Polyphagy and diversification in tussock moths: Support for the oscillation hypothesis from extreme generalists

Abstract

Theory on plasticity driving speciation, as applied to insect-plant interactions (the oscillation hypothesis), predicts more species in clades with higher diversity of host use, all else being equal. Previous support comes mainly from specialized herbivores such as butterflies, and plasticity theory suggests that there may be an upper host range limit where host diversity no longer promotes diversification. The tussock moths (Erebidae: Lymantriinae) are known for extreme levels of polyphagy. We demonstrate that this system is also very different from butterflies in terms of phylogenetic signal for polyphagy and for use of specific host orders. Yet we found support for the generality of the oscillation hypothesis, in that clades with higher diversity of host use were found to contain more species. These clades also consistently contained the most polyphagous single species. Comparing host use in Lymantriinae with related taxa shows that the taxon indeed stands out in terms of the frequency of polyphagous species. Comparative evidence suggests that this is most probably due to its nonfeeding adults, with polyphagy being part of a resulting life history syndrome. Our results indicate that even high levels of plasticity can drive diversification, at least when the levels oscillate over time.

Keywords: Lymantriinae; Nymphalidae; host plant range; plasticity; speciation.

Figure 1

Examples of tussock moths (subfamily Lymantriinae). (a–e): adults. (a): Lymantria similis (tribe Lymantriini) (b). Pida minensis (Locharnini) (c): Euproctis conistica (Nygmiini). (d): Calliteara contexta (Orgyiini) E. Arctornis sp.(Nygmiini) (f–g): larvae. (f): Artaxa angulate (Nygmiini). (g): Arna bicostata (Nygmiini). Photo: Houshuai Wang

Figure 2

(a) Character optimization of two levels of polyphagy on a phylogeny of the tussock moths subfamily Lymantriinae from (Wang et al. 2015) (b). Character optimization of two levels of polyphagy on a phylogeny of the butterfly subfamily Nymphalinae, based on (Wahlberg, Brower, & Nylin, 2005) and (Nylin & Wahlberg, 2008), with modifications from (Long, Thomson, & Shapiro, 2014). Taxa were coded as having the state 3+ orders if at least one species feed on three orders or more, and 7+ orders if at least one species feed on seven orders or more

Figure 3

(a) Phylogenetic correlation between host diversity and species richness among genus‐level taxa of Lymantriinae moths. (b) Phylogenetic correlation between maximum species‐level polyphagy and species richness among the same taxa. (c) The same analysis as in (a), but restricted to well‐studied faunas