SPECIFIC STATUS OF AQUARIUS-CINEREUS (PUTON) AND A-NAJAS (DEGEER) (HEMIPTERA, GERRIDAE) AND THE EXTENT OF HYBRIDIZATION IN THE MEDITERRANEAN REGION

Results of field observations, cross-breeding experiments, morphometry , and starch gel electrophoresis suggest that A. cinereus and A. najas are specifically distinct. Cross-breeding experiments between the two taxa in the laboratory were only successful when using males of A. ci nereus and females of A. najas. Field observations on mixed population s in spring did not reveal any pre- or postcopulae involving partners of the two taxa (assigned by body length). Instead all pairs observed consisted of conspecific males and females. Morphometry of field caugh t adults of A. najas and A. cinereus yielded a clearly bimodal distrib ution of body length with only very few individuals of intermediate si ze. Therefore, body length alone allows reasonable separation of the t wo taxa in natural populations. Although F1-hybrids from laboratory re arings (photoperiod 18L:6D, temperature approximately 23-degrees-C) di ffered only marginally in body length from A. najas reared under the s ame conditions, backcrosses and also F2-hybrids were of intermediate s ize. In the field individuals of both taxa are predominantly wingless, but both hybrids and backcrosses from the laboratory showed a net shi ft in morph ratio with many longwinged specimens. Vertical starch gel electrophoresis of three monomorphic (Apk, Got-2, Mdh-2), six polymorp hic loci with low levels of polymorphism (Idh-1, Fum, Got-1, Mdh-1, Me , 6-Pgd) and three highly polymorphic loci (Es-4, Idh-2, Pgm) suggests that the two taxa do not share a common gene pool. Gene differentiati on among all A. cinereus populations as measured by Nei's G(ST) values is very low. Between Tyrrhenian and continental A. najas populations, however, gene differentiation is very high, and in the same order of magnitude as gene differentiation between the two taxa A. cinereus and A. najas in sympatric populations. Our field observations, together w ith morphometric data, laboratory rearings, and information on wing po lymorphism, suggest that hybridization in natural populations is rare.