Molecular and Morphological Characters Discriminate Sitophilus oryzae and S. zeamais (Coleoptera: Curculionidae) and Confirm Reproductive Isolation

Abstract

The Rice Weevil, Sitophilus oryzae (L.), and the maize weevil, S. zeamais Motschulsky, are sibling species of grain weevils that are usually distinguished by grain preferences and subtle differences in morphology. Previous findings of successful laboratory hybridization, genetic similarity of allozymes and chromosomes, and identity of aggregation pheromones raised questions about the validity of S. oryzae and S. zeamais as reproductively isolated biological species. We used molecular techniques to test the hypothesis that individuals assigned as S. oryzae or S. zeamais by morphological criteria represent members of 2 distinct gene pools, and hence are reproductively isolated species. Weevils from 18 different localities, which were collected from Africa, Australia, Asia, the south Pacific, and North America, were studied. All individuals were scored for the presence or absence of morphological characters that have been used historically for species determination. In most cases, determinations for the same individual were not consistent, depending on the morphological character used. The polymerase chain reaction (PCR) was used on the same specimens to analyze randomly amplified polymorphic DNA (RAPD-PCR) markers and to amplify selectively regions of mitochondrial DNA for analysis of restriction site polymorphisms with restriction endonucleases (RFLP-PCR). Both methods yielded markers that were consistently associated with either presence or absence of specific genitalic characters in both males and females. This correlation of molecular markers with genitalic morphotypes was consistent in all specimens studied, whether collected sympatrically from the same farms or from widely separated geographic populations, and supports a model of 2 reproductively isolated species. Other morphological characters involving pronotal punctures proved unreliable as correlates with genetic markers and are not useful for species diagnosis.