Hybridization produces novelty when the mapping of form to function is many to one

Abstract

Background: Evolutionary biologists want to explain the origin of novel features and functions. Two recent but separate lines of research address this question. The first describes one possible outcome of hybridization, called transgressive segregation, where hybrid offspring exhibit trait distributions outside of the parental range. The second considers the explicit mapping of form to function and illustrates manifold paths to similar function (called many to one mapping, MTOM) when the relationship between the two is complex. Under this scenario, functional novelty may be a product of the number of ways to elicit a functional outcome (i.e., the degree of MTOM). We fuse these research themes by considering the influence of MTOM on the production of transgressive jaw biomechanics in simulated hybrids between Lake Malawi cichlid species.

Results: We characterized the component links and functional output (kinematic transmission, KT) of the 4-bar mechanism in the oral jaws of Lake Malawi cichlids. We demonstrated that the input and output links, the length of the lower jaw and the length of the maxilla respectively, have consistent but opposing relationships with KT. Based on these data, we predicted scenarios in which species with different morphologies but similar KT (MTOM species) would produce transgressive function in hybrids. We used a simple but realistic genetic model to show that transgressive function is a likely outcome of hybridization among Malawi species exhibiting MTOM. Notably, F2 hybrids are transgressive for function (KT), but not the component links that contribute to function. In our model, transgression is a consequence of recombination and assortment among alleles specifying the lengths of the lower jaw and maxilla.

Conclusion: We have described a general and likely pervasive mechanism that generates functional novelty. Simulations of hybrid offspring among Lake Malawi cichlids exhibiting MTOM produce transgressive function in the majority of cases, and at appreciable frequency. Functional transgression (i) is a product of recombination and assortment between alleles controlling the lengths of the lower jaw and the maxilla, (ii) occurs in the absence of transgressive morphology, and (iii) can be predicted from the morphology of parents. Our genetic model can be tested by breeding Malawi cichlid hybrids in the laboratory and examining the resulting range of forms and functions.

Figure 1

The components of the 4-bar linkage system are illustrated on cleared and stained cichlid heads of (A) Copadichromis eucinostomus (representative high KT) and (B) Chilotilapia rhoadesii (representative low KT). Yellow is the lower jaw (input) link; black is the fixed link, blue is the nasal link and green is the maxillary (output) link. Note differences in relative lengths of input and output links for these high and low KT exemplars.

Figure 2

The distribution of kinematic transmission (KT) for 169 individuals from 86 Lake Malawi species shows that the majority of individuals have KT values between 0.65 – 0.80.

Figure 3

A simple ratio of input (lower jaw) to output (maxilla) links is strongly positively correlated with the fully parameterized calculation of KT.

Figure 4

The distribution of KT values for simulated F₂ hybrids from the intercross of Cynotilapia afra and Pseudotropheus elongatus highlights an appreciable frequency (29%) of individuals transgressive for function. Arrows indicate the KT of the parents and values ± 1 or 2 SD from the mean.

Figure 5

Bubble plots demonstrate the boundary conditions of transgression for Cynotilapia afra × Pseudotropheus elongatus (A, B) and Protomelas fenestratus × Protomelas ornatus (C, D). For both crosses, transgressive KT lower than (A, C) and higher than (B, D) the parents is dependent on allelic combinations of lower jaw and maxillary links.

Figure 6

Box plots demonstrate a relationship between MTOM and transgression. Crosses that produce transgressive F₂ are between parental species with less difference in starting KT (a proxy for MTOM) than crosses that do not produce transgressive F₂ (t-test, p < 0.001). The bar is the median value, the box is the 25^th-75^th percentile, whiskers are the 10^th and 90^th percentiles and the dots are outliers beyond the 5^th and 95^th percentiles.