Two Species within Dendroctonus frontalis (Coleoptera: Curculionidae): Evidence from Morphological, Karyological, Molecular, and Crossing Studies

Dendroctonus frontalis Zimmermann is considered one of the most important economic and ecological forest pests in the United States, Mexico, and Central America. Recently, two apparent morphological variants of this species were discovered occurring syntopically in Central America and southern Mexico. Morphotype A beetles lack a series of fine parallel ridges on the episternal area of the prothorax that are present on morphotype B. The goal of the present work was to clarify the taxonomic status of the morphotypes of the D. frontalis species complex. Geometric morphometric analyses of seminal rod and spermatheca shape together with the characterization of 16 attributes of external morphology revealed differences in quantitative and qualitative characters that distinguished adults of the two morphotypes from each other as well as from the closely related species Dendroctonus vitei Wood and Dendroctonus mexicanus Hopkins. Karyotype analysis of morphotype B revealed a chromosomal formula (5AA + Xyp) distinct from that found in morphotype A previously reported for D. frontalis (7AA + Xyp). In the laboratory, forced intermorphotype crosses produced F1 progeny but at lower frequency than intramorphotype pairings, and dissections of spermatheca revealed a lower frequency of insemination at least one type of heterotypic cross. Phylogenetic analysis of the D. frontalis species complex based on 786 bp of the cytochrome oxidase I gene indicated that morphotypes B and A are two independent groups with 98% nodal support within D. frontalis. These data provide compelling evidence that the two syntopic morphotypes represent two distinct sibling species.