Experimental evidence of host race formation in Mitoura butterfl ies ( Lepidoptera : Lycaenidae )

A population of herbivorous insects that shifts to a novel host can experience selection pressures that result in adaptation to the new resource. Host race formation, considered an early stage of the speciation process, may result. ! e current study investigates host shifts and variation in traits potentially involved in the evolution of reproductive isolation among populations of the juniper hairstreak butterfl y, Mitoura gryneus . Mitoura are closely associated with their host trees (Cupressaceae) and exhibit host plant fi delity: in addition to larval development and oviposition, host trees support male leks and mating. Female oviposition preference for the natal host, and diff erential fi tness of larvae when reared on natal versus alternate hosts, are indications that specialization and local adaptation to the natal host plant are occurring. Populations with single host plant associations ( Juniperus ashei , J. pinchotii and J. virginiana ) as well as populations with multiple hosts (both J. ashei and J. pinchotii ) were examined. Concordance between female preference and larval performance was found for J. ashei -associated populations. Population-level variation in the patterns of female preference and larval performance, both within and among host associations, may refl ect diff erences in the timing and direction of colonization of hosts. For a single nominal species that otherwise exhibits no morphological or phenological diff erences, the experimental assessment of specialization and host fi delity in M. gryneus provides strong support for the hypothesis of ongoing host race formation in these butterfl ies. Oikos 120: 1165–1174, 2011 doi: 10.1111/j.1600-0706.2010.19290.x © 2011 ! e Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Daniel Gruner. Accepted 24 November 2010