation enhanc-
ing their body temperature in autumnal (or mountain) environments such
that these larvae grow more rapidly than the green morphs and, although
they have light weight at the onset of fifth instar, they undergo a “catch-
up” growth during the fifth and sixth larval stages so that they reach pupation
without delaying development or impairing pupal size ( Goulson, 1994 ).
7.2. Molecular mechanism for polyphenism induction
Interestingly, the hormonal mechanism underlying the production of poly-
phenism in Manduca moths might offer insight into the molecular basis
underlying both control and evolution of larval “diapause-bound” morphs.
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