Downloaded By: [University of Washington] At: 23:15 20 June 2007 ORIGINAL ARTICLE Environment-specific heterozygote deficiency and developmental instability in hybrid Mytilus STEVAN A. SPRINGER 1 & DANIEL D. HEATH 2 1 Department of Biology, University of Washington, Seattle, WA 98195, USA, 2 Department of Biological Sciences and Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario N9B 3P4, Canada Abstract The multiple discrete hybrid zones that characterize Mytilus blue mussels allow a novel, non-manipulative, examination of the selective pressures that create and maintain species. If endogenous genetic incompatibility is solely responsible for post- zygotic isolation, then individuals of a specified hybrid genotype are expected to show similar average fitness across environments. However, if hybrid fitness differs across environments, then exogenous selection is implicated, either via ecological selection or environment-specific expression of intrinsic genetic incompatibilities. Correspondence between developmental instability of hybrids and heterozygote deficiency, estimated in two M. trossulus M. galloprovincialis hybrid zones on the coast of North America, indicates that environment-dependent selection against hybrids may contribute to reproductive isolation among Pacific Mytilus species. Key words: Developmental stability, environment-dependent selection, heterozygote deficiency, Mytilus Introduction Scenarios of the origin and maintenance of species implicitly assume that exogenous or endogenous selective forces determine hybrid fitness, and the two most common classes of hybrid zone models reflect this dichotomy (Barton & Hewitt 1985; Moore & Price 1993). In exogenous (ecological) selection models, the coherence of taxa is main- tained by reduced hybrid fitness due to ecological circumstance (Moore & Price 1993; Kruuk et al. 1999). In contrast, endogenous (genetic) selection models are built without environmental variables; hybrid dysfunction is caused by the fusion of incompatible genetic systems and hybrid fitness is reduced relative to parental types in all environments (Barton & Hewitt 1985; Kruuk et al. 1999). A third alternative is that genetic incompatibilities are con- ditional, and cause reduced hybrid fitness in an environment-dependent manner (Kondrashov & Houle 1994; Rundle & Whitlock 2001). The im- portance of conditional genetic incompatibility in speciation is perhaps underestimated, as most tests of reproductive isolation occur in artificial condi- tions. All three models predict heterozygote defi- ciency, relative to Hardy  Weinberg equilibrium, when selection acts to reduce hybrid fitness. How- ever, the effects of strict genetic incompatibility are expected to be relatively stable across environments (Rundle & Whitlock 2001). Examining the fitness of natural hybrids of the same parental taxa in different environments can therefore implicate ecological selection in species formation. Decreased hybrid fitness manifest as increased developmental stress in hybrid individuals is consis- tent with both exogenous and endogenous hybrid zone models. However, the cause of developmental stress differs in each case. Exogenous speciation involves divergent adaptation to different environ- ments. Thus, hybrid individuals suffer increased developmental stress as a result of reduced compe- titive ability in parental environments or increased competition among hybrid individuals (Shykoff & Møller 1999). The degree of exogenous develop- mental stress experienced by a hybrid individual depends on local environmental conditions and hybrid zones resulting from the same two parental Correspondence: S. A. Springer, Department of Biology, University of Washington, Seattle, WA 98195, USA. E-mail: stevan@u.washington.edu Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark Marine Biology Research, 2007; 3: 182  187 (Accepted 26 February 2007; Printed 11 June 2007) ISSN 1745-1000 print/ISSN 1745-1019 online # 2007 Taylor & Francis DOI: 10.1080/17451000701320564