The Questions of Developmental Biology

*The importance of substrates for larval settlement and metamorphosis was first demonstrated in 1880, when William
Keith Brooks, an embryologist at Johns Hopkins University, was asked to help the ailing oyster industry of Chesapeake
Bay. For decades, oysters had been dredged from the bay, and there had always been a new crop to take their place. But
recently, each year brought fewer oysters. What was responsible for the decline? Experimenting with larval oysters,
Brooks discovered that the American oyster (unlike its better-studied European cousin) needed a hard substrate on
which to metamorphose. For years, oystermen had thrown the shells back into the sea, but with the advent of suburban
sidewalks, the oystermen were selling the shells to the cement factories. Brooks's solution: throw the shells back into
the bay. The oyster population responded, and the Baltimore wharves still sell their descendants.
The ability of environmental cues to induce phenotypic change should be considered "tertiary induction." Primary
induction involves the establishment of a single field within the embryo (such that one egg gives rise to just one
embryo). Secondary induction concerns those cascades of inductive events within the embryo by which the organs are
formed. Tertiary induction is the induction of developmental changes by factors in the environment.
Although seasonal polyphenism is usually considered adaptive, there are times when it does not increase the fitness of
the organism. For instance, the photoperiod can cause a hare's color to change from brown to white, but if it doesn't
snow, the hare will be conspicuous against the dark background.
The fovea is a depression in the center of the retina where only cones are present and the rods and blood vessels are
absent. Here it serves as a convenient landmark.
Recent studies (Colman et al. 1997) have shown that divergence in neurotransmitter release results in changes in
synaptic adhesivity and causes the withdrawal of the axon providing the weaker stimulation. Studies in mice (Huang et
al. 1999; Katz 1999) suggest that brain-derived neurotropic factor (BDNF) is crucial during the critical period.
Genetic Assimilation
In discussing the trade-offs between uninduced and
induced morphs, we mentioned that if an induced morph did not
have a significant trade-off, one would expect it to become the
predominant form of the species. C. H. Waddington and I. I.
Schmalhausen independently made this prediction to explain
how some species could rapidly evolve in particular directions
(see Gilbert 1994). Both scientists were impressed by the
calluses of the ostrich. Most mammalian skin has the ability to
form calluses on those areas that are abraded by the ground or
some other surface.* The skin cells respond to friction by
proliferating. While such examples of environmentally induced
callus formation are widespread, the ostrich is born with calluses
where it will touch the ground (Figure 21.17). Waddington and
Schmalhausen hypothesized that since the skin cells are already
competent to be induced by friction, they could be induced by
other things as well. As ostriches evolved, a mutation (or a
particular combination of alleles) appeared that enabled the skin cells to respond to a substance
within the embryo. Waddington (1942) wrote:
Presumably its skin, like that of other animals, would react directly to external pressure and
rubbing by becoming thicker. . . . This capacity to react must itself be dependent upon genes. . . .It
may then not be too difficult for a gene mutation to occur which will modify some other area in
the embryo in such a way that it takes over the function of external pressure, interacting with the
skin so as to "pull the trigger" and set off the development of callosities.