The Questions of Developmental Biology

The hindwing pigments of the short-day
forms are darker than those of the long-day
butterflies. This has a functional advantage during
the colder months of spring: The darker pigments
absorb sunlight more efficiently, raising the body
temperature faster than lighter pigments, so the
darker short-day butterflies can use their pigments
to heat themselves up between flights (Figure
21.7; Shapiro 1968, 1978; Watt 1968, 1969;
Hoffmann 1973; see also Nijhout 1991).
In tropical parts of the world, there is often a hot wet season and a cool dry season.
In Africa, the Malawian butterfly Bicyclus anynana has a polyphenism that is adaptive to
seasonal changes. It occurs in two phenotypes (morphs). The dry-season morph is cryptic,
resembling the dead brown leaves of its habitat. The wet-season morph is more active, and it has
ventral hindwing eyespots that deflect attacks from predatory birds and lizards (Figure 21.8).
The determining factor appears to be the temperature during pupation. Low temperatures produce
the dry-season morph; high temperatures produce the wet-season morph (Brakefield and Reitsma
1991). The development of butterfly eyespots begins in the late larval stages, when the
transcription of the Distal-less gene is restricted to a small focus that will become the center of
each eyespot. During the early pupal stage, Distal-less expression is seen in a wider area, and this
expression is thought to constitute the activating signal that determines the size of the spot. Last,
the cells receiving the signal determine the color they will take. The seasonal Bicyclus morphs
appear to diverge at the later stages of signal activation and color differentiation (Figure 21.9;
Brakefield et al. 1996).
Nutritional polyphenism
Not all polyphenisms are controlled by the seasons. In bees, the size of the female larva at
its metamorphic molt determines whether the individual is to be a worker or a queen. A larva fed
nutrient-rich "royal jelly" retains the activity of her corpora allata during her last instar stage.
The juvenile hormone secreted by these organs delays pupation, allowing the resulting bee to
emerge larger and (in some species) more specialized in her anatomy (Brian 1974, 1980;
Plowright and Pendrel 1977). The JH level of larvae destined to become queens is 25 times that
of larvae destined to become workers, and application of JH onto worker larvae can transform
them into queens as well (Wirtz 1973; Rachinsky and Hartfelder 1990).
Similarly, ant colonies are predominantly female, and the females can be extremely
polymorphic (Figure 21.10A). The two major types of females are the worker and the gyne.
The gyne is a potential queen. In more specialized species, a larger worker, the soldier, is also
seen.