Host-parasite cospeciation, in which parasite divergence occurs in response to host divergence, is commonly proposed as a driver of parasite diversification, yet few empirical examples of strict cospeciation exist. Host-parasite co-evolutionary histories commonly reflect complex mosaics of cospeciation, dispersal, lineage extinction and other phenomena. The episodic host-switching model of parasite diversification accounts for complexity by suggesting that diversification and faunal assembly is a consequence of fluctuation between environmental disruption and environmental stability. The phylogeographic predictions of the strict cospeciation and episodic host-switching models were tested using the North American pika/parasite assemblage, with a primary focus on the American pika, Ochotona princeps (Richardson, 1828), and a suite of its endoparasitic cestodes and nematodes. This approach integrating phylogeographic and demographic methods with inferences drawn from species distribution modelling revealed that the parasite community of pikas has been shaped by climate-driven range fluctuation of hosts and bouts of geographic and host colonization by parasites associated with transitions between glacial and interglacial phases.
The trichostrongylid nematode Travassostrongylus scheibelorum sp. n. from the Linnaeus' mouse opossum, Marmosa murina (Linnaeus) (type host), and the woolly mouse opossum, Marmosa demerarae (Thomas), from French Guiana is described. The nematodes have a synlophe with ridges frontally oriented from right to left, six dorsal and six ventral, at midbody; seven dorsal and seven ventral posterior to the vulva, and two cuticular thickenings within the lateral spaces; a long dorsal ray and a pointed cuticular flap covering the vulva. This is the 12th species of Travassostrongylus Orloff, 1933, which includes species featuring ridges around the synlophe and a didelphic condition. These traits contrast with those in other genera in the Viannaiidae Neveu-Lemaire, 1934, which feature ventral ridges on the synlophe of adults and a monodelphic condition. Members of the family are chiefly Neotropical and are diagnosed based on the presence of a bursa of the type 2-2-1, 2-1-2 or irregular, and cuticle without ridges on the dorsal side (at least during one stage of their development). Herein, we present a reconstruction of the ancestral states of the didelphic/monodelphic condition and the cuticular ridges that form the synlophe in opossum-dwelling trichostrongyles, namely Travassostrongylus and Viannaia Travassos, 1914. Our investigations suggest they are not reciprocal sister taxa and that the change from didelphy to monodelphy and the loss of dorsal ridges, occurred in the common ancestor of species of Viannaia. These results suggest a synlophe with three ventral ridges is not plesiomorphic in the opossum dwelling trichostrongylids.