Catherine P. Blair

CRYPTIC SPECIATION AND HOST‐RACE FORMATION IN A PURPORTEDLY GENERALIST TUMBLING FLOWER BEETLE

Abstract Host‐race formation remains controversial as a source of herbivorous insect diversity, and examples of host races are still fairly scarce. In this study, analysis of five enzyme loci in the ostensibly generalist tumbling flower beetle Mordellistena convicta (Coleoptera: Mordellidae) revealed hidden host‐plant and plant‐organ related genetic differentiation. Mordellistena convicta turned out to be a complex of cryptomorphic species, each with fewer hosts than the nominal species. These cryptic species, in turn, were divided into taxa that showed host‐race characteristics: samples from different host plants and organs exhibited (1) genetic indications of partial reproductive isolation, (2) differences in size and emergence timing that suggested divergent host‐related selection, and (3) among‐host selective differences in mortality from parasitoids. Host‐race formation in M. convicta, which has a somewhat different life history from the well‐studied host races, enlarges the group of insects considered likely to undergo this process. The widespread sympatry of the M. convicta species complex, along with its spectrum of host‐correlated genetic differentiation, suggests that these host specialist taxa developed in sympatry.

Sequential radiation of unrelated organisms: the gall fly Eurosta solidaginis and the tumbling flower beetle Mordellistena convicta

Host shifts and the formation of insect‐host races are likely common processes in the speciation of herbivorous insects. The interactions of goldenrods Solidago (Compositae), the gall fly Eurosta solidaginis (Diptera: Tephritidae) and the beetle Mordellistena convicta (Coleoptera: Mordellidae) provide behavioural, ecological and genetic evidence of host races that may represent incipient species forming via sympatric speciation. We summarize evidence for Eurosta host races and show that M. convicta has radiated from goldenrod stems to Eurosta galls to form host‐part races and, having exploited the gallers host shift, has begun to differentiate into host races within galls. Thus, host‐race formation has occurred in two interacting, but unrelated organisms representing two trophic levels, resulting in ‘sequential radiation’ (escalation of biodiversity up the trophic system). Distributions of host races and their behavioural isolating mechanisms suggest sympatric differentiation. Such differentiation suggests host‐race formation and subsequent speciation may be an important source of biodiversity.

ONE HOST SHIFT LEADS TO ANOTHER? EVIDENCE OF HOST-RACE FORMATION IN A PREDACEOUS GALL-BORING BEETLE

Abstract.— We show that a predator, the tumbling flower beetle Mordellistena convicta (Coleoptera: Mordellidae), has formed host races in response to a host‐plant shift and subsequent host‐race formation by its prey, the gall‐inducing fly Eurosta solidaginis (Diptera: Tephritidae). This fly has formed two host races, one that induces stem galls on the ancestral host plant, Solidago altissima (Compositae), and another that induces stem galls on the closely related S. gigantea. We found that subpopulations of M. convicta that attack E. solidaginis galls on the different host plants have significantly different emergence times and, although slight, these allochronic differences are consistent across a range of temperatures. More importantly, we found that beetles assortatively mate according to their natal host plants, and female M. convicta preferentially attack and/or their offspring have higher survival in galls on natal host plants. Our data suggest that subpopulations of M. convicta that attack E. solidaginis galls on S. altissima and S. gigantea have formed host races. This is one of the first studies to demonstrate that a host shift and subsequent host‐race formation by an herbivorous insect may have resulted in subsequent diversification by one of its natural enemies.

Testing the nutrition hypothesis for the adaptive nature of insect galls: does a non-adapted herbivore perform better in galls?

Abstract 1. The nutrition hypothesis for the adaptive nature of galls states that gall‐inducing insects control the nutrient levels in galls to their own benefit. Although the nutrition hypothesis is widely accepted, there have been few empirical tests of this idea.

Nutrition as a facilitator of host-race formation: the shift of a stem-boring beetle to a gall host.

1. The importance of host‐race formation to herbivorous insect diversity depends on the likelihood that successful populations can be established on a new plant host. A previously unexplored ecological aid to success on a novel host is better nutritional quality. The role of nutrition was examined in the shift of the stem‐boring beetle Mordellistena convicta to fly‐induced galls on goldenrod and the establishment there of a genetically distinct gall host race.

The role of olfactory cues in the sequential radiation of a gall-boring beetle, Mordellistena convicta

1. Herbivorous insects often have close associations with specific host plants, and their preferences for mating and ovipositing on a specific host‐plant species can reproductively isolate populations, facilitating ecological speciation. Volatile emissions from host plants can play a major role in assisting herbivores to locate their natal host plants and thus facilitate assortative mating and host‐specific oviposition.