Assortative mating preferences between colour morphs of the endemic Lake Tanganyika cichlid genus Tropheus

Female mate preferences effectuate reproductive isolation among and sexual selection within species. Both mechanisms have been associated with the diversification and speciation of cichlid species flocks of the East African Great Lakes. In Lake Tanganyika, the endemic genus Tropheus has diversified into > 100 geographic colour morphs. Although distributed allopatrically at present, water level fluctuations have repeatedly displaced and merged the benthic, rock-dwelling populations. Tests for assortative mating were performed to explore the potential for reproductive isolation between morphs in secondary contact, and to assess the importance of sexual selection for the diversification of this group. In contrast to other haplochromine cichlids, Tropheus is a sexually monochromatic, territorial and maternally mouthbrooding fish, which establishes temporary pair bonds prior to spawning. Female mate preference trials involved two-way choices between a homotypic and a heterotypic male and were conducted on allopatric populations of red and blue morphs from the southern part of Lake Tanganyika. Female affiliation time near each male's compartment did not predict the mate preferences subsequently expressed in unrestrained interactions after removal of the compartment separators (spawning, pseudospawning and courtship). Consequently, mate preferences were inferred from unrestrained interactions with one test male at a time in replicate observation sessions. Of the 23 females tested, 13 courted, pseudospawned or spawned with the homotypic male, one blue female courted a red male, and nine females expressed no sexual motivation. The assortative mate preferences in the experiments (P < 0.01) suggest that colour differentiation between Tropheus populations can effectuate reproductive isolation, and is consistent with the notion that sexual selection contributed to the diversification of the genus.