Malformed Frogs: The Collapse of Aquatic Ecosystems

1. What is an Amphibian Malformation?

one WHAT IS AN AMPHIBIAN MALFORMATION? Symmetry is an unmistakable sign that there’s relevant information in a place. That’s because symmetry is a property shared by a relatively small number of things in the landscape, all of them of keen interest to us. . . . Symmetry is also a sign of health in a creature, since mutations and environmental stresses can easily disturb it. MICHAEL POLLAN1 The word “malformation” literally means “bad form.” Bad form in most animals means an unintended lack of symmetry, or an imbalance in structure , color, or other quality. A lack of symmetry can arise through one of three mechanisms:2 Genetic Genes are flawed, or the expression of genes during development is flawed. Albinism (in which animals have white bodies and pink eyes) is a familiar genetically determined malformation. Albinism is often caused by a mutation that results in the failure of embryonic neural crest cells to migrate. Similar genetic mutations in northern leopard frogs produce blue or partially blue axanthic individuals (Color Plate 1),3 the kandiyohi pigment morph (which has tight blotches rather than spots; Color Plate 2), and the burnsi pigment morph (which lacks dorsal body spots; Color Plate 3).4 These mutations are not fatal and in the cases of the kandiyohi and burnsi morphs are caused by an autosomal dominant allele and need only be passed on by one parent. Roughly 50% of all offspring between one of these morphs and a normally pigmented leopard frog inherit the pattern; from 75–100% of offspring between two of these pigment morphs inherit the pattern. Pieter Johnson and his colleagues5 have suggested using the term “deformity” for malformations such as these that arise from genetic mechanisms. Not everyone has followed this suggestion, although such precision in thought guides understanding. 7 Epigenetic Genes and gene expression are normal but some deviation from expected environmental circumstances occurs where and at the time when genes are being expressed (due perhaps to a lack of nutrition, to the presence of toxins that disturb cell-to-cell interactions, to the presence of other chemicals that disrupt the endocrine system or cause chromosomal aberrations, or to mechanical disruptions). Both genetic and epigenetic problems are apparent before development is complete, and are called congenital abnormalities when present at birth in humans. Epigenetic mechanisms, as we shall see below, are the most likely cause of the current malformed frog phenomenon. Trauma (or other post-developmental mechanisms) Injuries or disease. Injuries can occur during failed predation attempts, territory or mate defense, amplexus (mating), or environmental exposure (desiccation or frostbite). Infections secondary to injury can also occur and have permanent effects.6 It can be difficult to determine the cause of any given malformation, but a sound knowledge of natural history offers perspective. For example, in Minnesota and other areas of the Upper Midwest, and in New England, most malformed frog hotspots are isolated wetlands; separated from other known hotspots by tens, if not hundreds, of miles. But frogs are mobile, they travel from wetland to wetland, and genes get spread among populations . If genetic mechanisms are the cause of malformations we would expect—in much the same way that burnsi and kandiyohi northern leopard frog color morphs are distributed—frogs in wetland clusters to exhibit malformations. Further, within any wetland hotspot, more than one species can be affected and many of the malformations found in these species are of the same type.7 The chances of the same genetic mutation arising in several species at a single site without any sign of mutations in populations from nearby wetlands (and these nearby wetlands are frequently close, well within the home range of individual frogs) must be astonishingly low. These observations suggest a site-based (epigenetic or trauma-based) cause. In fact, Dave Hoppe has performed the definitive experiment.8 He collected a newly laid cluster of mink frog (Rana septentrionalis) eggs and raised half the eggs in his laboratory, in water known to be suitable for normal development, and half in a pond known to produce malformed frogs. In these genetically similar animals, the lab-reared tadpoles developed normally, the pond-reared tadpoles developed extra legs as well as 8 what is an amphibian malformation? other malformations. Hoppe’s conclusion was unavoidable—these malformations were not genetically based, the cause was in the water. In deciding between epigenetic and trauma-based causes, trauma oftentimes seems the least likely. Trauma can...