Relaxed trait covariance in interspecific cichlid hybrids predicts morphological diversity in adaptive radiations

Abstract

The process of adaptive radiation involves multiple events of speciation in short succession, associated with ecological diversification. Understanding this process requires identifying the origins of heritable phenotypic variation that allows adaptive radiation to progress. Hybridization is one source of genetic and morphological variation that may spur adaptive radiation. We experimentally explored the potential role of hybridization in facilitating the onset of adaptive radiation. We generated first-and second-generation hybrids of four species of African cichlid fish, extant relatives of the putative ancestors of the adaptive radiations of Lakes Victoria and Malawi. We compared patterns in hybrid morphological variation with the variation in the lake radiations. We show that significant fractions of the interspecific morphological variation and the major trajectories in morphospace that characterize whole radiations can be generated in second-generation hybrids. Furthermore, we show that covariation between traits is relaxed in secondgeneration hybrids, which may facilitate adaptive diversification. These results support the idea that hybridization can provide the heritable phenotypic diversity necessary to initiate adaptive radiation.

Intermediate Kinematics Produce Inferior Feeding Performance in a Classic Case of Natural Hybridization

“…However, transgressive phenotypes have never been demonstrated in the Centrarchidae, even in intergeneric hybrids. Future work in other systems with more complex trophic traits may be necessary to demonstrate whether such relationships exist in nature and how they affect speciation processes (Selz et al 2014).…”

“…Hybridization is often thought to give rise to novel hybrid phenotypes via a phenomenon called transgressive segregation, which occurs when hybrid individuals express trait values that are more extreme than those of either parental species (Bell and Travis 2005). Transgressive segregation has been demonstrated in several of the rapidly evolving cichlid species flocks in East Africa's Rift Valley, suggesting that hybridization may play an important role in rapid adaptive radiations (Seehausen 2004;Selz et al 2014).Most work on hybridization focuses on reproductive characters or high-level fitness traits such as growth rate, but less work has been done on the proximal components of organismal performance. However, in order to understand the role of hybridization in evolution, it is crucially important to examine the functional capabilities of naturally occurring hybrids.…”

“…Hybridization is often thought to give rise to novel hybrid phenotypes via a phenomenon called transgressive segregation, which occurs when hybrid individuals express trait values that are more extreme than those of either parental species (Bell and Travis 2005). Transgressive segregation has been demonstrated in several of the rapidly evolving cichlid species flocks in East Africa's Rift Valley, suggesting that hybridization may play an important role in rapid adaptive radiations (Seehausen 2004;Selz et al 2014).…”

Mechanical Transgressive Segregation and the Rapid Origin of Trophic Novelty

“…body size) outside the parental phenotypic distribution89. Empirical studies of transgressive segregation have generally led to an appreciation that hybridization can rapidly produce novel phenotypes through sorting of alternative alleles at the multiple loci underlying quantitative trait differences between species10111213 (Table 1). Based on a similar principal, transgression can also occur from novel combinations of skeletal components that contribute non-additively to the function of mechanical systems1415 (Table 1).…”

Effects of interspecific gene flow on the phenotypic variance–covariance matrix in Lake Victoria Cichlids

“…The scores of the two leading PC axes were then compared among the five cichlid species using an ANOVA with a TukeyHSD post hoc test. To further infer where individuals from other islands would fall in the Makobe Island morphospace, they were subsequently projected into the Makobe morphospace following an approach described in (Selz et al, 2014;Lucek et al, 2014c). In short, this method takes the PC axes that were calculated using the Makobe Island individuals and subsequently infers where a given individual from another island would be placed in the mutli-species morphospace of Makobe Island.…”