Integration and evolution of the cichlid mandible: the molecular basis of alternate feeding strategies

Abstract

African cichlid fishes have repeatedly evolved highly specialized modes of feeding through adaptations of their oral jaws. Here, we explore the molecular genetic basis of the opening and closing lever mechanisms of the cichlid lower jaw, which have traditionally been used to describe the mechanics of feeding behavior in bony fishes. Quantitative genetic analyses demonstrate that the opening and closing mechanisms are genetically modular and therefore free to evolve independently. Bmp4 (bone morphogenetic protein 4) is one of two loci that segregate with the mechanical advantage of closing and that together account for >30% of the phenotypic variance in this trait. Species-specific differences in jaw shape are obvious early in cichlid larval development and are correlated with patterns of bmp4 expression in the mandibular primordium. When bmp4 is overexpressed in the obligate suction feeder Danio rerio, mandibular morphology exhibits specific transformations of opening and closing lever ratios. We conclude that patterns of morphological integration of the cichlid jaw reflect a balance among conflicting functional demands. Further, we demonstrate that bmp4 has the potential to alter mandibular morphology in a way that mimics adaptive variation among fish species.

Fig. 1.

Genetic basis of jaw opening and closing in cichlids. (a) Lever systems of the lower jaw. The out-lever is shown in blue, the closing in-lever is shown in purple, and the opening in-lever is shown in green. (b) Cichlids linkage groups that show significant associations with functional morphology of the jaw. Bars indicate regions exceeding the 95% genome-wide significance threshold for the corresponding QTL. Colors represent QTL affecting different traits. (c–e) Genetic correlation of traits in the F₂ population. (c) The out-lever is negatively correlated with the closing in-lever. (d) The out-lever is also negatively correlated with the opening in-lever, although the correlation coefficient is significantly less than that for the closing in-lever (q = 4.21; P < 0.01; ref. 40). (e) Closing and opening in-levers are not correlated in the F₂ population.

Fig. 2.

Venn diagram depicting regional differences in the level of integration of the cichlid mandible. The out-lever and closing in-lever share 2/3 QTL and exhibit high levels of integration. The out-lever and opening in-lever only share 1/5 QTL and exhibit low levels of integration. The closing and opening in-levers have no QTL in common (0/8) and are genetically decoupled.

Fig. 3.

Ontogeny of biomechanical lever systems in cichlids. (a) Adult LF and MZ (>1 year) show significant differences in the mechanical advantage of closing and opening (n = 29). (b and c) This difference is evident in juveniles at 21 dpf (n = 6) (b) and in larvae as early as 7 dpf (n = 10) (c).

Fig. 4.

LF and MZ embryos exhibit different levels of bmp4 expression in the mandibular arch. (a) At the high-pec stage, MZ pharyngula express bmp4 at the distal tip of the first arch (red arrowhead). In Left, the black arrows indicate the first and second pharyngeal arches. In Right, the black arrow indicates the mandibular arch. (b) Similarly staged LF embryos express bmp4 throughout the mandibular mesenchyme (red arrowhead). In Left, the black arrow indicates the second arch. In Right, the black arrow indicates the mandibular arch. Differences in bmp4 expression were not size-dependent. At no stage did MZ embryos show a level of mandibular bmp4 expression comparable to what was observed in LF. (Scale bars, 20 μm.)

Fig. 5.

Bmp4 expression and jaw morphogenesis in the zebrafish, D. rerio.(a) Expression of zebrafish bmp4 at 36 h postfertilization. Throughout much of the pharyngula period of embryonic development, bmp4 was expressed in the developing heart (h), ear (otic vesicle, ov), and pectoral fin (pf). However, very little bmp4 was detected in the mandibular arch (black arrowhead). (b) In 6-dpf wild-type embryos, Meckel's cartilage (Mk) possessed a distinct retroarticular process (rp) but lacked a discernable coronoid process (cp).

Fig. 6.

Distinct morphological transformation of Meckel's cartilage (Mk) by bmp4 overexpression. Landmark-based morphometric analysis revealed concomitant growth of the coronoid and retroarticular processes in 6-dpf zebrafish larvae when embryos were injected with 100 or 150 ng/μl translation-competent bmp4 mRNA at the one- to two-cell stage, compared with gfp-injected negative control embryos.