Collaborator: John S. Sparks (AMNH)

For biomechanical systems consisting of multiple parts, different morphological configurations can often achieve the same functional output. This is known as many-to-one mapping (MTOM) of form to function and it has potential to influence the distribution and nature of morphological diversification within clades. In cichlids, and many other ancanthomorph fishes, the mechanics of the anterior jaw can be modeled with a fourbar linkage (Fig. 1-left), which has been shown to display MTOM with respect to a functional output, maxillary kinematic transmission (MKT).

jaw fucntional phylomorphospace
Figure 1. Fourbar linkage (left) used to evaluate form and function of the oral jaw. PCs 1 and 2 (right) displaying variation in jaw shape in Malagasy cichlids, with the functional trait, maxillary kinematic transmission (MKT) mapped in the background. Contour lines represent fourbar shapes with equivalent MKT, displaying MTOM of form to function.

For this project, we evaluated evolutionary relationships between body shape (Fig. 2) and jaw functional morphology (Fig. 1) in two subfamilies of endemic Malagasy cichlid, Etroplinae and Ptychochrominae. We were interested in testing whether the ability of jaws to vary in morphology but not function (i.e., exhibit MTOM) would influence the relationships between cranial and post cranial components of the body plan.

cichlid body phylomorphospace copy
Figure 2. PCs 1 & 2 of body shape in Malagasy cichlids, showing parallel trajectories of body shape variation (from deep to shallow-bodied) in the subfamilies Etroplinae and Ptychochrominae.

We found that the two subfamilies used different strategies to mitigate impacts of body shape variation on MKT. Ptychochromines displayed modularity, or a level of evolutionary independence, between cranial and post cranial morphologies. In contrast, etroplines maintained a strong relationship between body and jaw shapes (i.e., integration), with jaw morphologies varying in a manner that limited change in MKT (Fig 3.). This research showed that, unlike modularity, MTOM can allow traits to retain strong evolutionary covariation while still reducing impacts on functionality. These results suggest that MTOM, and its influence on the evolution of correlated traits, is likely much more widespread than is currently understood.

jaw function copy
Figure 3. Comparisons of maxillary kinematic transmission of the oral jaw fourbar linkage in Malagasy cichlids. Note the narrow range in MKT in the etroplines in comparison to the ptychochromines.

 

Relevant Publications:

Martinez CM & Sparks JS. 2017. Malagasy cichlid subfamilies differentially limit impacts of body shape evolution on oral jaw form and function. Evolution. 10.1111/evo.13298.

Martinez CM, Arroyave J & Sparks JS. 2015. A new species of Ptychochromis from southeastern Madagascar (Teleostei: Cichlidae). Zootaxa. 4044, 79-92.

 

Funding Sources:

Gerstner Scholarship & Lerner Gray Postdoctoral Fellowship (AMNH)

NSF grants, IOS-0749943 and DEB-1257555 (awarded to JSS)