The Questions of Developmental Biology

Other species, such as A. tigrinum, metamorphose only if given cues from the
environment. Otherwise, they become neotenic, successfully mating as larvae. In part of its range,
A. tigrinum is a neotenic salamander, paddling its way through the cold ponds of the Rocky
Mountains. Its gonads and germ cells mature while the rest of the body retains its larval form.
However, in the warmer region of its range, the larval form of A. tigrinum is transitory, leading to
the land-dwelling tiger salamander. Neotenic populations from the Rockies can be induced to
undergo metamorphosis simply by placing them in water at higher temperatures. It appears that
the hypothalamus of this species cannot produce thyrotropin-releasing factor at low temperatures.
Some salamanders are permanently neotenic, even in the laboratory. Whereas T 3 is able
to produce the long-lost adult form of A. mexicanum, the neotenic species of Nec-turus and Siren
remain unresponsive to thyroid hormones (Frieden 1981); their neoteny is permanent. Yaoita and
Brown (1990) noted that the mRNA for TRβ is absent in Necturus, so that this species cannot
respond to T 3 . The genetic changes thought to be responsible for neoteny in several salamander
species are shown in Figure 18.9.
De Beer (1940) and Gould (1977) have speculated that neoteny is a major factor in the
evolution of more complex taxa. By retarding the development of somatic tissues, neoteny may
give natural selection a flexible substrate. According to Gould (1977, p. 283), neoteny would
"provide an escape from specialization. Animals can relinquish their highly specialized adult
forms, return to the lability of youth, and prepare themselves for new evolutionary directions."
Progenesis