Novel Jaw Morphology in Hybrids between Pseudotropheus zebra and Labeotropheus fuelleborni (Teleostei: Cichlidae) from Lake Malawi, Africa

Interspecific hybridization among haplochromine cichlids of the East African Great Lakes has been proposed as a mechanism for the evolution of new taxa in these highly speciose faunas; however, no obviously hybrid individuals have been collected, nor is it clear whether one could recognize hybrids if they did occur. Here we report on the morphology of experimentally produced F, hybrids between Pseudotropheus zebra and Labeotropheusfuelleborni, two common Lake Malawi rock-dwelling haplochromine taxa (mbuna) that differ substantially in trophic morphology. We qualitatively describe the morphology of hybrids, and quantitatively analyze 13 descriptors of oral morphology using both univariate and multivariate procedures. Hybrids, although morphologically distinct, more closely resemble P. zebra than L.fuelleborni. Three individual oral jaw characters are intermediate in, and thus diagnostic of, hybrids; however, hybrids display unique patterns of expression for five characters. Discriminant function and canonical variates analysis clearly separate all three groups. The position of hybrids along canonical variate II suggests that hybrids are neither intermediate nor mosaic but rather display a novel multivariate phenotype. Our results suggest that (1) interspecific hybridization between these taxa does not occur under current conditions in Lake Malawi, (2) the genetic control of mbuna trophic morphology may involve a significant dominance component, and (3) hybrids (when produced) may lie along an ecomorphological trajectory distinct from that separating parents. As a consequence, we propose that phenotypic shifts in cichlid populations may be discontinuous and involve limited regulatory genetic change and that hybridization (if of historical importance) could result in the production of novel phenotypes with unique evolutionary potentials.