The Questions of Developmental Biology

Diapause
Many species of insects have evolved a strategy called diapause. Diapause is a
suspension of development that can occur at the embryonic, larval, pupal, or adult stage,
depending on the species. The overwintering eggs of the hickory aphid provide an example of this
strategy. In some species, diapause is facultative and occurs only when induced by environmental
conditions; in other species, diapause has become an obligatory part of the life cycle. The latter is
often seen in temperate-zone insects, in which diapause is induced by changes in the photoperiod
(the relative lengths of day and night). The day length at which 50 percent of the population has
entered diapause is called the critical day length, and this usually occurs quite suddenly.
The critical day length is a genetically determined property (Danilevskii 1965; Tauber et al.
1986).
Diapause is not a physiological response brought about by harsh conditions. Rather, it is
brought about by token stimuli that presage a change in the environment, beginning before the
severe conditions actually arise. Diapause is especially important for temperate-zone insects,
enabling them to survive the winter. The silkworm moth Bombyx mori overwinters as an embryo,
entering diapause just before segmentation. The gypsy moth Lymantia dispar initiates its
diapause as a fully formed larva, ready to hatch as soon as diapause ends.
Gravity and pressure
An environmental constant that can be used in development is gravity and movement.
In Chapters 10 and 11 we saw that gravity was critical for frog and chick axis formation.
Moreover, there are several bones whose formation is dependent on stresses occasioned by the
movement of the embryo. Such stresses have been known to be responsible for the formation of
the human patella (kneecap) after birth. But more recently it has been shown that several bones in
the chicken do not form if embryonic movement is suppressed in the chick egg. One of these
bones is the fibular crest. This bone connects the tibia to the fibula, and it allows the force of the
iliofibularis muscle to pull directly from the femur to the tibia. This direct connection is thought
to be important in the evolution of birds, and the fibular crest is a universal feature of the bird
hindlimb (Müller and Steicher 1989). When the bird is prevented from moving within its egg, this
bone fails to develop (Figure 21.5; Wu 1996; Newman and Müller, in press). Therefore, even in
the formation of important bones, the environment can play a critical role.