Journal of Fish Biology (2009) 74, 1804–1815 doi:10.1111/j.1095-8649.2009.02256.x, available online at http://www.blackwell-synergy.com No barriers to gene flow among sympatric polychromatic ‘small’ Telmatherina antoniae from Lake Matano, Indonesia R. P. Walter*, G. D. Haffner and D. D. Heath Great Lakes Institute for Environmental Research, University of Windsor, 401 Sunset Ave, Windsor, Ontario N9B 3P4, Canada (Received 28 July 2008, Accepted 5 March 2009) Genetic divergence, assortative courtship and intermale aggression were assessed between sympatric colour morphs of the sailfin silverside Telmatherina antoniae, endemic to Lake Matano, Indonesia. Genetic analysis using microsatellite markers showed no barriers to gene flow among T. antoniae primary colour morphs (blue and yellow) within sampling sites, sympatric populations or at the lake- wide level. Low but significant genetic differentiation was found between yellow morphs and mixed (blue – yellow) morphs. Behavioural surveys indicated assortative courtship does occur along primary colour lines; however, intermale aggression among paired and intruding male morphs appeared equal with respect to male colour. These observations support the hypothesis that males view other males as threats to their courtship regardless of their colour. This study supports recent work suggesting that assortative mating is present in T. antoniae despite a lack of reproductive isolation among colour morphs. © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles Key words: colour; gene flow; intermale aggression; Lake Matano; microsatellites; Telmatherina . INTRODUCTION Since the formulation of the theory of natural selection, the maintenance of phe- notypic polymorphisms within a population has been a subject of great interest to biologists. The presence of colour polymorphisms within a species offers an opportunity for crypsis, predation avoidance, assortative mating and the poten- tial for sexual selection for a particular colour morph. Sexual selection, through female mating preferences or male competitive behaviour, has been implicated as a means of maintaining colour polymorphisms in natural populations (Kingston et al., 2003; Gray & McKinnon, 2007). Because many species occupy variable habitats throughout their lifetimes, their relative conspicuousness to both poten- tial mates and predators also varies (Gamble et al., 2003; Chunco et al., 2007). Frequency and density-dependent selection for particular colour morphs have been shown to maintain colour polymorphisms in differing habitats and social * Author to whom correspondence should be addressed. Tel.: +1 519 253 3000 ext. 4730; fax: +519 971 3616; email: walterj@uwindsor.ca 1804 © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles