Evidence of a fixed polymorphism of one-year and two-year larval growth in the myrmecophilous butterfly Maculinea rebeli

1. Maculinea (= Phengaris) species of butterfly have complex life-styles in which the larvae feed briefly on specific foodplants before entering the final instar, after which they live as social parasites within Myrmica ant colonies and acquire 98–99% of their ultimate biomass by either preying on ant brood (predatory species) or being fed by worker ants (cuckoo species). Cuckoo species demonstrate two growth forms, whereby some individuals develop to pupate in 11 months and others remain 23 months in the nest, a strategy that in theory represents the most efficient exploitation of resources within Myrmica societies. Early studies suggested that development rates in Maculinea rebeli (referred to by other authors as the xerophilous form of Maculinea alcon) were not plastic but represented an unusual fixed polymorphism, in which eggs were genetically predetermined as 1- or 2-year larvae. 2. We tested this idea in lab experiments using M. rebeli from the French HautesAlpes, with these results: (i) the growth strategy of every individual larva is predetermined by the mother either genetically or, less probably, during oogenesis. (ii) Intra-nest competition and host fitness affects survival and growth but does not influence the inherent growth strategy. (iii) The ratio of 1- and 2-year developing larvae is initially about 50:50, although differential survival in crowded nests may alter the ratio of surviving individuals. (iv) All females tested laid eggs that hatched into both 1- or 2-year developers. (v) Growth patterns vary between different regions. Larvae from the Spanish Pyrenees initially grow less than those from the Alps, with a smaller initial differentiation between the size classes