Beet Aphid Species Differentiation And Bean Aphid Complex Taxonomic Status

Aphis fabae Scopoli and Aphis craccivora complex belong to the Black aphid group and Black backed aphid group of genus Aphis (Aphidinae, Aphididae) respectively. Both of them are important agroforestry pests. Aphis fabae mainly dominates the temperate region of the Northern Hemisphere. It is also found in some areas of South America and African. Aphis craccivora complex are widespread throughout the world.The lack of valid morphological characters is a large obstacle for their classification. Most of them are identified by their host plants. So different taxonomist have different views on their state. Remaudiere and Remaudiere (1997) divided the Aphis fabae into 6 subspecies,including Aphis fabae fabae Scopoli, Aphis fabae cirsiiacanthoidis Scopoli, Aphis fabae evonymi Fabricius, Aphis fabae mordvilkoi Borner and Janich, Aphis fabae solanella Theobald and Aphis fabae eryngii E. E. Blanchard. There has been controversy about identifying them by their plants. Zhang et al. (1981) divided Aphis craccivora complex into 6 species/subspecies, including Aphis sophoricola Zhang, Aphis atrata Zhang, Aphis craccivora craccivora Koch, Aphis craccivora usuana Zhang, Aphis robiniae robiniae Macchiati and Aphis robiniae canavaliae Zhang. However, Remaudiere and Remaudiere (1997) thought the other species belonged to Aphis craccivora Koch except Aphis sophoricola Zhang. Combined morphological characters, molecular data and host plants, the differentiation of Aphis fabae and the state of Aphis craccivora complex are discussed here. The main conclusions are summarized as the following.1. Aphis fabaeIn this study,4 subspecies of Aphis fabae were selected, including Aphis fabae fabae, Aphis fabae cirsiiacanthoidis, Aphis fabae mordvilkoi and Aphis fabae solanella.10 important characters of 114 specimens were used in morphological comparison. The analysis indicated that the ranges of characters were highly overlapped. They are not very useful in identifying the four subspecies.34 samples (4 subspecies) were selected in mitochondrial gene analysis. The results indicated the pair-wise genetic distances of mitochondrial cytochorme b (Cytb) and cytochrome oxidase I (COI) sequences are from 0.00%～1.30%. The range of intraspecies sequences divergences of them was from 0.00%～0.03%. Phylogenetic analysis revealed that the 4 subspecies clustered into two clades with high support (bootstrap value 100%). The genetic distances between the two clades are 1.10%～1.30%. One of the clades included Aphis fabae fabae. Aphis fabae mordvilkoi and Aphis fabae solanella. The other clade was mainly Aphis fabae cirsiiacanthoidis. While combined morphological characters and molecular data analysis indicate that Aphis fabae may be on the way to differentiation, the grouping into 4 subspecies is not supported.2. Aphis craccivora complex6 species/subspecies of Aphis craccivora complex were selected for this study.11 important characters (182 specimens) were used in morphological comparison among samples from various host plants. Discriminant analysis and principal component analysis implied all of these morphological characters were useless to discriminate them. In addition,67 specimens were selected for analysis of their pair-wise genetic distances. The results indicated the intraspecies sequences divergences in the Aphis craccivora complex is from 0.00% to 1.30% for COI gene (DNA Barcoding fragment),0.00%～1.70% for the other fragment of COI gene,0.00%～1.00% for COII and Cytb gene. The results also indicated that the mean intraspecific sequences divergences of the four genes were lower than 0.50%. The 6 species/subspecies of Aphis craccivora complex assembled a clade with high support (bootstrap value 99%～100%) in NJ and maximum-parsimony (MP) analysis. Significant relationships between host plants and the species/subspecies of Aphis craccivora complex have not been found. In conclusion, current results do not justify the separation of Aphis craccivora complex into 6 sepcies/subspecies. It is suggested that the 6 sepcies/subspecies should be synonymized as a single species—Aphis craccivora Koch.