Distinct species or colour polymorphism? Life history, morphology and sequence data separate two Pyrrhalta elm beetles (Coleoptera: Chrysomelidae)

The elm leaf beetles Pyrrhalta maculicollis (Motschulsky, 1853) and P. aenescens (Fairmaire, 1878) (Chrysomelidae: Galerucinae) are considered to be two separate species that occur in sympatry and synchrony on the same plant host. Conventionally they are distinguished by differences in elytron and pronotum colour, but this variation could just represent intra-specific polymorphism. To test the status of P. maculicollis and P. aenescens we compared their biology, larval and adult morphological characters, and molecular genetics. Using laboratory-bred cohorts of either colour type, biological and morphological analyses showed great similarities in life cycle, body size, external morphology of eggs and pupae, and the presence of spines on the internal sac of the aedeagus not seen in other species of Pyrrhalta. However, both groups showed consistent differences in colour patterns, tubercles and setae of the larvae, while the number of spines on the internal sac also differed consistently. The molecular phylogenetic analysis based on mitochondrial (COI, COII) and nuclear (ITS2) markers recovered both colour types as deeply separated, reciprocally monophyletic lineages, and in addition found P. maculicollis to be split into two divergent subgroups by both markers. A time-calibrated tree that included several further species of Pyrrhalta and the related Galerucella indicated that P. maculicollis and P. aenescens separated in the Miocene, at around 7.5 Ma (95% CI, 9.5-5.6 Ma).