Molecular phylogenetics and reproductive incompatibility in a complex of cryptic species of aphid parasitoids.

We infer the phylogeny of a complex of cryptic species and populations of parasitic wasps and examine how reproductive incompatibility maps onto the molecular phylogeny. We used four nuclear (28S-D2, ITS1, ITS2, ArgK) and two mitochondrial (COI, COII) gene regions to analyze relationships among populations in the Aphelinus varipes species complex (Hymenoptera: Aphelinidae) from throughout Eurasia (France, Georgia, Israel, China, Korea and Japan) and from three aphid hosts (Aphis glycines, Diuraphis noxia and Rhopalosiphum padi; Hemiptera: Aphididae). A combined analysis of 21 genotypes of Aphelinus resulted in six most-parsimonious trees, and successive approximations character-weighting selected two of these as best supported by the data. All six gene regions were necessary to fully resolve the relationships among taxa. Four clades within the A. varipes complex were distinguished: (1) Aphelinus kurdjumovi, (2) Aphelinus hordei, (3) Aphelinus atriplicis, Aphelinus varipes, and Aphelinus albipodus, and (4) Aphelinus certus (populations from China, Korea, and Japan). Based on rates of nucleotide substitutions, these clades diverged between 78 and 526 thousand years ago during a period of repeated glaciations in Eurasia. In laboratory crosses, A. kurdjumovi, A. hordei, and A. varipes were reproductively incompatible with one another and all other populations. A. atriplicis was incompatible with these three species, but not with A. certus. The populations of A. certus from China, Japan, and Korea were reproductively compatible with one another but not with the other populations. Thus, with one exception, entities that were phylogenetically distinct were also reproductively incompatible with one another. Our evidence on molecular differentiation and reproductive incompatibility supports recognition of at least five cryptic species in the A. varipes complex. We discuss likely reasons for the high rate of speciation in this complex.