Species Of Spadefoot Toads Research Articles

Investigators have long been familiar with the marked variation in labial tooth-row formulas of larvae of spadefoot toads. To ascertain whether the observed variations are nongenetic (environmental) modifications or the result of different hereditary constitutions, progeny of members of different populations of Scaphiopus holbrooki from southeastern Louisiana were raised under identical laboratory conditions. Even in a uniform environment, the offspring of a given mated pair exhibited a wide array of labial counts. The progeny were thus genetically heterogeneous. The differentiation of the mouthparts is most likely governed by many heritable factors (polygenes); diverse phenotypes are promoted by extensive polygenic recombination. Certain labial formulas (5/5 and 6/5) prevail in a given population; the commonest counts probably represent the optimal or better adapted phenotypic expressions. The pattern of variation was essentially similar among the progeny of different mated pairs from any single locality, i.e., intrapopulation variability was negligible. However, interpopulation differences were found, indicative of microgeographical genetical differentiation. In particular, the distributions of tooth-row counts in offspring from two localities separated by 8 miles (Varnado and Bogalusa) were equivalent, whereas the frequencies of counts in progeny from Slidell (32 miles distant from Bogalusa) differed significantly from those of the Varnado and Bogalusa samples. INTRODUCTION Larvae of the genus Scaphiopus are notable for the wide variation in the number and arrangement of labial teeth in the oral disc. As an instance, the labial tooth-row formulas of S. bombi/rons, as recorded by Bragg and Bragg (1958), range erratically from one extreme to the other, from 0/0 to 6/6. An inordinate array of toothrow counts has been reported for other species of spadefoot toads (Bragg, 1944a,b; Bresler and Bragg, 1954; Gosner and Black, 1954; Stebbins, 1951; Taylor, 1942; Wright, 1929). rhe factors responsible for the diversity have not been elucidated. Environmental conditions in nature, acting during morphogenesis. may be instrtimental in modifying greatly the phenotypic expression of the complex of genes regulating the differentiation of the mouthparts. Inspection alone can not reveal the extent of nongenetic or environmental modifications. The relative influences of heredity and environment can be assessed by simple experimentation. One need only obtain adults from a natural population, breed them in a laboratory, and rear the progeny under uniform laboratory conditions. Differences among the offspring that arise in a common environment are ascribable to dissimilar genetic constitutions. This type of experimentation also yields meaningful information concerning the basic causes of interpopulation variability. If, in a controlled environment, larvae derived from parents from different localities are essentially alike, then the conclusion is warranted that the observed differences in nature are the direct outcome of local environmental variations.

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