Hormone regulation and the evolution of frog metamorphic diversity

A vast body of work exists on the endocrine regulation of frog metamorphosis ( Dodd and Dodd 1976 , Shi 1999 , Denver et al. 2002 , Buchholz et al. 2006 , Furlow and Neff 2006 , Brown and Cai 2007 ), and a separate set of literature describes the dramatic larval period diversity found among amphibians ( Duellman and Trueb 1994 , McDiarmid and Altig 1999 ). This chapter begins to unite these topics by addressing endocrine and molecular mechanisms underlying life history evolution in larval amphibians. Amphibian metamorphic life history diversity includes species differences in time to, and size at, metamorphosis, evolution of direct developing forms from typical free-living larvae, and neoteny. Ecological aspects of this diversity are not covered, nor is the diversity of reproductive modes. Metamorphic life history parameters are under neuroendocrine control, including both central and peripheral control mechanisms ( Fig. 7-1 ). Despite limited evidence, theory suggests mechanistic changes at the central level of control, whereas evolution of control at the peripheral level has been more adequately demonstrated. We present mechanisms of tissue responsiveness to thyroid hormone from model frog species in order to shed light on potential sites of evolutionary change in endocrine physiology that may underlie life history differences found within amphibians. The larval period of the amphibian life cycle occurs from the beginning of post-embryogenesis (i.e., completion of organogenesis) through metamorphosis to the production of a juvenile frog. Within tadpoles, life history variation includes larval period duration (ranging from 8 days in Scaphiopus couchii to 2–3 years in Heleophryne sp., Ascaphus , and some Rana ) and size at metamorphosis (ranging from <1 cm in many species to adult size in Pseudis sp.). Even though virtually every environmental factor affects larval period duration and size at metamorphosis ( Wilbur and Collins 1973 , Denver 2009 ), species diversity in larval period and metamorph size is not completely accounted for by phenotypic plasticity, as determined by rearing different species under identical laboratory conditions ( Lieps and Travis 1994 , Buchholz and Hayes 2002 ). Direct development and neoteny are dramatic evolutionary departures from the free-living transitory larva. Direct developers lack a free-living larval period and hatch from the egg as a juvenile. Evolutionary loss of larval features in direct developers varies widely across species, from Eleutherodactylus , with its vestigial and highly modifi ed larval structures, to other species which hatch from the egg with the appearance of a tadpole but have enough nutrition from yolk to complete metamorphosis ( Callery et al. 2001 , Thibaudeau and Altig 1999 ). Conversely, neoteny is reproductive maturation in the larval form, found in salamanders but not frogs or caecilians ( Dent et al. 1968 ). Depending on the species, neotenic salamanders do or do not undergo metamorphosis in nature, and can or cannot be induced to undergo metamorphosis by hormone injections.