VARIATION AND COVARIATION OF CHARACTERS OF ALATE PEMPHIGUS POPULI-TRANSVERSUS IN EASTERN NORTH AMERICA,

This paper is based on a study of the variation and covariation of 18 morphological characters of the aphid, Pemphigus populi-transversus Riley, from 23 localities in eastern North America. The variation and covariation of these characters are separately studied within galls, among galls within localities, and among localities. By means of factor analysis, covariation is restated in terms of fewer dimensions (factors). The factors are compared at the different levels of variation. This paper continues an earlier study (Sokal, 1952) in which the author examined 21 characters of a single population of this gall-forming aphid from Ripley, Ohio. Of this local population, 28 galls containing four alate aphids each were studied. Since the alates within any one gall are isogenic and the different galls presumably represent different genotypes, the variance of the characters could be partitioned into a non-hereditary, intragall portion and an intergall portion which is assumed to be partly genetic and partly environmental in origin. Several characters, representing independent dimensions of intergall variation, were chosen by analysis of covariance. The purpose behind this procedure was the recognition of fewer than the original number of characters for a further study of intraspecific variation. Three problems emerged concerning the use of the characters chosen by the previous analysis. The first is the possibility, suggested by Clark's (1941) findings on Peromyscus, that the intergall correlations on which the former study is based might vary geographically, thus presenting a different pattern of significant characters in other areas. Average intergall correlations for many localities are therefore computed below. The second problem is whether some characters lacking intergall variance in the Ripley population might yet possess significant interlocality variance. As will be seen below, this proved to be the case for several meristic characters. Thirdly, and most important, are the characters representing genetic and environmental differences among galls necessarily the same as those describing genetic and environmental differences among localities? This study will show that there are real differences between interlocality and intergall factor patterns. As the study progressed a number of other objectives came into view, and assumed progressively increasing importance. The unusual hierarchic structure of the data at hand (isogenic alate aphids descended from a single stem mother, morphologically distinct but genetically identical, all contained within an individual gall; galls putatively representing different genotypes found within one locality; and finally localities presumed to differ genetically from each other) permitted a detailed and quantitative separation of sources of variation in this study. Since correlations can be computed at each of these levels of covariation, the differences between these correlation matrices and between the factors into which they were resolved eventually, provide a novel Part 2 of a study of variation in the aphid genus Pemphigus. Part 1 is Sokal (1952), part 3 is Sokal and Rinkel (1962). 2 Contribution No. 1125 from the Department of Entomology of The University of Kansas, Lawrence, Kansas.