Freshwater Biology (2007) 52, 1881–1897 doi:10.1111/j.1365-2427.2007.01813.x
Population genetic structure reveals terrestrial affinities for a headwater stream insect
DEBRA S. FINN, MICHAEL S. BLOUIN AND DAVID A. LYTLE Department of Zoology, Oregon State University, Corvallis, OR, U.S.A.
SUMMARY 1. The spatial distribution of stream-dwelling organisms is often considered to be limited primarily according to the hierarchical structure of the hydrologic network, and previous conceptual models of population genetic structure have reflected this generality. Headwater specialists, however, are confined to short upstream sections of the network, and therefore are unlikely to respond in the same way as species with a broader range of habitat tolerance. 2. Here, we propose a model to describe spatial patterns of genetic diversity in headwater specialists with a limited ability for among-stream dispersal. The headwater model predicts a partitioning of genetic variance according to higher-elevation ‘islands’ of terrestrial habitat that provide required headwater stream conditions. The model therefore expects a geographic pattern of genetic variance similar to that expected for low-dispersal terrestrial species occupying the adjacent habitat. 3. Using a 1032-bp mitochondrial DNA fragment encompassing parts of the COI and COII genes, we demonstrate that Madrean Sky Islands populations of the giant water bug Abedus herberti conform to the proposed headwater model. Furthermore, they exhibit phylogeo- graphic patterns broadly concordant with those shown for several terrestrial species in the region, including a major zone of discontinuity in the Chiricahua mountain range. 4. Overall, populations are highly isolated from one another, and a nested clade analysis suggested that A. herberti population structure, similarly to terrestrial Sky Islands species studied previously, has been influenced by Pleistocene climatic cycles causing expansion and contraction of temperate woodland habitat. 5. Because they have no ability to disperse among present-day mountaintop habitat islands, A. herberti and other headwater species with limited dispersal ability are vulnerable to the projected increasing rate of climatic warming in this region.
Keywords: Belostomatidae, headwater streams, nested clade analysis, population genetics, stream hierarchy model
1988; see also Grant, Lowe & Fagan, 2007). For aquatic Introduction organisms with limited terrestrial (trans-basin) dis- The hierarchical structure of stream networks and persal propensity, gene flow typically is thought to drainage basins provides a natural spatial framework occur primarily within versus among drainage basins for considering the effects of dispersal and resulting (e.g. Bilton, Freeland & Okamura, 2001). Under this gene flow in resident species (Meffe & Vrijenhoek, scenario, population genetic variance is expected to partition significantly according to drainage basin Correspondence: Debra S. Finn, Department of Zoology, hierarchical structure. This pattern has been demon- Oregon State University, 3029 Cordley Hall, strated in several low-dispersal stream insects, partic- Corvallis, OR 97331, U.S.A. ularly for species in mountainous systems, where the E-mail: fi[email protected] intervening steep topography is expected to render