Inhibition of thyroxine-induced metamorphosis by prolactin in tadpoles, Rana catesbeiana.

Thyroxine (T4)-prolactin interactions on hepatic arginase and ornithine transcarbamylase (OTC) as well as hind legs, tail, digestive tract and median eminence were investigated in tadpoles, Rana catesbeiana. Prolactin completely blocked T4-induced tail resorption, but failed to suppress hind-leg growth, shortening of digestive tract and promotion by T4 of the median eminence development. Prolactin blocked T4-induced increase in hepatic arginase activity but not in hepatic OTC activity. A possibility that T4 and prolactin are regulating the hepatic arginase indirectly is discussed.     

Retardation of thyroxine-induced metamorphosis by Amphenone B in toad tadpoles

The effect of Amphenone B, an inhibitor of corticoid synthesis, on thyroxine (T4)-induced metamorphosis was studied in toad tadpoles kept in thiourea. Amphenone injections retarded T4-induced tail resorption markedly. The effect of Amphenone was nullified by aldosterone and corticosterone added to the water in which tadpoles were kept. Steroidogenic cells of adrenals in Amphenone-injected animals were enlarged markedly as compared with those in the saline-injected tadpoles or the Amphenone-injected tadpoles which were supplemented with corticoids. The results strongly suggest that endogenous corticoids act together with thyroid hormone to accelerate metamorphosis.     

Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine-induced, spontaneous, and prolactin-inhibited metamorphosis in Rana pipiens tadpoles

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.     

Selection for phenotypic plasticity in Rana sylvatica tadpoles

The hypothesis that phenotypic plasticity is an adaptation to environmental variation rests on the two assumptions that plasticity improves the performance of individuals that possess it, and that it evolved in response to selection imposed in heterogeneous environments. The first assumption has been upheld by studies showing the beneficial nature of plasticity. The second assumption is difficult to test since it requires knowing about selection acting in the past. However, it can be tested in its general form by asking whether natural selection currently acts to maintain phenotypic plasticity. We adopted this approach in a study of plastic morphological traits in larvae of the wood frog, Rana sybatica. First we reared tadpoles in artificial ponds for 18 days, in either the presence or absence of Anax dragonfly larvae (confined within cages to prevent them from killing the tadpoles). These conditioning treatments produced dramatic differences in size and shape: tadpoles from ponds with predators were smaller and had relatively short bodies and deep tail fins. We estimated selection by Anax on the two kinds of tadpoles by testing for non-random mortality in overnight predation trials. Dragonflies imposed strong selection by preferentially killing individuals with relatively shallow and short tail fins, and narrow tail muscles. The same traits that exhibited the strongest plasticity were under the strongest selection, except that tail muscle width exhibited no plasticity but experienced strong increasing selection. A laboratory competition experiment, testing for selection in the absence of predators, showed that tadpoles with deep tail fins grew relatively slowly. In the cattle tanks, where there were also no free predators, the predator-induced phenotype survived more poorly and developed slowly, but this cost was apparently not associated with particular morphological traits. These results indicate that selection is currently promoting morphological plasticity in R. sylvatica, and support the hypothesis that plasticity represents an adaptation to variable predator environments.     

The development of nerve-muscle junctions in Rana catesbeiana tadpoles

The physiological properties of developing nerve-muscle junctions in Rana catesbeiana tadpoles are described. Developing neurons at different stages of ontogeny formed functional synaptic connections with a section of tail muscle implanted in place of the hind limb bud. Transmission is quantal in nature, sensitive in normal ways to calcium and magnesium concentrations, and conforms to a Poisson distribution. The quantal content is initially low and increases with development. Mepp's occur randomly and have low frequencies which increase slightly with development. The size of a single quantum of transmitter does not change during development. The muscle fibers are multiply innervated, resulting in Epp's with distinct peaks and complex skewed mepp amplitude histograms. No significant increases were observed in the level of differentiation of the developing motor neurons as a result of their having innervated a portion of mature tail muscle. The numbers of developing motor neurons increased in the experimental lateral motor column, and a lag in their maturity was observed relative to motor neurons in the control lateral motor column.     

