PATTERNS OF POSTMATING REPRODUCTIVE ISOLATION IN A NEWLY DISCOVERED SPECIES PAIR, AQUARIUS-REMIGIS AND AQUARIUS-REMIGOIDES (HEMIPTERA, GERRIDAE)

Aquarius remigoides Gallant and Fairbairn has recently been described as specifically distinct from A. remigis (Say) based upon genetic and morphological data. Both species are common, semiaquatic bugs (Hemipte ra; Gerridae) found on the surface of streams and small rivers. Allozy me studies have shown them to be more distantly related than most othe r congeneric species in the Gerridae, with significant barriers to gen e flow where their ranges abut. We assess postmating reproductive isol ation between A. remigoides and A. remigis, using a bracket cross desi gn with five sampling sites along a north-south dine traversing the hy brid zone. We also report the results of long-distance conspecific cro sses of A. remigis, using populations from California and Quebec. Neit her these nor the intraspecific bracket crosses reveal any evidence of isolation by distance within species. However, heterospecific crosses show significantly reduced fertility, hatching success, survival of b oth sexes to eclosion (final moult) and percentage of males. Egg produ ction is not influenced by cross type, and we found no evidence of hyb rid sterility in either sex. Analyses of genotypic frequencies at thre e isozyme loci in eight hybrid and 22 pure populations reveal signific ant deficiencies of heterozygotes in hybrid populations. The proportio n of males is also significantly lower in hybrid populations than in p ure populations. The laboratory and field results indicate that postma ting isolation between these two species occurs in the form of severe reductions in both the fertility of heterospecific crosses and the via bility of hybrids, particularly hybrid males. Genetic mechanisms for H aldane's rule and asymmetries in the reciprocal heterospecific crosses are discussed.