Genetic differentiation in Tetranychus urticae (Acari : Tetranychidae): polymorphism, host races or sibling species ?

Based on allozyme electrophoresis at the Pgm locus and nuclear ribosomal DN A (ITS2) sequences, we studied the genetic variation of the two-spotted spi der mite Tetranychus urticae Koch collected on rose bay, Nerium oleander L. (Apocynaceae), from several localities around the Mediterranean basin. In addition, we compared these results with those of Navajas et al. (1998) and Tsagkarakou (1997) who collected from several other host plants from the M editerranean. In the western part of this area (Spain, France, Tunisia), we found the individuals collected from rose bay to be clearly genetically di fferentiated from other samples. No evidence of such host-associated differ entiation was detected in the eastern Mediterranean (Italy and Greece). The genetic differentiation of mites collected on rose bay was investigated fu rther by studying the reproductive incompatibilities between populations in Greece and in France and a laboratory strain reared on bean, Phaseolus vul garis, in France. Reciprocal crosses performed between these strains reveal ed variable levels of incompatibility, spanning from partial to complete re productive isolation. In all cases incompatibility was asymmetric. We desig ned a test based on double-mating to establish the fertilization status of females in fully incompatible crosses. These crosses showed that the female s had been inseminated, which suggests that the barrier to reproduction is not of a prezygotic behavioral nature. The data raises the question of the relative role of ecological factors (host plant) and geographical distance, in the ongoing differentiation process potentially leading to speciation.