Functional differentiation of the tail muscle fibers in Rana temporaria tadpoles

Studies have been made on changes in the electrical properties of muscle membrane and lipid content of two types of myotomal fibers in the tail of tadpoles during metamorphosis. It was shown that during premetamorphosis, peripheral and inner muscle fibers do not differ with respect to their effective resistance, time constant of the membrane and lipid content; the resting membrane potential is higher in the inner fibers. During further development of the tadpoles, differentiation of muscle fibers takes place, and to the beginning of the climax the inner fibers attain lower values of the effective resistance and time constant, as well as lower content of lipids in their sarcoplasm; the difference in the level of resting membrane potential between the peripheral and inner fibers increases. The data obtained suggest that the inner fibers may be referred to as fast ones, whereas the peripheral ones--as slow. These data also reveal specific features in neurotrophic regulation of functional properties of muscle fibers in tadpoles.     

Predation- and competition-mediated brain plasticity in Rana temporaria tadpoles

An increasing number of studies have demonstrated phenotypic plasticity in brain size and architecture in response to environmental variation. However, our knowledge on how brain architecture is affected by commonplace ecological interactions is rudimentary. For example, while intraspecific competition and risk of predation are known to induce adaptive plastic modifications in morphology and behaviour in a wide variety of organisms, their effects on brain development have not been studied. We studied experimentally the influence of density and predation risk on brain development in common frog (Rana temporaria) tadpoles. Tadpoles grown at low density and under predation risk developed smaller brains than tadpoles at the other treatment combinations. Further, at high densities, tadpoles developed larger optic tecta and smaller medulla oblongata than those grown at low densities. These results demonstrate that ecological interactions - like intraspecific competition and predation risk - can have strong effects on brain development in lower vertebrates.     

Low molecular weight immunoglobulins in Rana catesbeiana tadpoles.

Rana catesbeiana tadpoles formed high and low m.w. antibodies in response to immunization with a bacteriophage. Although the neutralizing activity associated with the low m.w. immunoglobulins was relatively weak, the existence of antibodies in this class was clearly demonstrated by radioimmunoelectrophoresis. Moreover, two antigenically distinct variants of the low m.w. antibodies were detected. These were serologically indistinguishable from the two types of low m.w. immunoglobulin previously isolated from the serum of adult frogs of this species.     

Differential performance among LDH-B genotypes in Rana lessonae tadpoles

The European pool frog, Rana lessonae, is widely polymorphic for two common alleles (b,e) at the lactate dehydrogenase-B (LDH-B) locus. We compared fitness-related larval life-history traits among LDH-B genotypes, which originated from segregation in heterozygous parents, in an artificial pond experiment where tadpoles of R. lessonae from a Swiss population were raised together with tadpoles of the hemiclonal hybrid R. esculenta at two densities. In R. lessonae, LDH-B e/e homozygotes at each density had a higher proportion of metamorphs among survivors, reached metamorphosis earlier, and were heavier at metamorphosis than b/b homozygotes; b/e heterozygotes had intermediate values. That e/e individuals were superior to b/b in both time to and mass at metamorphosis is surprising because these two life-history traits are thought to reflect a performance trade-off; e/e genotypes apparently compensated for shorter time to metamorphosis by a higher growth rate. The two alleles showed the same performance ranking when combined in hybrids with a R. ridibunda allele: When R. esculenta from Swiss populations reared in the same ponds had received the e allele rather than the b allele from their R. lessonae parent, they reached metamorphosis earlier, but did not differ in mass at metamorphosis. The degree of linkage disequilibrium in the source population of the eight R. lessonae used as parents of the R. lessonae tadpoles is unknown, so we cannot exclude the possibility that the performance differences are caused by some anonymous tightly linked gene, rather than the LDH-B locus, that constitutes the genomically localized target of natural selection. A causal involvement of LDH-B is plausible, nevertheless, because this enzyme takes part in the central energy-metabolizing processes and has been reported to underlie fitness differences in other animals; also, differential performance of LDH-B genotypes has been observed in R. lessonae larvae from another population. The present results suggest strong directional selection for allele e; the sum of available data, including an independent laboratory experiment, suggests that partial environment-dependent overdominance combined with balancing selection favoring e/e homozygotes under some and b/b homozygotes under other conditions may be partially responsible for the broad maintenance of the LDH-B polymorphism in R. lessonae.