Variation in a single locus can alter multiple aspects of a dynamic system

Traditionally, the focus of evolutionary developmental biology was to link genes with straightforward shifts in morphology, but in order for a species to adapt to new environments, a divergence in resource use must also occur. This divergence often involves variation in multiple components of complex functional systems. Based on this idea, Dr Albertson and his colleagues predicted that, for evo-devo research, the elucidation of broad evolutionary processes should investigate more dynamic traits within this framework.

Dr Albertson’s work focuses on the diversification of craniofacial morphology and during vertebrate evolution. The adaptive variation of craniofacial structure occurs in response to different food sources and habitats, which in turn plays a role in niche partitioning and speciation. Actually, a majority of the morphological and functional divergence between vertebrates occurs in the craniofacial region. Thus, it’s no surprise that numerous studies have examined the patterns of craniofacial divergence in a wide variety of animals. Of the animals studied, East African cichlids are considered an excellent system to study the genetic and developmental mechanisms that promote craniofacial divergence, because they exhibit a large degree of variation in craniofacial morphology.

To study variations in the craniofacial skeleton, Dr Albertson and his team combined traditional quantitative trait locus mapping, population genetics and experimental embryology to understand how differences in gene expression during larval development in African cichlids contributes to adaptive morphological variation in adults. In one study, Dr Albertson’s team showed that a gene in the Hedgehog (Hh) signalling pathway mediates widespread variation in bones that affects the kinematics of lower jaw depression in cichlids. This seemingly simple action is controlled by a complex arrangement of bones and tendons, and the efficiency of this system has major implications for how well an animal can feed. This finding contributes to the one of the key questions in evolutionary studies, which asks how genetic variation translates into ecomorphological adaptation and ultimately, fitness. Dr Albertson’s study presents experimental data that variation in a single gene affects various aspects of a dynamic mechanical system. Ultimately, this work links adaptive variations at genetic, developmental and functional levels.

Epigenetics – a divergence from the Central Dogma

Epigenetics is the study of cellular and physiological phenotypic trait variations that are caused by external or environmental factors that turn genes on and off. While the study above highlights the genetic roles for adaptive variation in the jaw, these genetic effects only contribute to a relatively small percentage of the phenotypic variation that is observed. Cichlids rapidly evolve and display a large spectrum of variation in their craniofacial skeletons, and Dr Albertson is interested in how these changes occur at the genetic level. However, his team also found that there is an epigenetic origin for the adaptive variation that occurs in the cichlid jaw.

The team revealed that cichlid larvae begin gaping their mouths right after the cartilaginous lower jaw forms, which is just prior to the commencement of bone development. They showed further that the frequency of gaping varies between species in a manner that predicts variations in bone deposition. Moreover, when the gaping is disrupted in the fast gaping species, the jaw forms in a manner that is similar to the slow gaping species. In contrast, when the gaping is forced to occur at a higher frequency, the jaw of the slow gaping species develops to resemble that of the fast gaping species. Finally, Dr Albertson and his colleagues showed that these epigenetic changes also occur through the Hh signalling pathway. Thus, both the genetic and epigenetic path to jaw shape variation appears to leverage the same pathway. Taken together, these results emphasise the complexity of how craniofacial shape takes place and propose a novel experimental framework for investigating sources of phenotypic variation beyond those determined by changes in gene sequences.

 

‘While we know a lot about how the head develops, we know much less about how the complex geometry of the skull is determined’

 

Environmental influences on gene deployment during trait development

Phenotypic plasticity refers to the ability of organisms to alter their phenotype in response to changes in the environment. Recently, phenotypic plasticity has been a hot topic in discussions of evolutionary potential, but a complete understanding of the genetic basis of plasticity is lacking. Dr Albertson and his team investigated the genetic basis of phenotypic plasticity in cichlid fishes. They crossed two divergent species to generate a genetic mapping population. These hybrid families, at early juvenile stages, were split and raised in alternate foraging environments that mimicked benthic/scraping or limnetic/sucking modes of feeding. The different environments produced variations in morphology that were largely comparable to the major axis of divergence among the cichlids, which supported the flexible stem theory of adaptive radiation – a theory proposing that patterns of plasticity in a population will shape future patterns of morphological evolution. In addition, the study revealed that the genetic architecture of nearly every morphological trait examined was highly sensitive to the environment. In other words, different genes underlie shape variation in different environments. These results have major implications for research aimed at identifying the genes that underlie species divergence. They suggest that results could be misleading if the experiments are not performed in the ‘correct’ environment, which is to say the environment in which evolutionary divergence occurred.

Natural selection acts upon phenotypic variation. This notion has not changed since the time of Darwin. Recent work in Dr Albertson’s lab highlights the importance of genes in generating variation, however it also underscores the immense significance of the environment. As the field moves forward, it will be necessary to incorporate both variables (genes and environment) into a common framework in order to gain a comprehensive and mechanistic understanding of how species evolve.