INTERMEDIATE-WINGED APHIDS AND THE TIME-OF-DETERMINATION THEORY

¹ COE COLLEGE, CEDAR RAPIDS, IOWA

In two strains of aphids of the species Macrosiphum solanifolii, intermediates between winged and wingless females were experimentally produced, and a histological study was made of these. Aphids may show internal intermediacy when there is no evidence of it externally, therefore the degree of winged-wingless intermediacy cannot be safely judged by external observations alone.

Two theories attempting to explain the combination of characters (wings, ocelli, wing muscle) found in intermediate-winged aphids are discussed. The first theory postulates that the combination of characters in intermediates is due to the fact that the differentiating organs are determined at different times in the embryogeny (time-of-determination theory), while the second theory assumes that the structural character of intermediates results from a given concentration of some substance (physiological level theory) in development.

It could not be ascertained from the conditions under which these aphids were reared what direction of change (winged to wingless or wingless to winged), assuming there was a change, produced a given winged-wingless intermediate, but evidence is given which indicates that the character of the wing muscle is correlated with some definite change in the direction of embryonic development. The evidence makes it appear probable that degenerate wing muscle means that the intermediate aphid is the result of some physiological change from a winged to wingless form during the embryogeny; whereas the intermediate with normal wing muscle appears to be the result of a change from wingless to winged during its development.

If the time-of-determination theory were perfectly applicable to intermediate-winged aphids they should all fit one (the same one) of the six possible assumptions for the time at which the wings, ocelli and wing muscles may be determined in the embryogeny. More than two-thirds of the aphids of this study did fit one assumption (Assumption IV) which is evidence that in a general way the composition of intermediates is determined by the segregation of their differentiating structures at different times in the embryogeny. This indicates that organs are determined in a serial order in aphids, as has been found true for certain vertebrates. The order of embryonic determination of the structures distinguishing winged from wingless is wings, ocelli, and wing muscles.

The small differences between the amount of development in wings, ocelli, and wing muscles found in many cases is taken to indicate that the time of determination of these organs is close together. However, that the period of embryonic segregation for these three structures is of considerable duration is suggested by the large number of intergrades.

Most of the intermediates which did not conform to Assumption IV appeared to owe their irregular combination of characters to a shift in the time of determination of the ocelli. The simplest explanation of these irregularities is to postulate that the time of segregation may vary for a given organ in different individuals.

An interpretation of the combination of differentiating characters of intermediate-winged aphids based on the physiological level theory would involve the necessity of assuming that the threshold of stimulation for the development of these organs varies in different individuals. Moreover, the threshold of stimulation would have to show a series of regular variations to produce aphids fitting one direction of development, as winged to wingless, at one time and another direction, as wingless to winged, at another time. The difficulties involved in making such an assumption renders this hypothesis highly improbable.

The results of this investigation support the time-of-determination theory with the modification of frequent variations in the time at which an organ may be embryonically determined.