Clinal Variation in Morphology in Eastern Populations of the Stream-Dwelling Waterstrider, Aquarius remigis (Hemiptera: Gerridae)

Brennan, Julie

ABSTRACT: I examined morphological variation among populations of the stream-dewelling waterstrider, Aquarius remigis. A previous analysis of allozyme variation along a transect from southern Quebec to southern North Carolina revealed two distinct 'types' of A. remigis, north and south of a transition zone in southern New York and northern Pennsylvania. To assess the concordance of morphological and allozymic differentiation, adults were sampled from 15 populations along the original transect, as well as 11 additional populations in New York, Connecticut and New Jersey. Eight measurements of leg and body size were taken of preserved individuals. Univariate and multivariate analyses (Multiple regression, PCA, discriminant analysis) reveal significant differentiation between populations from Quebec and New York and populations from Pennsylvania, Connecticut, New Jersey, Virginia and North Carolina. Strong clines in body size and shape within each region and a sharp transition between regions supports previous suggestions that there are two distinct 'types' of this species in eastern North America. Increased within-population variance in the transitional populations supports the earlier conclusion of a restriction of gene flow within the zone. The transition zone in morphology coincides with that revealed by the allozyme variation, but extends further south within Pennsylvania. A greater proportion of northern 'type' females than males in the transitional population suggests more introgression of northern alleles in females (consistent with Haldane's rule). The covariance of allozyme frequency and morphometric traits suggests that differentiation has occurred through vicariance and secondary contact, and that in this system, cladistic and adaptive differentiation are coupled.