Interspecific Fertile Hybrids of Haplochromine Cichlidae (Teleostei) and Their Possible Importance for Speciation

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Abstract
Hybridization tests were carried out with the allopatric and sympatric haplochromine cichlids Haplochromis burtoni (Lake Tanganyika), H. nubilus (L. Victoria), H. elegans (L. George) and H. "black lividus" (L. Victoria). Female H. nubilus crossed with male H. burtoni produce lethal hybrids which do not survive the larval stage. The reciprocal cross, female H. burtoni x H. nubilus, produces F1, F2, F3 hybrids with a normal sex ratio, and fertile backcrosses F1 x H. burtoni and F1 x H. nubilus with a skewed sex ratio. In behavioral tests female H. nubilus spawn readily with H. burtoni males (lethal cross), whereas female H. burtoni spawn rarely with male H. nubilus (fertile cross). The inheritance of coloration, the "egg-dummies" pattern on the anal fin, and reproductive vigor in the hybrids are analysed in detail. Crosses which involve male H. elegans x female H. nubilus and male H. "black lividus" x female H. nubilus are fertile but have skewed sex ratios in favor of the females. The importance of environmental instability, learning and hybridization in cichlid fish speciation are discussed.

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