The colour/pattern polymorphism of Philaenus spumarius (L.) (Homoptera: Cercopidae) in England and Wales

Abstract

The spittlebug Philaenus spumarius is a common and widespread Palaearctic insect exhibiting a striking dorsal colour/pattern polymorphism, which is conveniently divisible into eight ‘melanic’ and five ‘non-melanic’ phenotypes. The polymorphism is controlled by seven alleles at a single autosomal locus with complete or partial female-limitation and variable penetrance in males of certain melanic phenotypes. It is a universal polymorphism but, despite much investigation, little is known of the factors which maintain it or influence morph frequency. Patterns of geographic variation are useful in both these contexts. This study presents an extensive survey of morph frequency variation in 548 populations of P. spumarius across England and Wales. Frequencies of individual phenotypes within the melanic category vary heterogeneously; frequency variation for a ‘core’ group of five phenotypes is broadly in parallel, whereas the other three vary independently. Furthermore, the degree of penetrance in males varies between the melanic phenotypes, which in turn varies between populations; one consequence is that higher melanic frequencies are associated with enhanced expression of certain melanic phenotypes in males. Comparisons are made with previously reported populations around a point source of serious gaseous and particulate pollution in the Cynon Valley, South Wales and with intensively studied populations in southern Finland. The data indicate a pronounced ‘urban effect’ on morph frequencies, with populations in major conurbations exhibiting higher mean melanic frequencies ( x̄ = 19.1%) than rural ones ( x̄ = 7.3%). Populations around three (of the eight investigated) point sources of atmospheric pollution show elevated melanic frequencies, although all are substantially below those recorded from the Cynon Valley Although Finnish populations exhibit substantially higher melanic frequencies than analogous rural populations in Britain the strict female-limitation of certain melanic phenotypes in Finland is relaxed in British populations, where male melanics are approximately 20-fold more frequent. The universality and stability of this polymorphism strongly suggest the overriding influence of natural selection in its maintenance. The predominance of the phenotypes GIB and LCE (mostly black, but with pale heads) over the completely black form (LOP) in populations with the highest overall melanic frequencies demonstrates that the selective agent(s) favouring melanics in intensely polluted areas cannot be operating simply for melanism per se . The near-equality of melanic frequencies between the sexes in the majority of populations reported here suggests that the model of genetic control established for populations in the Cynon Valley also applies to most British populations and remains distinct from the model applying to Finnish populations.