Evolution of host- and habitat association in the wood-feeding cockroach, Cryptocercus

Members of the cockroach genus Cryptocercus are subsocial insects that live in temperate forests and feed on decomposing logs. At present, seven species are recognized worldwide: four in the eastern USA, one in the western USA, and one each in Russia and China. Genetic variation within and among the Nearctic species has been characterized extensively in previous studies. However, whether there has been a corresponding divergence in the host and habitat association of Cryptocercus species is not known. Here, we report on differences in host and habitat association among six of the seven Cryptocercus species, estimated from field observations, elevation data, and land cover data. Our results indicated that the eastern and western USA species differ from one another in their distribution patterns, abundance, and habitat association. The eastern USA species are associated largely with deciduous forests, whereas the western USA and the Russian species are associated with evergreen forests. Thus, the eastern USA species, which are evolutionarily the most recent ones, have adapted to a different set of tree species relative to the basal species. There were also differences in the habitat association of the various species. Specifically, in the eastern USA, Cryptocercus darwini, evolutionarily the most recent species, occupied a habitat that is predominantly at low elevation [<400m above sea level (ASL)] while all the other Nearctic species and the Russian species occupied a habitat that is at relatively higher elevations (>400m ASL). Mapping of the above traits on a phylogenetic tree revealed that the evolutionary trend in Cryptocercus with regard to host and habitat association has been toward the utilization of low elevation habitats dominated by deciduous forests.