© 2007 The Authors Journal compilation © 2007 The Royal Entomological Society 515 Insect Molecular Biology (2007) 16(5), 515–524 doi: 10.1111/j.1365-2583.2007.00746.x Blackwell Publishing Ltd Genetic basis for reproductive diapause is correlated with life history traits within the Culex pipiens complex A. Mori, J. Romero-Severson and D. W. Severson Center for Global Health and Infectious Diseases, Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, USA Abstract The evolution of late season reproductive arrest (diapause) among female Culex pipiens mosquitoes allows them to overwinter in temperate climates, while females of the sibling species Culex quinquefasciatus do not exhibit the diapause phenotype. We present results for quantitative trait loci (QTL) analyses of two independent segregating populations derived from crosses between C. pipiens and C. quinquefasciatus. QTL for diapause and three life history traits were identified and compared for genome positions and gene effects. Using a combination of composite interval mapping, single-marker analysis and all possible subsets regression, we identified multiple QTL for each trait, totalling 14 and 17 QTL for each population, respectively. Individual QTL across traits often mapped to similar genome locations, suggesting these traits may be controlled in part by genes with pleiotropic effects or multiple tightly linked genes. The majority of QTL were intermediate in magnitude in that they explained 10–25% of the phenotypic variation. The majority of QTL showed overdominance effects. We suggest that this could impact natural populations, as increased heterosis across hybrid zones may allow populations to adapt to environmental conditions via stabilizing selection, and yet maintain species identity outside these regions because of strong morphological integration, wherein related traits evolve as an integrated unit. Keywords: heterosis, overdominance, photoperiod, QTL, West Nile virus. Introduction Diapause is a state of programmed developmental arrest that facilitates survival under adverse environmental conditions. Individuals in diapause exhibit characteristic changes in morphology, physiology and behaviour com- pared to nondiapausing individuals (Tauber et al., 1986; Danks, 1987). Adult diapause is a common adaptation among some insects for overwintering in temperate climates, and involves arrested reproductive development, typically evidenced in females as delayed ovary development following adult eclosion (Danks, 1987). Diapause status in receptive individuals is controlled by the interaction between environmental cues, namely photoperiod and temperature, and insect hormone activity (Beck, 1980). Short daylength coupled with low ambient temperatures result in reduced juvenile hormone secretion by the corpus allatum, which suppresses reproduction (Denlinger, 1985). The Culex pipiens complex represents a group of closely related mosquito species that are distributed nearly world- wide (Vinogradova, 2000). The major species are delineated largely by their geographical distribution, with C. pipiens (identified as C. pipiens pallens in the Far East) largely associated with temperate zones and C. quinquefasciatus largely associated with tropical zones. Members of the C. pipiens complex have been implicated as primary vectors for the West Nile virus in North America (Fonseca et al., 2004), and diapausing females can serve as reservoirs for West Nile virus to re-initiate vertebrate infections in the spring (Nasci et al., 2001; Farajollahi et al., 2005). Inter- species hybrids are known to occur in areas where their distributions overlap (Barr, 1957; Jakob et al., 1979, 1980; Tabachnik & Powell, 1983; Pryor & Daly, 1991; Cornel et al., 2003; Fonseca et al., 2004). Investigations of hybrid zones have confirmed that, although significant introgres- sion occurs, fitness clines largely related to temperature gradients are evident (Urbanelli et al., 1995, 1997; Humeres et al., 1998). Culex pipiens females are induced into ovarian diapause, wherein ovarian follicles remain largely undeveloped, by decreasing photoperiod and low ambient temperatures in late summer or autumn (Eldridge, 1968; Sanburg & Larsen, 1973). Additionally, females in diapause are characterized Received 6 December 2006; accepted following revision 5 February 2007; first published online 17 July 2007. Correspondence: Dr David W. Severson, 137 Galvin Life Sciences, University of Notre Dame, Notre Dame, IN 46556–5645, USA. Tel.: +1 574 631 3826; fax: +1 574 631 7413; e-mail: severson.1@nd.edu